Concepts of Biology

Chapter 11 | Evolution and Its Processes 249

11 | EVOLUTION AND ITS

PROCESSES

Figure 11.1 The diversity of life on Earth is the result of evolution, a continuous process that is still occurring. (credit
“wolf”: modification of work by Gary Kramer, USFWS; credit “coral”: modification of work by William Harrigan, NOAA;
credit “river”: modification of work by Vojtěch Dostál; credit “protozoa”: modification of work by Sharon Franklin,
Stephen Ausmus, USDA ARS; credit “fish” modification of work by Christian Mehlführer; credit “mushroom”, “bee”:
modification of work by Cory Zanker; credit “tree”: modification of work by Joseph Kranak)

Chapter Outline
11.1: Discovering How Populations Change
11.2: Mechanisms of Evolution
11.3: Evidence of Evolution
11.4: Speciation
11.5: Common Misconceptions about Evolution

Introduction
All species of living organisms—from the bacteria on our skin, to the trees in our yards, to the birds outside—evolved at
some point from a different species. Although it may seem that living things today stay much the same from generation to
generation, that is not the case: evolution is ongoing. Evolution is the process through which the characteristics of species
change and through which new species arise.
The theory of evolution is the unifying theory of biology, meaning it is the framework within which biologists ask
questions about the living world. Its power is that it provides direction for predictions about living things that are borne
out in experiment after experiment. The Ukrainian-born American geneticist Theodosius Dobzhansky famously wrote that
[1]
“nothing makes sense in biology except in the light of evolution.” He meant that the principle that all life has evolved

1. Theodosius Dobzhansky. “Biology, Molecular and Organismic.” American Zoologist 4, no. 4 (1964): 449.
250 Chapter 11 | Evolution and Its Processes

and diversified from a common ancestor is the foundation from which we understand all other questions in biology. This
chapter will explain some of the mechanisms for evolutionary change and the kinds of questions that biologists can and
have answered using evolutionary theory.

11.1 | Discovering How Populations Change

By the end of this section, you will be able to:
• Explain how Darwin’s theory of evolution differed from the current view at the time
• Describe how the present-day theory of evolution was developed
• Describe how population genetics is used to study the evolution of populations

The theory of evolution by natural selection describes a mechanism for species change over time. That species change had
been suggested and debated well before Darwin. The view that species were static and unchanging was grounded in the
writings of Plato, yet there were also ancient Greeks that expressed evolutionary ideas.
In the eighteenth century, ideas about the evolution of animals were reintroduced by the naturalist Georges-Louis Leclerc,
Comte de Buffon and even by Charles Darwin’s grandfather, Erasmus Darwin. During this time, it was also accepted that
there were extinct species. At the same time, James Hutton, the Scottish naturalist, proposed that geological change occurred
gradually by the accumulation of small changes from processes (over long periods of time) just like those happening today.
This contrasted with the predominant view that the geology of the planet was a consequence of catastrophic events occurring
during a relatively brief past. Hutton’s view was later popularized by the geologist Charles Lyell in the nineteenth century.
Lyell became a friend to Darwin and his ideas were very influential on Darwin’s thinking. Lyell argued that the greater age
of Earth gave more time for gradual change in species, and the process provided an analogy for gradual change in species.
In the early nineteenth century, Jean-Baptiste Lamarck published a book that detailed a mechanism for evolutionary change
that is now referred to as inheritance of acquired characteristics. In Lamarck’s theory, modifications in an individual
caused by its environment, or the use or disuse of a structure during its lifetime, could be inherited by its offspring and, thus,
bring about change in a species. While this mechanism for evolutionary change as described by Lamarck was discredited,
Lamarck’s ideas were an important influence on evolutionary thought. The inscription on the statue of Lamarck that stands
at the gates of the Jardin des Plantes in Paris describes him as the “founder of the doctrine of evolution.”

Charles Darwin and Natural Selection

The actual mechanism for evolution was independently conceived of and described by two naturalists, Charles Darwin
and Alfred Russell Wallace, in the mid-nineteenth century. Importantly, each spent time exploring the natural world on
expeditions to the tropics. From 1831 to 1836, Darwin traveled around the world on H.M.S. Beagle, visiting South America,
Australia, and the southern tip of Africa. Wallace traveled to Brazil to collect insects in the Amazon rainforest from 1848
to 1852 and to the Malay Archipelago from 1854 to 1862. Darwin’s journey, like Wallace’s later journeys in the Malay
Archipelago, included stops at several island chains, the last being the Galápagos Islands (west of Ecuador). On these
islands, Darwin observed species of organisms on different islands that were clearly similar, yet had distinct differences.
For example, the ground finches inhabiting the Galápagos Islands comprised several species that each had a unique beak
shape (Figure 11.2). He observed both that these finches closely resembled another finch species on the mainland of South
America and that the group of species in the Galápagos formed a graded series of beak sizes and shapes, with very small
differences between the most similar. Darwin imagined that the island species might be all species modified from one
original mainland species. In 1860, he wrote, “Seeing this gradation and diversity of structure in one small, intimately
related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been
[2]
taken and modified for different ends.”

2. Charles Darwin, Journal of Researches into the Natural History and Geology of the Countries Visited during the Voyage of H.M.S. Beagle Round the
World, under the Command of Capt. Fitz Roy, R.N, 2nd. ed. (London: John Murray, 1860), http://www.archive.org/details/journalofresea00darw.

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Chapter 11 | Evolution and Its Processes 251

Figure 11.2 Darwin observed that beak shape varies among finch species. He postulated that the beak of an ancestral
species had adapted over time to equip the finches to acquire different food sources. This illustration shows the beak
shapes for four species of ground finch: 1. Geospiza magnirostris (the large ground finch), 2. G. fortis (the medium
ground finch), 3. G. parvula (the small tree finch), and 4. Certhidea olivacea (the green-warbler finch).

Wallace and Darwin both observed similar patterns in other organisms and independently conceived a mechanism to explain
how and why such changes could take place. Darwin called this mechanism natural selection. Natural selection, Darwin
argued, was an inevitable outcome of three principles that operated in nature. First, the characteristics of organisms are
inherited, or passed from parent to offspring. Second, more offspring are produced than are able to survive; in other words,
resources for survival and reproduction are limited. The capacity for reproduction in all organisms outstrips the availability
of resources to support their numbers. Thus, there is a competition for those resources in each generation. Both Darwin and
Wallace’s understanding of this principle came from reading an essay by the economist Thomas Malthus, who discussed
this principle in relation to human populations. Third, offspring vary among each other in regard to their characteristics
and those variations are inherited. Out of these three principles, Darwin and Wallace reasoned that offspring with inherited
characteristics that allow them to best compete for limited resources will survive and have more offspring than those
individuals with variations that are less able to compete. Because characteristics are inherited, these traits will be better
represented in the next generation. This will lead to change in populations over generations in a process that Darwin called
“descent with modification.”
Papers by Darwin and Wallace (Figure 11.3) presenting the idea of natural selection were read together in 1858 before the
Linnaean Society in London. The following year Darwin’s book, On the Origin of Species, was published, which outlined
in considerable detail his arguments for evolution by natural selection.
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Figure 11.3 (a) Charles Darwin and (b) Alfred Wallace wrote scientific papers on natural selection that were presented
together before the Linnean Society in 1858.

Demonstrations of evolution by natural selection can be time consuming. One of the best demonstrations has been in the
very birds that helped to inspire the theory, the Galápagos finches. Peter and Rosemary Grant and their colleagues have
studied Galápagos finch populations every year since 1976 and have provided important demonstrations of the operation
of natural selection. The Grants found changes from one generation to the next in the beak shapes of the medium ground
finches on the Galápagos island of Daphne Major. The medium ground finch feeds on seeds. The birds have inherited
variation in the bill shape with some individuals having wide, deep bills and others having thinner bills. Large-billed birds
feed more efficiently on large, hard seeds, whereas smaller billed birds feed more efficiently on small, soft seeds. During
1977, a drought period altered vegetation on the island. After this period, the number of seeds declined dramatically: the
decline in small, soft seeds was greater than the decline in large, hard seeds. The large-billed birds were able to survive
better than the small-billed birds the following year. The year following the drought when the Grants measured beak sizes
in the much-reduced population, they found that the average bill size was larger (Figure 11.4). This was clear evidence
for natural selection (differences in survival) of bill size caused by the availability of seeds. The Grants had studied the
inheritance of bill sizes and knew that the surviving large-billed birds would tend to produce offspring with larger bills,
so the selection would lead to evolution of bill size. Subsequent studies by the Grants have demonstrated selection on and
evolution of bill size in this species in response to changing conditions on the island. The evolution has occurred both to
larger bills, as in this case, and to smaller bills when large seeds became rare.

Figure 11.4 A drought on the Galápagos island of Daphne Major in 1977 reduced the number of small seeds available
to finches, causing many of the small-beaked finches to die. This caused an increase in the finches’ average beak size
between 1976 and 1978.

Variation and Adaptation

Natural selection can only take place if there is variation, or differences, among individuals in a population. Importantly,
these differences must have some genetic basis; otherwise, selection will not lead to change in the next generation. This
is critical because variation among individuals can be caused by non-genetic reasons, such as an individual being taller
because of better nutrition rather than different genes.
Genetic diversity in a population comes from two main sources: mutation and sexual reproduction. Mutation, a change in
DNA, is the ultimate source of new alleles or new genetic variation in any population. An individual that has a mutated
gene might have a different trait than other individuals in the population. However, this is not always the case. A mutation
can have one of three outcomes on the organisms’ appearance (or phenotype):

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Chapter 11 | Evolution and Its Processes 253

• A mutation may affect the phenotype of the organism in a way that gives it reduced fitness—lower likelihood of
survival, resulting in fewer offspring.
• A mutation may produce a phenotype with a beneficial effect on fitness.
• Many mutations, called neutral mutations, will have no effect on fitness.
Mutations may also have a whole range of effect sizes on the fitness of the organism that expresses them in their phenotype,
from a small effect to a great effect. Sexual reproduction and crossing over in meiosis also lead to genetic diversity: when
two parents reproduce, unique combinations of alleles assemble to produce unique genotypes and, thus, phenotypes in each
of the offspring.
A heritable trait that aids the survival and reproduction of an organism in its present environment is called an adaptation.
An adaptation is a “match” of the organism to the environment. Adaptation to an environment comes about when a
change in the range of genetic variation occurs over time that increases or maintains the match of the population with its
environment. The variations in finch beaks shifted from generation to generation providing adaptation to food availability.
Whether or not a trait is favorable depends on the environment at the time. The same traits do not always have the same
relative benefit or disadvantage because environmental conditions can change. For example, finches with large bills were
benefited in one climate, while small bills were a disadvantage; in a different climate, the relationship reversed.
Patterns of Evolution
The evolution of species has resulted in enormous variation in form and function. When two species evolve in different
directions from a common point, it is called divergent evolution. Such divergent evolution can be seen in the forms of the
reproductive organs of flowering plants, which share the same basic anatomies; however, they can look very different as a
result of selection in different physical environments, and adaptation to different kinds of pollinators (Figure 11.5).

Figure 11.5 Flowering plants evolved from a common ancestor. Notice that the (a) dense blazing star and (b) purple
coneflower vary in appearance, yet both share a similar basic morphology. (credit a, b: modification of work by Cory
Zanker)

In other cases, similar phenotypes evolve independently in distantly related species. For example, flight has evolved in
both bats and insects, and they both have structures we refer to as wings, which are adaptations to flight. The wings
of bats and insects, however, evolved from very different original structures. When similar structures arise through
evolution independently in different species it is called convergent evolution. The wings of bats and insects are called
analogous structures; they are similar in function and appearance, but do not share an origin in a common ancestor.
Instead they evolved independently in the two lineages. The wings of a hummingbird and an ostrich are homologous
structures, meaning they share similarities (despite their differences resulting from evolutionary divergence). The wings
of hummingbirds and ostriches did not evolve independently in the hummingbird lineage and the ostrich lineage—they
descended from a common ancestor with wings.

The Modern Synthesis

The mechanisms of inheritance, genetics, were not understood at the time Darwin and Wallace were developing their
idea of natural selection. This lack of understanding was a stumbling block to comprehending many aspects of evolution.
In fact, blending inheritance was the predominant (and incorrect) genetic theory of the time, which made it difficult to
understand how natural selection might operate. Darwin and Wallace were unaware of the genetics work by Austrian monk
Gregor Mendel, which was published in 1866, not long after publication of On the Origin of Species. Mendel’s work was
rediscovered in the early twentieth century at which time geneticists were rapidly coming to an understanding of the basics
of inheritance. Initially, the newly discovered particulate nature of genes made it difficult for biologists to understand how
gradual evolution could occur. But over the next few decades genetics and evolution were integrated in what became known
254 Chapter 11 | Evolution and Its Processes

as the modern synthesis—the coherent understanding of the relationship between natural selection and genetics that took
shape by the 1940s and is generally accepted today. In sum, the modern synthesis describes how evolutionary pressures,
such as natural selection, can affect a population’s genetic makeup, and, in turn, how this can result in the gradual evolution
of populations and species. The theory also connects this gradual change of a population over time, called microevolution,
with the processes that gave rise to new species and higher taxonomic groups with widely divergent characters, called
macroevolution.

Population Genetics
Recall that a gene for a particular character may have several variants, or alleles, that code for different traits associated
with that character. For example, in the ABO blood type system in humans, three alleles determine the particular blood-type
protein on the surface of red blood cells. Each individual in a population of diploid organisms can only carry two alleles for
a particular gene, but more than two may be present in the individuals that make up the population. Mendel followed alleles
as they were inherited from parent to offspring. In the early twentieth century, biologists began to study what happens to all
the alleles in a population in a field of study known as population genetics.
Until now, we have defined evolution as a change in the characteristics of a population of organisms, but behind that
phenotypic change is genetic change. In population genetic terms, evolution is defined as a change in the frequency of an
allele in a population. Using the ABO system as an example, the frequency of one of the alleles, IA, is the number of copies
of that allele divided by all the copies of the ABO gene in the population. For example, a study in Jordan found a frequency
[3]
of IA to be 26.1 percent. The IB, I0 alleles made up 13.4 percent and 60.5 percent of the alleles respectively, and all of the
frequencies add up to 100 percent. A change in this frequency over time would constitute evolution in the population.
There are several ways the allele frequencies of a population can change. One of those ways is natural selection. If a given
allele confers a phenotype that allows an individual to have more offspring that survive and reproduce, that allele, by virtue
of being inherited by those offspring, will be in greater frequency in the next generation. Since allele frequencies always
add up to 100 percent, an increase in the frequency of one allele always means a corresponding decrease in one or more
of the other alleles. Highly beneficial alleles may, over a very few generations, become “fixed” in this way, meaning that
every individual of the population will carry the allele. Similarly, detrimental alleles may be swiftly eliminated from the
gene pool, the sum of all the alleles in a population. Part of the study of population genetics is tracking how selective forces
change the allele frequencies in a population over time, which can give scientists clues regarding the selective forces that
may be operating on a given population. The studies of changes in wing coloration in the peppered moth from mottled white
to dark in response to soot-covered tree trunks and then back to mottled white when factories stopped producing so much
soot is a classic example of studying evolution in natural populations (Figure 11.6).

Figure 11.6 As the Industrial Revolution caused trees to darken from soot, darker colored peppered moths were
better camouflaged than the lighter colored ones, which caused there to be more of the darker colored moths in the
population.

In the early twentieth century, English mathematician Godfrey Hardy and German physician Wilhelm Weinberg
independently provided an explanation for a somewhat counterintuitive concept. Hardy’s original explanation was in
response to a misunderstanding as to why a “dominant” allele, one that masks a recessive allele, should not increase in
frequency in a population until it eliminated all the other alleles. The question resulted from a common confusion about
what “dominant” means, but it forced Hardy, who was not even a biologist, to point out that if there are no factors that affect
an allele frequency those frequencies will remain constant from one generation to the next. This principle is now known
as the Hardy-Weinberg equilibrium. The theory states that a population’s allele and genotype frequencies are inherently
stable—unless some kind of evolutionary force is acting on the population, the population would carry the same alleles in
the same proportions generation after generation. Individuals would, as a whole, look essentially the same and this would
3. Sahar S. Hanania, Dhia S. Hassawi, and Nidal M. Irshaid, “Allele Frequency and Molecular Genotypes of ABO Blood Group System in a Jordanian
Population,” Journal of Medical Sciences 7 (2007): 51-58, doi:10.3923/jms.2007.51.58

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Chapter 11 | Evolution and Its Processes 255

be unrelated to whether the alleles were dominant or recessive. The four most important evolutionary forces, which will
disrupt the equilibrium, are natural selection, mutation, genetic drift, and migration into or out of a population. A fifth
factor, nonrandom mating, will also disrupt the Hardy-Weinberg equilibrium but only by shifting genotype frequencies, not
allele frequencies. In nonrandom mating, individuals are more likely to mate with like individuals (or unlike individuals)
rather than at random. Since nonrandom mating does not change allele frequencies, it does not cause evolution directly.
Natural selection has been described. Mutation creates one allele out of another one and changes an allele’s frequency
by a small, but continuous amount each generation. Each allele is generated by a low, constant mutation rate that will
slowly increase the allele’s frequency in a population if no other forces act on the allele. If natural selection acts against
the allele, it will be removed from the population at a low rate leading to a frequency that results from a balance between
selection and mutation. This is one reason that genetic diseases remain in the human population at very low frequencies.
If the allele is favored by selection, it will increase in frequency. Genetic drift causes random changes in allele frequencies
when populations are small. Genetic drift can often be important in evolution, as discussed in the next section. Finally, if
two populations of a species have different allele frequencies, migration of individuals between them will cause frequency
changes in both populations. As it happens, there is no population in which one or more of these processes are not operating,
so populations are always evolving, and the Hardy-Weinberg equilibrium will never be exactly observed. However, the
Hardy-Weinberg principle gives scientists a baseline expectation for allele frequencies in a non-evolving population to
which they can compare evolving populations and thereby infer what evolutionary forces might be at play. The population
is evolving if the frequencies of alleles or genotypes deviate from the value expected from the Hardy-Weinberg principle.
Darwin identified a special case of natural selection that he called sexual selection. Sexual selection affects an individual’s
ability to mate and thus produce offspring, and it leads to the evolution of dramatic traits that often appear maladaptive
in terms of survival but persist because they give their owners greater reproductive success. Sexual selection occurs in
two ways: through male–male competition for mates and through female selection of mates. Male–male competition takes
the form of conflicts between males, which are often ritualized, but may also pose significant threats to a male’s survival.
Sometimes the competition is for territory, with females more likely to mate with males with higher quality territories.
Female choice occurs when females choose a male based on a particular trait, such as feather colors, the performance of a
mating dance, or the building of an elaborate structure. In some cases male–male competition and female choice combine in
the mating process. In each of these cases, the traits selected for, such as fighting ability or feather color and length, become
enhanced in the males. In general, it is thought that sexual selection can proceed to a point at which natural selection against
a character’s further enhancement prevents its further evolution because it negatively impacts the male’s ability to survive.
For example, colorful feathers or an elaborate display make the male more obvious to predators.

11.2 | Mechanisms of Evolution

By the end of this section, you will be able to:
• Describe the four basic causes of evolution: natural selection, mutation, genetic drift, and gene flow
• Explain how each evolutionary force can influence the allele frequencies of a population

The Hardy-Weinberg equilibrium principle says that allele frequencies in a population will remain constant in the absence
of the four factors that could change them. Those factors are natural selection, mutation, genetic drift, and migration (gene
flow). In fact, we know they are probably always affecting populations.

Natural Selection
Natural selection has already been discussed. Alleles are expressed in a phenotype. Depending on the environmental
conditions, the phenotype confers an advantage or disadvantage to the individual with the phenotype relative to the other
phenotypes in the population. If it is an advantage, then that individual will likely have more offspring than individuals with
the other phenotypes, and this will mean that the allele behind the phenotype will have greater representation in the next
generation. If conditions remain the same, those offspring, which are carrying the same allele, will also benefit. Over time,
the allele will increase in frequency in the population.

Mutation
Mutation is a source of new alleles in a population. Mutation is a change in the DNA sequence of the gene. A mutation
can change one allele into another, but the net effect is a change in frequency. The change in frequency resulting from
mutation is small, so its effect on evolution is small unless it interacts with one of the other factors, such as selection. A
mutation may produce an allele that is selected against, selected for, or selectively neutral. Harmful mutations are removed
256 Chapter 11 | Evolution and Its Processes

from the population by selection and will generally only be found in very low frequencies equal to the mutation rate.
Beneficial mutations will spread through the population through selection, although that initial spread is slow. Whether
or not a mutation is beneficial or harmful is determined by whether it helps an organism survive to sexual maturity and
reproduce. It should be noted that mutation is the ultimate source of genetic variation in all populations—new alleles, and,
therefore, new genetic variations arise through mutation.

Genetic Drift
Another way a population’s allele frequencies can change is genetic drift (Figure 11.7), which is simply the effect of
chance. Genetic drift is most important in small populations. Drift would be completely absent in a population with infinite
individuals, but, of course, no population is this large. Genetic drift occurs because the alleles in an offspring generation
are a random sample of the alleles in the parent generation. Alleles may or may not make it into the next generation
due to chance events including mortality of an individual, events affecting finding a mate, and even the events affecting
which gametes end up in fertilizations. If one individual in a population of ten individuals happens to die before it leaves
any offspring to the next generation, all of its genes—a tenth of the population’s gene pool—will be suddenly lost. In a
population of 100, that 1 individual represents only 1 percent of the overall gene pool; therefore, it has much less impact on
the population’s genetic structure and is unlikely to remove all copies of even a relatively rare allele.
Imagine a population of ten individuals, half with allele A and half with allele a (the individuals are haploid). In a stable
population, the next generation will also have ten individuals. Choose that generation randomly by flipping a coin ten times
and let heads be A and tails be a. It is unlikely that the next generation will have exactly half of each allele. There might
be six of one and four of the other, or some different set of frequencies. Thus, the allele frequencies have changed and
evolution has occurred. A coin will no longer work to choose the next generation (because the odds are no longer one half
for each allele). The frequency in each generation will drift up and down on what is known as a random walk until at one
point either all A or all a are chosen and that allele is fixed from that point on. This could take a very long time for a large
population. This simplification is not very biological, but it can be shown that real populations behave this way. The effect
of drift on frequencies is greater the smaller a population is. Its effect is also greater on an allele with a frequency far from
one half. Drift will influence every allele, even those that are being naturally selected.

Figure 11.7 Genetic drift in a population can lead to the elimination of an allele from a population by chance. In
each generation, a random set of individuals reproduces to produce the next generation. The frequency of alleles
in the next generation is equal to the frequency of alleles among the individuals reproducing.

Do you think genetic drift would happen more quickly on an island or on the mainland?

Genetic drift can also be magnified by natural or human-caused events, such as a disaster that randomly kills a large portion
of the population, which is known as the bottleneck effect that results in a large portion of the genome suddenly being
wiped out (Figure 11.8). In one fell swoop, the genetic structure of the survivors becomes the genetic structure of the entire

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Chapter 11 | Evolution and Its Processes 257

population, which may be very different from the pre-disaster population. The disaster must be one that kills for reasons
unrelated to the organism’s traits, such as a hurricane or lava flow. A mass killing caused by unusually cold temperatures at
night, is likely to affect individuals differently depending on the alleles they possess that confer cold hardiness.

Figure 11.8 A chance event or catastrophe can reduce the genetic variability within a population.

Another scenario in which populations might experience a strong influence of genetic drift is if some portion of the
population leaves to start a new population in a new location, or if a population gets divided by a physical barrier of
some kind. In this situation, those individuals are unlikely to be representative of the entire population which results in the
founder effect. The founder effect occurs when the genetic structure matches that of the new population’s founding fathers
and mothers. The founder effect is believed to have been a key factor in the genetic history of the Afrikaner population of
Dutch settlers in South Africa, as evidenced by mutations that are common in Afrikaners but rare in most other populations.
This is likely due to a higher-than-normal proportion of the founding colonists, which were a small sample of the original
population, carried these mutations. As a result, the population expresses unusually high incidences of Huntington’s disease
(HD) and Fanconi anemia (FA), a genetic disorder known to cause bone marrow and congenital abnormalities, and even
[4]
cancer.

Visit this site (http://openstaxcollege.org/l/genetic_drift2) to learn more about genetic drift and to run simulations of
allele changes caused by drift.

Gene Flow
Another important evolutionary force is gene flow, or the flow of alleles in and out of a population resulting from the
migration of individuals or gametes (Figure 11.9). While some populations are fairly stable, others experience more flux.
Many plants, for example, send their seeds far and wide, by wind or in the guts of animals; these seeds may introduce alleles
common in the source population to a new population in which they are rare.

4. A. J. Tipping et al., “Molecular and Genealogical Evidence for a Founder Effect in Fanconi Anemia Families of the Afrikaner Population of South
Africa,” PNAS 98, no. 10 (2001): 5734-5739, doi: 10.1073/pnas.091402398.
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Figure 11.9 Gene flow can occur when an individual travels from one geographic location to another and joins a
different population of the species. In the example shown here, the brown allele is introduced into the green population.

11.3 | Evidence of Evolution

By the end of this section, you will be able to:
• Explain sources of evidence for evolution
• Define homologous and vestigial structures

The evidence for evolution is compelling and extensive. Looking at every level of organization in living systems, biologists
see the signature of past and present evolution. Darwin dedicated a large portion of his book, On the Origin of Species,
identifying patterns in nature that were consistent with evolution and since Darwin our understanding has become clearer
and broader.

Fossils
Fossils provide solid evidence that organisms from the past are not the same as those found today; fossils show a progression
of evolution. Scientists determine the age of fossils and categorize them all over the world to determine when the organisms
lived relative to each other. The resulting fossil record tells the story of the past, and shows the evolution of form over
millions of years (Figure 11.10). For example, highly detailed fossil records have been recovered for sequences of species
in the evolution of whales and modern horses. The fossil record of horses in North America is especially rich and many
contain transition fossils: those showing intermediate anatomy between earlier and later forms. The fossil record extends
back to a dog-like ancestor some 55 million years ago that gave rise to the first horse-like species 55 to 42 million years
ago in the genus Eohippus. The series of fossils tracks the change in anatomy resulting from a gradual drying trend that
changed the landscape from a forested one to a prairie. Successive fossils show the evolution of teeth shapes and foot and
leg anatomy to a grazing habit, with adaptations for escaping predators, for example in species of Mesohippus found from
40 to 30 million years ago. Later species showed gains in size, such as those of Hipparion, which existed from about 23 to
2 million years ago. The fossil record shows several adaptive radiations in the horse lineage, which is now much reduced to
only one genus, Equus, with several species.

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Chapter 11 | Evolution and Its Processes 259

Figure 11.10 This illustration shows an artist’s renderings of these species derived from fossils of the evolutionary
history of the horse and its ancestors. The species depicted are only four from a very diverse lineage that contains
many branches, dead ends, and adaptive radiations. One of the trends, depicted here is the evolutionary tracking of
a drying climate and increase in prairie versus forest habitat reflected in forms that are more adapted to grazing and
predator escape through running. Przewalski's horse is one of a few living species of horse.

Anatomy and Embryology

Another type of evidence for evolution is the presence of structures in organisms that share the same basic form. For
example, the bones in the appendages of a human, dog, bird, and whale all share the same overall construction (Figure
11.11). That similarity results from their origin in the appendages of a common ancestor. Over time, evolution led to changes
in the shapes and sizes of these bones in different species, but they have maintained the same overall layout, evidence of
descent from a common ancestor. Scientists call these synonymous parts homologous structures. Some structures exist in
organisms that have no apparent function at all, and appear to be residual parts from a past ancestor. For example, some
snakes have pelvic bones despite having no legs because they descended from reptiles that did have legs. These unused
structures without function are called vestigial structures. Other examples of vestigial structures are wings on flightless
birds (which may have other functions), leaves on some cacti, traces of pelvic bones in whales, and the sightless eyes of
cave animals.

Figure 11.11 The similar construction of these appendages indicates that these organisms share a common ancestor.

Click through the activities at this interactive site (http://openstaxcollege.org/l/bone_structure2) to guess which bone
structures are homologous and which are analogous, and to see examples of all kinds of evolutionary adaptations that
illustrate these concepts.
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Another evidence of evolution is the convergence of form in organisms that share similar environments. For example,
species of unrelated animals, such as the arctic fox and ptarmigan (a bird), living in the arctic region have temporary white
coverings during winter to blend with the snow and ice (Figure 11.12). The similarity occurs not because of common
ancestry, indeed one covering is of fur and the other of feathers, but because of similar selection pressures—the benefits of
not being seen by predators.

Figure 11.12 The white winter coat of (a) the arctic fox and (b) the ptarmigan’s plumage are adaptations to their
environments. (credit a: modification of work by Keith Morehouse)

Embryology, the study of the development of the anatomy of an organism to its adult form also provides evidence of
relatedness between now widely divergent groups of organisms. Structures that are absent in some groups often appear in
their embryonic forms and disappear by the time the adult or juvenile form is reached. For example, all vertebrate embryos,
including humans, exhibit gill slits at some point in their early development. These disappear in the adults of terrestrial
groups, but are maintained in adult forms of aquatic groups such as fish and some amphibians. Great ape embryos, including
humans, have a tail structure during their development that is lost by the time of birth. The reason embryos of unrelated
species are often similar is that mutational changes that affect the organism during embryonic development can cause
amplified differences in the adult, even while the embryonic similarities are preserved.

Biogeography
The geographic distribution of organisms on the planet follows patterns that are best explained by evolution in conjunction
with the movement of tectonic plates over geological time. Broad groups that evolved before the breakup of the
supercontinent Pangaea (about 200 million years ago) are distributed worldwide. Groups that evolved since the breakup
appear uniquely in regions of the planet, for example the unique flora and fauna of northern continents that formed from
the supercontinent Laurasia and of the southern continents that formed from the supercontinent Gondwana. The presence of
Proteaceae in Australia, southern Africa, and South America is best explained by the plant family’s presence there prior to
the southern supercontinent Gondwana breaking up (Figure 11.13).

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Chapter 11 | Evolution and Its Processes 261

Figure 11.13 The Proteacea family of plants evolved before the supercontinent Gondwana broke up. Today, members
of this plant family are found throughout the southern hemisphere (shown in red). (credit “Proteacea flower”:
modification of work by “dorofofoto”/Flickr)

The great diversification of the marsupials in Australia and the absence of other mammals reflects that island continent’s
long isolation. Australia has an abundance of endemic species—species found nowhere else—which is typical of islands
whose isolation by expanses of water prevents migration of species to other regions. Over time, these species diverge
evolutionarily into new species that look very different from their ancestors that may exist on the mainland. The marsupials
of Australia, the finches on the Galápagos, and many species on the Hawaiian Islands are all found nowhere else but on
their island, yet display distant relationships to ancestral species on mainlands.

Molecular Biology
Like anatomical structures, the structures of the molecules of life reflect descent with modification. Evidence of a common
ancestor for all of life is reflected in the universality of DNA as the genetic material and of the near universality of
the genetic code and the machinery of DNA replication and expression. Fundamental divisions in life between the three
domains are reflected in major structural differences in otherwise conservative structures such as the components of
ribosomes and the structures of membranes. In general, the relatedness of groups of organisms is reflected in the similarity
of their DNA sequences—exactly the pattern that would be expected from descent and diversification from a common
ancestor.
DNA sequences have also shed light on some of the mechanisms of evolution. For example, it is clear that the evolution
of new functions for proteins commonly occurs after gene duplication events. These duplications are a kind of mutation
in which an entire gene is added as an extra copy (or many copies) in the genome. These duplications allow the free
modification of one copy by mutation, selection, and drift, while the second copy continues to produce a functional protein.
This allows the original function for the protein to be kept, while evolutionary forces tweak the copy until it functions in a
new way.

11.4 | Speciation
By the end of this section, you will be able to:
• Describe the definition of species and how species are identified as different
• Explain allopatric and sympatric speciation
• Describe adaptive radiation

The biological definition of species, which works for sexually reproducing organisms, is a group of actually or potentially
interbreeding individuals. According to this definition, one species is distinguished from another by the possibility of
matings between individuals from each species to produce fertile offspring. There are exceptions to this rule. Many species
are similar enough that hybrid offspring are possible and may often occur in nature, but for the majority of species this
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rule generally holds. In fact, the presence of hybrids between similar species suggests that they may have descended from a
single interbreeding species and that the speciation process may not yet be completed.
Given the extraordinary diversity of life on the planet there must be mechanisms for speciation: the formation of two
species from one original species. Darwin envisioned this process as a branching event and diagrammed the process in the
only illustration found in On the Origin of Species (Figure 11.14a). For speciation to occur, two new populations must
be formed from one original population, and they must evolve in such a way that it becomes impossible for individuals
from the two new populations to interbreed. Biologists have proposed mechanisms by which this could occur that fall into
two broad categories. Allopatric speciation, meaning speciation in “other homelands,” involves a geographic separation
of populations from a parent species and subsequent evolution. Sympatric speciation, meaning speciation in the “same
homeland,” involves speciation occurring within a parent species while remaining in one location.
Biologists think of speciation events as the splitting of one ancestral species into two descendant species. There is no reason
why there might not be more than two species formed at one time except that it is less likely and such multiple events can
also be conceptualized as single splits occurring close in time.

Figure 11.14 The only illustration in Darwin’s On the Origin of Species is (a) a diagram showing speciation events
leading to biological diversity. The diagram shows similarities to phylogenetic charts that are drawn today to illustrate
the relationships of species. (b) Modern elephants evolved from the Palaeomastodon, a species that lived in Egypt
35–50 million years ago.

Speciation through Geographic Separation

A geographically continuous population has a gene pool that is relatively homogeneous. Gene flow, the movement of alleles
across the range of the species, is relatively free because individuals can move and then mate with individuals in their new
location. Thus, the frequency of an allele at one end of a distribution will be similar to the frequency of the allele at the other
end. When populations become geographically discontinuous that free-flow of alleles is prevented. When that separation
lasts for a period of time, the two populations are able to evolve along different trajectories. Thus, their allele frequencies
at numerous genetic loci gradually become more and more different as new alleles independently arise by mutation in
each population. Typically, environmental conditions, such as climate, resources, predators, and competitors, for the two
populations will differ causing natural selection to favor divergent adaptations in each group. Different histories of genetic
drift, enhanced because the populations are smaller than the parent population, will also lead to divergence.
Given enough time, the genetic and phenotypic divergence between populations will likely affect characters that influence
reproduction enough that were individuals of the two populations brought together, mating would be less likely, or if a
mating occurred, offspring would be non-viable or infertile. Many types of diverging characters may affect the reproductive
isolation (inability to interbreed) of the two populations. These mechanisms of reproductive isolation can be divided
into prezygotic mechanisms (those that operate before fertilization) and postzygotic mechanisms (those that operate after
fertilization). Prezygotic mechanisms include traits that allow the individuals to find each other, such as the timing of
mating, sensitivity to pheromones, or choice of mating sites. If individuals are able to encounter each other, character
divergence may prevent courtship rituals from leading to a mating either because female preferences have changed or
male behaviors have changed. Physiological changes may interfere with successful fertilization if mating is able to occur.
Postzygotic mechanisms include genetic incompatibilities that prevent proper development of the offspring, or if the
offspring live, they may be unable to produce viable gametes themselves as in the example of the mule, the infertile
offspring of a female horse and a male donkey.

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Chapter 11 | Evolution and Its Processes 263

If the two isolated populations are brought back together and the hybrid offspring that formed from matings between
individuals of the two populations have lower survivorship or reduced fertility, then selection will favor individuals that are
able to discriminate between potential mates of their own population and the other population. This selection will enhance
the reproductive isolation.
Isolation of populations leading to allopatric speciation can occur in a variety of ways: from a river forming a new branch,
erosion forming a new valley, or a group of organisms traveling to a new location without the ability to return, such as
seeds floating over the ocean to an island. The nature of the geographic separation necessary to isolate populations depends
entirely on the biology of the organism and its potential for dispersal. If two flying insect populations took up residence in
separate nearby valleys, chances are that individuals from each population would fly back and forth, continuing gene flow.
However, if two rodent populations became divided by the formation of a new lake, continued gene flow would be unlikely;
therefore, speciation would be more likely.
Biologists group allopatric processes into two categories. If a few members of a species move to a new geographical area,
this is called dispersal. If a natural situation arises to physically divide organisms, this is called vicariance.
Scientists have documented numerous cases of allopatric speciation taking place. For example, along the west coast of
the United States, two separate subspecies of spotted owls exist. The northern spotted owl has genetic and phenotypic
differences from its close relative, the Mexican spotted owl, which lives in the south (Figure 11.15). The cause of their
initial separation is not clear, but it may have been caused by the glaciers of the ice age dividing an initial population into
[5]
two.

Figure 11.15 The northern spotted owl and the Mexican spotted owl inhabit geographically separate locations with
different climates and ecosystems. The owl is an example of incipient speciation. (credit “northern spotted owl”:
modification of work by John and Karen Hollingsworth, USFWS; credit “Mexican spotted owl”: modification of work by
Bill Radke, USFWS)

Additionally, scientists have found that the further the distance between two groups that once were the same species, the
more likely for speciation to take place. This seems logical because as the distance increases, the various environmental
factors would likely have less in common than locations in close proximity. Consider the two owls; in the north, the climate
is cooler than in the south; the other types of organisms in each ecosystem differ, as do their behaviors and habits; also, the
hunting habits and prey choices of the owls in the south vary from the northern ones. These variances can lead to evolved
differences in the owls, and over time speciation will likely occur unless gene flow between the populations is restored.

5. Courtney, S.P., et al, “Scientific Evaluation of the Status of the Northern Spotted Owl,” Sustainable Ecosystems Institute (2004), Portland, OR.
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In some cases, a population of one species disperses throughout an area, and each finds a distinct niche or isolated
habitat. Over time, the varied demands of their new lifestyles lead to multiple speciation events originating from a single
species, which is called adaptive radiation. From one point of origin, many adaptations evolve causing the species to
radiate into several new ones. Island archipelagos like the Hawaiian Islands provide an ideal context for adaptive radiation
events because water surrounds each island, which leads to geographical isolation for many organisms (Figure 11.16). The
Hawaiian honeycreeper illustrates one example of adaptive radiation. From a single species, called the founder species,
numerous species have evolved, including the eight shown in Figure 11.16.

Figure 11.16 The honeycreeper birds illustrate adaptive radiation. From one original species of bird, multiple others
evolved, each with its own distinctive characteristics.

Notice the differences in the species’ beaks in Figure 11.16. Change in the genetic variation for beaks in response to natural
selection based on specific food sources in each new habitat led to evolution of a different beak suited to the specific food
source. The fruit and seed-eating birds have thicker, stronger beaks which are suited to break hard nuts. The nectar-eating
birds have long beaks to dip into flowers to reach their nectar. The insect-eating birds have beaks like swords, appropriate for
stabbing and impaling insects. Darwin’s finches are another well-studied example of adaptive radiation in an archipelago.

Click through this interactive site (http://openstaxcollege.org/l/bird_evolution) to see how island birds evolved; click
to see images of each species in evolutionary increments from five million years ago to today.

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Chapter 11 | Evolution and Its Processes 265

Speciation without Geographic Separation

Can divergence occur if no physical barriers are in place to separate individuals who continue to live and reproduce in the
same habitat? A number of mechanisms for sympatric speciation have been proposed and studied.
One form of sympatric speciation can begin with a chromosomal error during meiosis or the formation of a hybrid
individual with too many chromosomes. Polyploidy is a condition in which a cell, or organism, has an extra set, or sets,
of chromosomes. Scientists have identified two main types of polyploidy that can lead to reproductive isolation of an
individual in the polyploid state. In some cases a polyploid individual will have two or more complete sets of chromosomes
from its own species in a condition called autopolyploidy (Figure 11.17). The prefix “auto” means self, so the term means
multiple chromosomes from one’s own species. Polyploidy results from an error in meiosis in which all of the chromosomes
move into one cell instead of separating.

Figure 11.17 Autopolyploidy results when mitosis is not followed by cytokinesis.

For example, if a plant species with 2n = 6 produces autopolyploid gametes that are also diploid (2n = 6, when they
should be n = 3), the gametes now have twice as many chromosomes as they should have. These new gametes will be
incompatible with the normal gametes produced by this plant species. But they could either self-pollinate or reproduce
with other autopolyploid plants with gametes having the same diploid number. In this way, sympatric speciation can occur
quickly by forming offspring with 4n called a tetraploid. These individuals would immediately be able to reproduce only
with those of this new kind and not those of the ancestral species. The other form of polyploidy occurs when individuals
of two different species reproduce to form a viable offspring called an allopolyploid. The prefix “allo” means “other”
(recall from allopatric); therefore, an allopolyploid occurs when gametes from two different species combine. Figure 11.18
illustrates one possible way an allopolyploidy can form. Notice how it takes two generations, or two reproductive acts,
before the viable fertile hybrid results.

Figure 11.18 Alloploidy results when two species mate to produce viable offspring. In the example shown, a normal
gamete from one species fuses with a polyploid gamete from another. Two matings are necessary to produce viable
offspring.

The cultivated forms of wheat, cotton, and tobacco plants are all allopolyploids. Although polyploidy occurs occasionally
in animals, most chromosomal abnormalities in animals are lethal; it takes place most commonly in plants. Scientists have
discovered more than 1/2 of all plant species studied relate back to a species evolved through polyploidy.
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Sympatric speciation may also take place in ways other than polyploidy. For example, imagine a species of fish that lived in
a lake. As the population grew, competition for food also grew. Under pressure to find food, suppose that a group of these
fish had the genetic flexibility to discover and feed off another resource that was unused by the other fish. What if this new
food source was found at a different depth of the lake? Over time, those feeding on the second food source would interact
more with each other than the other fish; therefore they would breed together as well. Offspring of these fish would likely
behave as their parents and feed and live in the same area, keeping them separate from the original population. If this group
of fish continued to remain separate from the first population, eventually sympatric speciation might occur as more genetic
differences accumulated between them.
This scenario does play out in nature, as do others that lead to reproductive isolation. One such place is Lake Victoria in
Africa, famous for its sympatric speciation of cichlid fish. Researchers have found hundreds of sympatric speciation events
in these fish, which have not only happened in great number, but also over a short period of time. Figure 11.19 shows
this type of speciation among a cichlid fish population in Nicaragua. In this locale, two types of cichlids live in the same
geographic location; however, they have come to have different morphologies that allow them to eat various food sources.

Figure 11.19 Cichlid fish from Lake Apoyeque, Nicaragua, show evidence of sympatric speciation. Lake Apoyeque,
a crater lake, is 1800 years old, but genetic evidence indicates that the lake was populated only 100 years ago by a
single population of cichlid fish. Nevertheless, two populations with distinct morphologies and diets now exist in the
lake, and scientists believe these populations may be in an early stage of speciation.

Finally, a well-documented example of ongoing sympatric speciation occurred in the apple maggot fly, Rhagoletis
pomonella, which arose as an isolated population sometime after the introduction of the apple into North America. The
native population of flies fed on hawthorn species and is host-specific: it only infests hawthorn trees. Importantly, it also
uses the trees as a location to meet for mating. It is hypothesized that either through mutation or a behavioral mistake, flies
jumped hosts and met and mated in apple trees, subsequently laying their eggs in apple fruit. The offspring matured and
kept their preference for the apple trees effectively dividing the original population into two new populations separated
by host species, not by geography. The host jump took place in the nineteenth century, but there are now measureable
differences between the two populations of fly. It seems likely that host specificity of parasites in general is a common cause
of sympatric speciation.

11.5 | Common Misconceptions about Evolution

By the end of this section, you will be able to:
• Identify common misconceptions about evolution
• Identify common criticisms of evolution

Although the theory of evolution initially generated some controversy, by 20 years after the publication of On the Origin
of Species it was almost universally accepted by biologists, particularly younger biologists. Nevertheless, the theory of
evolution is a difficult concept and misconceptions about how it works abound. In addition, there are those that reject it as
an explanation for the diversity of life.

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Chapter 11 | Evolution and Its Processes 267

This website (http://openstaxcollege.org/l/misconception2) addresses some of the main misconceptions associated with
the theory of evolution.

Evolution Is Just a Theory

Critics of the theory of evolution dismiss its importance by purposefully confounding the everyday usage of the word
“theory” with the way scientists use the word. In science, a “theory” is understood to be a concept that has been extensively
tested and supported over time. We have a theory of the atom, a theory of gravity, and the theory of relativity, each of which
describes what scientists understand to be facts about the world. In the same way, the theory of evolution describes facts
about the living world. As such, a theory in science has survived significant efforts to discredit it by scientists, who are
naturally skeptical. While theories can sometimes be overturned or revised, this does not lessen their weight but simply
reflects the constantly evolving state of scientific knowledge. In contrast, a “theory” in common vernacular means a guess or
suggested explanation for something. This meaning is more akin to the concept of a “hypothesis” used by scientists, which
is a tentative explanation for something that is proposed to either be supported or disproved. When critics of evolution say
evolution is “just a theory,” they are implying that there is little evidence supporting it and that it is still in the process of
being rigorously tested. This is a mischaracterization. If this were the case, geneticist Theodosius Dobzhansky would not
[6]
have said that “nothing in biology makes sense, except in the light of evolution.”

Individuals Evolve
An individual is born with the genes it has—these do not change as the individual ages. Therefore, an individual cannot
evolve or adapt through natural selection. Evolution is the change in genetic composition of a population over time,
specifically over generations, resulting from differential reproduction of individuals with certain alleles. Individuals do
change over their lifetime, but this is called development; it involves changes programmed by the set of genes the individual
acquired at birth in coordination with the individual’s environment. When thinking about the evolution of a characteristic,
it is probably best to think about the change of the average value of the characteristic in the population over time. For
example, when natural selection leads to bill-size change in medium ground finches in the Galápagos, this does not mean
that individual bills on the finches are changing. If one measures the average bill size among all individuals in the population
at one time, and then measures the average bill size in the population several years later after there has been a strong
selective pressure, this average value may be different as a result of evolution. Although some individuals may survive
from the first time to the second, those individuals will still have the same bill size. However, there may be enough new
individuals with different bill sizes to change the average bill size.

Evolution Explains the Origin of Life

It is a common misunderstanding that evolution includes an explanation of life’s origins. Conversely, some of the theory’s
critics complain that it cannot explain the origin of life. The theory does not try to explain the origin of life. The theory of
evolution explains how populations change over time and how life diversifies—the origin of species. It does not shed light
on the beginnings of life including the origins of the first cells, which is how life is defined. The mechanisms of the origin
of life on Earth are a particularly difficult problem because it occurred a very long time ago, over a very long time, and
presumably just occurred once. Importantly, biologists believe that the presence of life on Earth precludes the possibility
that the events that led to life on Earth can be repeated because the intermediate stages would immediately become food for
existing living things. The early stages of life included the formation of organic molecules such as carbohydrates, amino
acids, or nucleotides. If these were formed from inorganic precursors today, they would simply be broken down by living
things. The early stages of life also probably included more complex aggregations of molecules into enclosed structures
with an internal environment, a boundary layer of some form, and the external environment. Such structures, if they were
formed now, would be quickly consumed or broken down by living organisms.

6. Theodosius Dobzhansky. “Biology, Molecular and Organismic.” American Zoologist 4, no. 4 (1964): 449.
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However, once a mechanism of inheritance was in place in the form of a molecule like DNA or RNA, either within a cell
or within a pre-cell, these entities would be subject to the principle of natural selection. More effective reproducers would
increase in frequency at the expense of inefficient reproducers. So while evolution does not explain the origin of life, it may
have something to say about some of the processes operating once pre-living entities acquired certain properties.

Organisms Evolve on Purpose

Statements such as “organisms evolve in response to a change in an environment,” are quite common. There are two
easy misunderstandings possible with such a statement. First of all, the statement must not be understood to mean that
individual organisms evolve, as was discussed above. The statement is shorthand for “a population evolves in response
to a changing environment.” However, a second misunderstanding may arise by interpreting the statement to mean that
the evolution is somehow intentional. A changed environment results in some individuals in the population, those with
particular phenotypes, benefiting and, therefore, producing proportionately more offspring than other phenotypes. This
results in change in the population if the characters are genetically determined.
It is also important to understand that the variation that natural selection works on is already in a population and does not
arise in response to an environmental change. For example, applying antibiotics to a population of bacteria will, over time,
select for a population of bacteria that are resistant to antibiotics. The resistance, which is caused by a gene, did not arise
by mutation because of the application of the antibiotic. The gene for resistance was already present in the gene pool of
the bacteria, likely at a low frequency. The antibiotic, which kills the bacterial cells without the resistance gene, strongly
selects for individuals that are resistant, since these would be the only ones that survived and divided. Experiments have
demonstrated that mutations for antibiotic resistance do not arise as a result of antibiotic application.
In a larger sense, evolution is also not goal directed. Species do not become “better” over time; they simply track their
changing environment with adaptations that maximize their reproduction in a particular environment at a particular time.
Evolution has no goal of making faster, bigger, more complex, or even smarter species. This kind of language is common
in popular literature. Certain organisms, ourselves included, are described as the “pinnacle” of evolution, or “perfected”
by evolution. What characteristics evolve in a species are a function of the variation present and the environment, both of
which are constantly changing in a non-directional way. What trait is fit in one environment at one time may well be fatal
at some point in the future. This holds equally well for a species of insect as it does the human species.

Evolution Is Controversial among Scientists

The theory of evolution was controversial when it was first proposed in 1859, yet within 20 years virtually every working
biologist had accepted evolution as the explanation for the diversity of life. The rate of acceptance was extraordinarily
rapid, partly because Darwin had amassed an impressive body of evidence. The early controversies involved both scientific
arguments against the theory and the arguments of religious leaders. It was the arguments of the biologists that were resolved
after a short time, while the arguments of religious leaders have persisted to this day.
The theory of evolution replaced the predominant theory at the time that species had all been specially created within
relatively recent history. Despite the prevalence of this theory, it was becoming increasingly clear to naturalists during the
nineteenth century that it could no longer explain many observations of geology and the living world. The persuasiveness
of the theory of evolution to these naturalists lay in its ability to explain these phenomena, and it continues to hold
extraordinary explanatory power to this day. Its continued rejection by some religious leaders results from its replacement
of special creation, a tenet of their religious belief. These leaders cannot accept the replacement of special creation by a
mechanistic process that excludes the actions of a deity as an explanation for the diversity of life including the origins of
the human species. It should be noted, however, that most of the major denominations in the United States have statements
supporting the acceptance of evidence for evolution as compatible with their theologies.
The nature of the arguments against evolution by religious leaders has evolved over time. One current argument is that
the theory is still controversial among biologists. This claim is simply not true. The number of working scientists who
reject the theory of evolution, or question its validity and say so, is small. A Pew Research poll in 2009 found that 97
[7]
percent of the 2500 scientists polled believe species evolve. The support for the theory is reflected in signed statements
from many scientific societies such as the American Association for the Advancement of Science, which includes working
scientists as members. Many of the scientists that reject or question the theory of evolution are non-biologists, such as
engineers, physicians, and chemists. There are no experimental results or research programs that contradict the theory.
There are no papers published in peer-reviewed scientific journals that appear to refute the theory. The latter observation
might be considered a consequence of suppression of dissent, but it must be remembered that scientists are skeptics and
that there is a long history of published reports that challenged scientific orthodoxy in unpopular ways. Examples include
the endosymbiotic theory of eukaryotic origins, the theory of group selection, the microbial cause of stomach ulcers, the

7. Pew Research Center for the People & the Press, Public Praises Science; Scientists Fault Public, Media (Washington, DC, 2009), 37.

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Chapter 11 | Evolution and Its Processes 269

asteroid-impact theory of the Cretaceous extinction, and the theory of plate tectonics. Research with evidence and ideas
with scientific merit are considered by the scientific community. Research that does not meet these standards is rejected.

Other Theories Should Be Taught

A common argument from some religious leaders is that alternative theories to evolution should be taught in public schools.
Critics of evolution use this strategy to create uncertainty about the validity of the theory without offering actual evidence.
In fact, there are no viable alternative scientific theories to evolution. The last such theory, proposed by Lamarck in the
nineteenth century, was replaced by the theory of natural selection. A single exception was a research program in the
Soviet Union based on Lamarck’s theory during the early twentieth century that set that country’s agricultural research back
decades. Special creation is not a viable alternative scientific theory because it is not a scientific theory, since it relies on an
untestable explanation. Intelligent design, despite the claims of its proponents, is also not a scientific explanation. This is
because intelligent design posits the existence of an unknown designer of living organisms and their systems. Whether the
designer is unknown or supernatural, it is a cause that cannot be measured; therefore, it is not a scientific explanation. There
are two reasons not to teach nonscientific theories. First, these explanations for the diversity of life lack scientific usefulness
because they do not, and cannot, give rise to research programs that promote our understanding of the natural world.
Experiments cannot test non-material explanations for natural phenomena. For this reason, teaching these explanations as
science in public schools is not in the public interest. Second, in the United States, it is illegal to teach them as science
because the U.S. Supreme Court and lower courts have ruled that the teaching of religious belief, such as special creation
or intelligent design, violates the establishment clause of the First Amendment of the U.S. Constitution, which prohibits
government sponsorship of a particular religion.
The theory of evolution and science in general is, by definition, silent on the existence or non-existence of the spiritual
world. Science is only able to study and know the material world. Individual biologists have sometimes been vocal atheists,
but it is equally true that there are many deeply religious biologists. Nothing in biology precludes the existence of a god,
indeed biology as a science has nothing to say about it. The individual biologist is free to reconcile her or his personal
and scientific knowledge as they see fit. The Voices for Evolution project (http://ncse.com/voices), developed through the
National Center for Science Education, works to gather the diversity of perspectives on evolution to advocate it being taught
in public schools.
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KEY TERMS
adaptation a heritable trait or behavior in an organism that aids in its survival in its present environment

adaptive radiation a speciation when one species radiates out to form several other species

allopatric speciation a speciation that occurs via a geographic separation

analogous structure a structure that is similar because of evolution in response to similar selection pressures resulting
in convergent evolution, not similar because of descent from a common ancestor

bottleneck effect the magnification of genetic drift as a result of natural events or catastrophes

convergent evolution an evolution that results in similar forms on different species

dispersal an allopatric speciation that occurs when a few members of a species move to a new geographical area

divergent evolution an evolution that results in different forms in two species with a common ancestor

founder effect a magnification of genetic drift in a small population that migrates away from a large parent population
carrying with it an unrepresentative set of alleles

gene flow the flow of alleles in and out of a population due to the migration of individuals or gametes

gene pool all of the alleles carried by all of the individuals in the population

genetic drift the effect of chance on a population’s gene pool

homologous structure a structure that is similar because of descent from a common ancestor

inheritance of acquired characteristics a phrase that describes the mechanism of evolution proposed by Lamarck in
which traits acquired by individuals through use or disuse could be passed on to their offspring thus leading to
evolutionary change in the population

macroevolution a broader scale of evolutionary changes seen over paleontological time

microevolution the changes in a population’s genetic structure (i.e., allele frequency)

migration the movement of individuals of a population to a new location; in population genetics it refers to the movement
of individuals and their alleles from one population to another, potentially changing allele frequencies in both the old
and the new population

modern synthesis the overarching evolutionary paradigm that took shape by the 1940s and is generally accepted today

natural selection the greater relative survival and reproduction of individuals in a population that have favorable
heritable traits, leading to evolutionary change

population genetics the study of how selective forces change the allele frequencies in a population over time

speciation a formation of a new species

sympatric speciation a speciation that occurs in the same geographic space

variation the variety of alleles in a population

vestigial structure a physical structure present in an organism but that has no apparent function and appears to be from a
functional structure in a distant ancestor

vicariance an allopatric speciation that occurs when something in the environment separates organisms of the same
species into separate groups

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Chapter 11 | Evolution and Its Processes 271

CHAPTER SUMMARY
11.1 Discovering How Populations Change

Evolution by natural selection arises from three conditions: individuals within a species vary, some of those variations are
heritable, and organisms have more offspring than resources can support. The consequence is that individuals with
relatively advantageous variations will be more likely to survive and have higher reproductive rates than those individuals
with different traits. The advantageous traits will be passed on to offspring in greater proportion. Thus, the trait will have
higher representation in the next and subsequent generations leading to genetic change in the population.
The modern synthesis of evolutionary theory grew out of the reconciliation of Darwin’s, Wallace’s, and Mendel’s thoughts
on evolution and heredity. Population genetics is a theoretical framework for describing evolutionary change in
populations through the change in allele frequencies. Population genetics defines evolution as a change in allele frequency
over generations. In the absence of evolutionary forces allele frequencies will not change in a population; this is known as
Hardy-Weinberg equilibrium principle. However, in all populations, mutation, natural selection, genetic drift, and
migration act to change allele frequencies.

11.2 Mechanisms of Evolution

There are four factors that can change the allele frequencies of a population. Natural selection works by selecting for
alleles that confer beneficial traits or behaviors, while selecting against those for deleterious qualities. Mutations introduce
new alleles into a population. Genetic drift stems from the chance occurrence that some individuals have more offspring
than others and results in changes in allele frequencies that are random in direction. When individuals leave or join the
population, allele frequencies can change as a result of gene flow.

11.3 Evidence of Evolution

The evidence for evolution is found at all levels of organization in living things and in the extinct species we know about
through fossils. Fossils provide evidence for the evolutionary change through now extinct forms that led to modern
species. For example, there is a rich fossil record that shows the evolutionary transitions from horse ancestors to modern
horses that document intermediate forms and a gradual adaptation o changing ecosystems. The anatomy of species and the
embryological development of that anatomy reveal common structures in divergent lineages that have been modified over
time by evolution. The geographical distribution of living species reflects the origins of species in particular geographic
locations and the history of continental movements. The structures of molecules, like anatomical structures, reflect the
relationships of living species and match patterns of similarity expected from descent with modification.

11.4 Speciation

Speciation occurs along two main pathways: geographic separation (allopatric speciation) and through mechanisms that
occur within a shared habitat (sympatric speciation). Both pathways force reproductive isolation between populations.
Sympatric speciation can occur through errors in meiosis that form gametes with extra chromosomes, called polyploidy.
Autopolyploidy occurs within a single species, whereas allopolyploidy occurs because of a mating between closely related
species. Once the populations are isolated, evolutionary divergence can take place leading to the evolution of reproductive
isolating traits that prevent interbreeding should the two populations come together again. The reduced viability of hybrid
offspring after a period of isolation is expected to select for stronger inherent isolating mechanisms.

11.5 Common Misconceptions about Evolution

The theory of evolution is a difficult concept and misconceptions abound. The factual nature of evolution is often
challenged by wrongly associating the scientific meaning of a theory with the vernacular meaning. Evolution is sometimes
mistakenly interpreted to mean that individuals evolve, when in fact only populations can evolve as their gene frequencies
change over time. Evolution is often assumed to explain the origin of life, which it does not speak to. It is often spoken in
goal-directed terms by which organisms change through intention, and selection operates on mutations present in a
population that have not arisen in response to a particular environmental stress. Evolution is often characterized as being
controversial among scientists; however, it is accepted by the vast majority of working scientists. Critics of evolution often
argue that alternative theories to evolution should be taught in public schools; however, there are no viable alternative
scientific theories to evolution. The alternative religious beliefs should not be taught as science because it cannot be
proven, and in the United States it is unconstitutional. Science is silent on the question of the existence of a god while
scientists are able to reconcile religious belief and scientific knowledge.
272 Chapter 11 | Evolution and Its Processes

ART CONNECTION QUESTIONS

1. Figure 11.7 Do you think genetic drift would happen
more quickly on an island or on the mainland?

REVIEW QUESTIONS
2. Which scientific concept did Charles Darwin and 7. In which of the following pairs do both evolutionary
Alfred Wallace independently discover? processes introduce new genetic variation into a
a. mutation population?
b. natural selection a. natural selection and genetic drift
c. overbreeding b. mutation and gene flow
d. sexual reproduction c. natural selection and gene flow
d. gene flow and genetic drift
3. Which of the following situations will lead to natural
selection? 8. The wing of a bird and the arm of a human are
a. The seeds of two plants land near each other and examples of ________.
one grows larger than the other. a. vestigial structures
b. Two types of fish eat the same kind of food, and b. molecular structures
one is better able to gather food than the other. c. homologous structures
c. Male lions compete for the right to mate with d. analogous structures
females, with only one possible winner. 9. The fact that DNA sequences are more similar in more
d. all of the above closely related organisms is evidence of what?
4. What is the difference between micro- and a. optimal design in organisms
macroevolution? b. adaptation
a. Microevolution describes the evolution of small c. mutation
organisms, such as insects, while d. descent with modification
macroevolution describes the evolution of large 10. Which situation would most likely lead to allopatric
organisms, like people and elephants. speciation?
b. Microevolution describes the evolution of a. A flood causes the formation of a new lake.
microscopic entities, such as molecules and b. A storm causes several large trees to fall down.
proteins, while macroevolution describes the c. A mutation causes a new trait to develop.
evolution of whole organisms. d. An injury causes an organism to seek out a new
c. Microevolution describes the evolution of food source.
populations, while macroevolution describes the
emergence of new species over long periods of 11. What is the main difference between dispersal and
time. vicariance?
d. Microevolution describes the evolution of a. One leads to allopatric speciation, whereas the
organisms over their lifetimes, while other leads to sympatric speciation.
macroevolution describes the evolution of b. One involves the movement of the organism,
organisms over multiple generations. whereas the other involves a change in the
environment.
5. Population genetics is the study of ________. c. One depends on a genetic mutation occurring,
a. how allele frequencies in a population change whereas the other does not.
over time d. One involves closely related organisms, whereas
b. populations of cells in an individual the other involves only individuals of the same
c. the rate of population growth species.
d. how genes affect embryological development
12. Which variable increases the likelihood of allopatric
6. Galápagos medium ground finches are found on Santa speciation taking place more quickly?
Cruz and San Cristóbal islands, which are separated by a. lower rate of mutation
about 100 km of ocean. Occasionally, individuals from b. longer distance between divided groups
either island fly to the other island to stay. This can alter c. increased instances of hybrid formation
the allele frequencies of the population through which of d. equivalent numbers of individuals in each
the following mechanisms? population
a. natural selection
b. genetic drift 13. The word “theory” in theory of evolution is best
c. gene flow replaced by ________.
d. mutation a. fact
b. hypothesis

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Chapter 11 | Evolution and Its Processes 273

c. idea b. there are no viable scientific alternatives

d. alternate explanation c. it is against the law
d. alternative scientific theories are suppressed by
14. Why are alternative scientific theories to evolution not
the science establishment
taught in public school?
a. more theories would confuse students

CRITICAL THINKING QUESTIONS

15. If a person scatters a handful of plant seeds from one 20. Two species of fish had recently undergone sympatric
species in an area, how would natural selection work in speciation. The males of each species had a different
this situation? coloring through which females could identify and choose
a partner from her own species. After some time, pollution
16. Explain the Hardy-Weinberg principle of equilibrium.
made the lake so cloudy it was hard for females to
17. Describe natural selection and give an example of distinguish colors. What might take place in this situation?
natural selection at work in a population.
21. How does the scientific meaning of “theory” differ
18. Why do scientists consider vestigial structures from the common, everyday meaning of the word?
evidence for evolution?
22. Explain why the statement that a monkey is more
19. Why do island chains provide ideal conditions for evolved than a mouse is incorrect.
adaptive radiation to occur?
Answer Key 601

that extends the 3' end, so a primer is synthesized and extended. Thus, the ends are protected. 16 The cell controls which protein
is expressed, and to what level that protein is expressed, in the cell. Prokaryotic cells alter the transcription rate to turn genes on
or off. This method will increase or decrease protein levels in response to what is needed by the cell. Eukaryotic cells change the
accessibility (epigenetic), transcription, or translation of a gene. This will alter the amount of RNA, and the lifespan of the RNA,
to alter the amount of protein that exists. Eukaryotic cells also change the protein’s translation to increase or decrease its overall
levels. Eukaryotic organisms are much more complex and can manipulate protein levels by changing many stages in the process.

Chapter 10
1 Figure 10.7 Because even though the original cell came from a Scottish Blackface sheep and the surrogate mother was a Scottish
Blackface, the DNA came from a Finn-Dorset. 2 B 4 A 6 C 8 D 10 The polymerase chain reaction is used to quickly produce
many copies of a specific segment of DNA when only one or a very few copies are originally present. The benefit of PCR is that
there are many instances in which we would like to know something about a sample of DNA when only very small amounts are
available. PCR allows us to increase the number of DNA molecules so that other tests, such as sequencing, can be performed with
it. 12 Genome mapping helps researchers to study disease-causing genes in humans. It also helps to identify traits of organisms
that can be used in applications such as cleaning up pollution.

Chapter 11
1 Figure 11.7 Genetic drift is likely to occur more rapidly on an island, where smaller populations are expected to occur. 2
B 4 C 6 C 8 C 10 A 12 B 14 B 15 The plants that can best use the resources of the area, including competing with other
individuals for those resources, will produce more seeds themselves and those traits that allowed them to better use the resources
will increase in the population of the next generation. 17 The theory of natural selection stems from the observation that some
individuals in a population survive longer and have more offspring than others, thus passing on more of their genes to the next
generation. For example, a big, powerful male gorilla is much more likely than a smaller, weaker gorilla to become the population’s
silverback, the pack’s leader who mates far more than the other males of the group. The pack leader will, therefore, father more
offspring, who share half of his genes, and are thus likely to also grow bigger and stronger like their father. Over time, the genes for
bigger size will increase in frequency in the population, and the population will, as a result, grow larger on average. 19 Organisms
of one species can arrive to an island together and then disperse throughout the chain, each settling into different niches, exploiting
different food resources and, evolving independently with little gene flow between different islands. 21 In science, a theory is a
thoroughly tested and verified set of explanations for a body of observations of nature. It is the strongest form of knowledge in
science. In contrast, a theory in common usage can mean a guess or speculation about something, meaning that the knowledge
implied by the theory may be very weak.

Chapter 12
1 Figure 12.3 Cats and dogs are part of the same group at five levels: both are in the domain Eukarya, the kingdom Animalia, the
phylum Chordata, the class Mammalia, and the order Carnivora. 3 C 5 D 7 B 9 A 11 B 13 The phylogenetic tree shows the
order in which evolutionary events took place and in what order certain characteristics and organisms evolved in relation to others.
It does not generally indicate time durations. 15 Dolphins are mammals and fish are not, which means that their evolutionary
paths (phylogenies) are quite separate. Dolphins probably adapted to have a similar body plan after returning to an aquatic lifestyle,
and therefore this trait is probably analogous. 17 The biologist looks at the state of the character in an outgroup, an organism
that is outside the clade for which the phylogeny is being developed. The polarity of the character change is from the state of the
character in the outgroup to the second state.

Chapter 13
1 Figure 13.6 A 2 B 4 D 6 C 8 D 10 C 12 C 14 Antibiotics kill bacteria that are sensitive to them; thus, only the resistant
ones will survive. These resistant bacteria will reproduce, and therefore, after a while, there will be only resistant bacteria, making
it more difficult to treat the diseases they may cause in humans. 16 Eukaryote cells arose through endosymbiotic events that
gave rise to energy-producing organelles within the eukaryotic cells, such as mitochondria and plastids. The nuclear genome of
eukaryotes is related most closely to the Archaea, so it may have been an early archaean that engulfed a bacterial cell that evolved
into a mitochondrion. Mitochondria appear to have originated from an alpha-proteobacterium, whereas chloroplasts originated
from a cyanobacterium. There is also evidence of secondary endosymbiotic events. Other cell components may have resulted from
endosymbiotic events. 18 The trypanosomes that cause this disease are capable of expressing a glycoprotein coat with a different
molecular structure with each generation. Because the immune system must respond to specific antigens to raise a meaningful
defense, the changing nature of trypanosome antigens prevents the immune system from ever clearing this infection. Massive
trypanosome infection eventually leads to host organ failure and death.

Chapter 14
1 Figure 14.19 B. The diploid zygote forms after the pollen tube has finished forming so that the male generative nucleus (sperm)
can fuse with the female egg. 3 A 5 A 7 D 9 A 11 A 13 The sporangium of plants protects the spores from drying out. Apical
Index 605

anaerobic, 292, 319 autosome, 170

INDEX anaerobic cellular respiration, autosomes, 165
113 autotroph, 118, 132, 563
A analogous structure, 270, 283, autotrophs, 535
absorption spectrum, 124, 132 288 axial skeleton, 426, 440
abyssal zone, 556, 563 analogous structures, 253 axon, 433, 440
acellular, 450, 472 anaphase, 140, 149
acetyl CoA, 104, 113 aneuploid, 165, 170 B
acid, 51 anion, 51
B cell, 472
Acid rain, 547 anions, 31
B cells, 460
acid rain, 563 anneal, 245
Basal angiosperms, 348
Acids, 38 annealing, 229
basal angiosperms, 351
acoelomate, 395 Annelida, 378, 395
basal ganglia, 436, 440
acoelomates, 360 anoxic, 292, 319
base, 51
Actinopterygii, 387, 395 anther, 344, 351
bases, 38
action potential, 432, 440 Anthophyta, 347, 351
Basic science, 22
activation energy, 97, 113 Anthropoids, 393
basic science, 24
active immunity, 461, 472 anthropoids, 395
Basidiomycota, 314
active site, 98, 113 antibody, 461, 472
basidiomycota, 319
Active transport, 81 antigen, 460, 472
benthic realm, 555, 563
active transport, 85 antigen-presenting cell (APC),
bicuspid valve, 417, 440
adaptation, 253, 270 462, 472
Bilateral symmetry, 359
Adaptive immunity, 460 Anura, 388, 395
bilateral symmetry, 395
adaptive immunity, 472 anus, 411, 440
Bile, 410
adaptive radiation, 264, 270 aorta, 417, 440
bile, 440
adhesion, 37, 51 apex consumer, 563
binary fission, 145, 149
adrenal gland, 440 apex consumers, 531
binomial nomenclature, 276,
adrenal glands, 423 aphotic zone, 555, 563
288
Age structure, 512 apical meristem, 329, 351
biodiversity, 568, 590
age structure, 525 Apoda, 388, 395
biodiversity hotspot, 586, 590
algal bloom, 560, 563 apoptosis, 453, 472
bioenergetics, 92, 113
allele, 194 appendicular skeleton, 428, 440
biofilm, 294, 319
alleles, 178 applied science, 22, 24
biogeochemical cycle, 537, 563
allergy, 469, 472 Archaeplastida, 306, 319
Biology, 5
Allopatric speciation, 262 Arctic tundra, 553
biology, 24
allopatric speciation, 270 arctic tundra, 563
Biomagnification, 536
allosteric inhibition, 100, 113 Arteries, 419
biomagnification, 563
alternation of generations, 155, artery, 440
biomarker, 243, 245
170 Arthropoda, 371, 395
biome, 531, 563
alternative RNA splicing, 219, Ascomycota, 314, 319
bioremediation, 301, 319
220 Asexual reproduction, 478
biosphere, 12, 24
alveoli, 415 asexual reproduction, 495
Biotechnology, 225
alveolus, 440 Asymmetrical, 358
biotechnology, 245
amino acid, 51 asymmetrical, 395
birth rate, 505, 525
Amino acids, 46 atom, 9, 24
Black Death, 297, 319
amniote, 395 atomic number, 28, 51
blastocyst, 483, 495
amniotes, 389 ATP, 102, 113
body plan, 356, 395
amoebocyte, 395 ATP synthase, 107, 113
bolus, 409, 440
Amoebocytes, 362 atrium, 417, 440
bones, 391
Amoebozoa, 306, 319 attenuation, 455, 472
boreal forest, 552, 563
Amphibia, 388, 395 auditory ossicles, 427, 440
bottleneck effect, 256, 270
ampulla of Lorenzini, 395 autoantibody, 470, 472
botulism, 299, 319
ampullae of Lorenzini, 387 Autoimmunity, 470
brachiation, 393, 395
amygdala, 437, 440 autoimmunity, 472
brainstem, 437, 440
amylase, 409, 440 autonomic nervous system, 437,
branch point, 279, 288
anabolic, 93, 113 440
606 Index

bronchi, 415, 440 chaetae, 379 complete digestive system, 370,

bronchiole, 440 channel, 561, 563 396
bronchioles, 415 chaparral, 550, 563 concentration gradient, 77, 85
budding, 363, 395, 495 chelicerae, 373, 395 cone, 351
Budding, 479 chemical bond, 51 cones, 339
buffer, 51 chemical bonds, 31 conifer, 351
Buffers, 38 chemical diversity, 569, 590 Conifers, 341
bulbourethral gland, 486, 495 chemiosmosis, 107, 113 conjugation, 296, 319
Bush meat, 578 chemoautotroph, 563 Continuous variation, 174
bush meat, 590 chemoautotrophs, 535 continuous variation, 194
chiasmata, 158, 170 control, 20, 24
C chitin, 41, 51, 370, 395 convergent evolution, 253, 270
chlorophyll, 120, 132 coral reef, 563
caecilian, 395
chlorophyll a, 124, 132 Coral reefs, 557
Caecilians, 389
chlorophyll b, 124, 132 corolla, 344, 351
Calvin cycle, 127, 132
chloroplast, 85, 120, 132 corpus callosum, 435, 441
calyx, 344, 351
Chloroplasts, 69 corpus luteum, 487, 495
canopy, 548, 563
choanocyte, 362, 395 cotyledon, 351
capillaries, 419
Chondrichthyes, 386, 395 cotyledons, 347
capillary, 440
Chordata, 382, 395 covalent bond, 32, 51
capsid, 451, 472
Chromalveolata, 306, 319 craniate, 396
capsule, 295, 319
chromosome inversion, 168, craniates, 385
carbohydrate, 51
170 Crocodilia, 390, 396
Carbohydrates, 40
chyme, 410, 441 crossing over, 158, 170
carbon fixation, 127, 132
chytridiomycosis, 580, 590 cryptofauna, 558, 563
cardiac cycle, 418, 440
Chytridiomycota, 314, 319 ctenidia, 375, 396
Cardiac muscle tissue, 430
cilia, 64 cutaneous respiration, 388, 396
cardiac muscle tissue, 440
cilium, 85 cyanobacteria, 292, 319
carpel, 344, 351
citric acid cycle, 105, 113 cycad, 351
carrying capacity, 505, 525
clade, 288 Cycads, 341
cartilaginous joint, 440
clades, 285 cytokine, 457, 472
Cartilaginous joints, 428
cladistics, 285, 288 Cytokinesis, 140
catabolic, 93, 113
class, 276, 288 cytokinesis, 149
cation, 51
cleavage furrow, 140, 149 cytopathic, 453, 472
cations, 31
climax community, 524, 525 cytoplasm, 63, 85
cell, 10, 24
clitellum, 380, 395 cytoskeleton, 63, 85
cell cycle, 137, 149
clitoris, 487, 495 cytosol, 63, 85
cell cycle checkpoints, 142, 149
cloning, 228, 245 cytotoxic T lymphocyte (TC),
cell plate, 140, 149
closed circulatory system, 417, 472
cell wall, 69, 85
441
cell-mediated immune D
club moss, 351
response, 460, 472
club mosses, 335
Cellulose, 41 dead zone, 544, 563
Cnidaria, 363, 395
cellulose, 51 death rate, 505, 525
cnidocyte, 395
central nervous system (CNS), Deductive reasoning, 19
cnidocytes, 363
435, 440 deductive reasoning, 24
codominance, 186, 194
central vacuole, 70, 85 demography, 500, 525
codon, 214, 220
centriole, 149 denaturation, 46, 51
coelom, 360, 395
centrioles, 138 dendrite, 441
cohesion, 36, 51
Cephalochordata, 383, 395 Dendrites, 432
colon, 411, 441
cephalothorax, 373, 395 dendritic cell, 462, 472
commensalism, 302, 319
cerebellum, 437, 441 density-dependent, 508
community, 12, 24
cerebral cortex, 435, 441 density-dependent regulation,
competitive exclusion principle,
cerebrospinal fluid (CSF), 435, 525
518, 525
441 density-independent, 508
competitive inhibition, 99, 113
chaeta, 395
complement system, 459, 472

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Index 607

density-independent regulation, ectotherms, 404 eutherian mammal, 396

525 effector cell, 472 Eutherian mammals, 393
deoxyribonucleic acid (DNA), effector cells, 464 eutrophication, 542, 564
49, 51 electrocardiogram (ECG), 419, evaporation, 35, 51
deoxyribose, 200, 220 441 evolution, 12, 24
depolarization, 432, 441 electrochemical gradient, 81, 85 Excavata, 306, 319
Descriptive, 19 electromagnetic spectrum, 123, exergonic, 113
descriptive science, 24 132 exergonic reactions, 96
desmosome, 85 electron, 28, 51 exocrine gland, 441
desmosomes, 72 electron transfer, 31, 51 Exocrine glands, 421
detrital food web, 534, 563 electron transport chain, 105, Exocytosis, 83
Deuteromycota, 319 113 exocytosis, 85
deuterostome, 396 element, 51 exon, 220
Deuterostomes, 360 elements, 28 exons, 212
diaphragm, 415, 441 Emergent vegetation, 562 Exotic species, 579
diastole, 418, 441 emergent vegetation, 563 exotic species, 590
dicot, 351 Endemic species, 571 exponential growth, 504, 525
dicots, 348 endemic species, 590 external fertilization, 481, 495
Diffusion, 77 endergonic, 113 extinction, 570, 590
diffusion, 85 endergonic reactions, 96 extinction rate, 590
dihybrid, 183, 194 endocrine gland, 441 extinction rates, 584
dioecious, 371, 396 endocrine glands, 421 extracellular digestion, 365, 396
diphyodont, 396 Endocytosis, 82 extracellular matrix, 70, 85
diphyodonts, 392 endocytosis, 85 extremophile, 319
diploblast, 396 endomembrane system, 64, 85 extremophiles, 294
diploblasts, 359 endoplasmic reticulum (ER), 65,
diploid, 136, 149 85 F
diploid-dominant, 155, 170 endosymbiosis, 319
F1, 175, 194
Diplontic, 327 endosymbiotic theory, 303
diplontic, 351 endotherm, 404, 441 F2, 175, 194
disaccharide, 51 environmental disturbance, 525 facilitated transport, 78, 85
Disaccharides, 41 environmental disturbances, fallout, 546, 564
discontinuous variation, 174, 523 falsifiable, 20, 24
194 enzyme, 51, 113 family, 276, 288
dispersal, 263, 270 Enzymes, 45 fat, 43, 51
divergent evolution, 253, 270 enzymes, 97 Feedback inhibition, 102
DNA ligase, 205, 220 epidemic, 319 feedback inhibition, 113
DNA polymerase, 205, 220 epidemics, 297 fermentation, 108, 113
domain, 288 epidermis, 364, 396 fern, 351
domains, 276 epigenetic, 216, 220 ferns, 336
Dominant, 177 epistasis, 192, 194 fertilization, 157, 170
dominant, 194 Equilibrium, 531 fibrous joint, 441
dorsal hollow nerve cord, 382, equilibrium, 563 fibrous joints, 428
396 esophagus, 408, 441 filament, 344, 351
double helix, 201, 220 essential nutrient, 441 Fission, 478
down feather, 396 essential nutrients, 413 fission, 495
down feathers, 391 estrogen, 491, 495 Flagella, 64
down-regulation, 422, 441 Estuaries, 559 flagellum, 85
estuary, 563 fluid mosaic model, 74, 85
E eucoelomate, 396 follicle stimulating hormone
eucoelomates, 360 (FSH), 490, 495
Echinodermata, 380, 396 food chain, 531, 564
eudicots, 347, 351
ecosystem, 12, 24, 530, 563 food web, 533, 564
eukaryote, 24
ecosystem diversity, 569, 590 foodborne disease, 299, 319
eukaryotes, 10
ecosystem services, 560, 563 Foundation species, 521
eukaryotic cell, 60, 85
ectotherm, 441 foundation species, 525
euploid, 165, 170
608 Index

founder effect, 257, 270 gestation period, 493, 495 homosporous, 327, 351
fragmentation, 363, 396, 495 gingkophyte, 351 homozygous, 178, 194
Fragmentation, 479 ginkgophyte, 342 hormone, 51, 441
frog, 396 glia, 432, 441 hormone receptors, 421
Frogs, 389 Glomeromycota, 314, 319 Hormones, 45, 421
frontal lobe, 436, 441 Glycogen, 41 hornwort, 351
FtsZ, 147, 149 glycogen, 51 hornworts, 333
Glycolysis, 103 horsetail, 351
G glycolysis, 113 Horsetails, 335
glycoprotein, 451, 472 host, 519, 525
G0 phase, 141, 149
gnathostome, 396 human beta chorionic
G1 phase, 137, 149 Gnathostomes, 386 gonadotropin (β-HCG), 493, 495
G2 phase, 138, 149 gnetophyte, 351 humoral immune response, 460,
gallbladder, 411, 441 Gnetophytes, 342 472
gametangia, 327 Golgi apparatus, 66, 86 hybridization, 194
gametangium, 351 gonadotropin-releasing hybridizations, 175
gamete, 149 hormone (GnRH), 490, 495 hydrogen bond, 33, 51
gametes, 136 Gram-negative, 295, 319 hydrophilic, 34, 52
gametophyte, 170, 327, 351 Gram-positive, 295, 319 hydrophobic, 34, 52
gametophytes, 157 granum, 121, 132 hydrosphere, 537, 564
gap junction, 85 grazing food web, 534, 564 hydrothermal vent, 293, 319
Gap junctions, 72 gross primary productivity, 535, hyoid bone, 427, 441
gastrodermis, 364, 396 564 hypersensitivity, 469, 472
gastrovascular cavity, 365, 396 gymnosperm, 351 hypertonic, 79, 86
gastrulation, 484, 495 Gymnosperms, 339 hypha, 312, 319
Gel electrophoresis, 226 gynoecium, 344, 351 hypothalamus, 437, 441
gel electrophoresis, 245 hypothesis, 18, 24
gemmule, 396 H hypothesis-based science, 19,
gemmules, 363 24
gene, 149 habitat heterogeneity, 572, 590
hypotonic, 79, 86
gene expression, 216, 220 hagfish, 396
gene flow, 257, 270 Hagfishes, 385 I
gene pool, 254, 270 haplodiplontic, 327, 351
Gene therapy, 233 haploid, 136, 149 immune tolerance, 468, 473
gene therapy, 245 haploid-dominant, 155, 170 Immunodeficiency, 469
genes, 136 Haplontic, 327 immunodeficiency, 473
genetic code, 214, 220 haplontic, 351 incomplete dominance, 186,
Genetic diversity, 569 heat energy, 94, 113 194
genetic diversity, 590 helicase, 205, 220 Inductive reasoning, 18
genetic drift, 255, 270 helper T lymphocyte (TH), 472 inductive reasoning, 24
genetic engineering, 232, 245 hemizygous, 189, 194 inferior vena cava, 417, 441
genetic map, 236, 245 hemocoel, 371, 396 inflammation, 457, 473
genetic testing, 245 herbaceous, 349, 351 inheritance of acquired
genetically modified organism, Hermaphroditism, 480 characteristics, 250, 270
232 hermaphroditism, 495 inhibin, 491, 495
genetically modified organism heterodont teeth, 392, 396 Innate immunity, 456
(GMO), 245 heterosporous, 327, 351 innate immunity, 473
genome, 136, 149 heterotroph, 132 inner cell mass, 483, 495
genomics, 236, 245 Heterotrophs, 118 interferon, 457, 473
genotype, 178, 194 heterozygous, 179, 194 interkinesis, 161, 170
genus, 276, 288 hippocampus, 436, 441 internal fertilization, 481, 495
germ cell, 170 homeostasis, 8, 24 interphase, 137, 149
germ cells, 155 homologous chromosomes, interstitial cell of Leydig, 495
germ layer, 396 136, 149 interstitial cells of Leydig, 485
germ layers, 359 homologous structure, 270 interstitial fluid, 406, 441
gestation, 493, 495 homologous structures, 253 intertidal zone, 555, 564

This OpenStax book is available for free at http://cnx.org/content/col11487/1.9

Index 609

intracellular, 421 life sciences, 18 meiosis I, 157, 170

intracellular digestion, 362, 396 life table, 525 Meiosis II, 157
intracellular hormone receptor, life tables, 500 meiosis II, 170
441 light-dependent reaction, 132 membrane potential, 442
intraspecific competition, 506, light-dependent reactions, 121 memory cell, 464, 473
525 limbic system, 437, 442 meninges, 435, 442
intron, 220 line, 387 menstrual cycle, 491, 495
introns, 212 linkage, 191, 194 mesoglea, 364, 397
ion, 31, 52 Lipids, 42 mesohyl, 362, 397
ionic bond, 32, 52 lipids, 52 mesophyll, 120, 132
Island biogeography, 521 litmus, 37 metabolism, 92, 114
island biogeography, 525 litmus paper, 52 Metagenomics, 240
isotonic, 80, 86 liver, 411, 442 metagenomics, 245
isotope, 52 liverwort, 352 metamerism, 379, 397
Isotopes, 29 Liverworts, 333 metaphase, 140, 149
locus, 136, 149 metaphase plate, 140, 149
J logistic growth, 505, 525 MHC class II molecule, 461
Lophotrochozoa, 374, 397 microbial mat, 293, 320
J-shaped growth curve, 505,
luteinizing hormone (LH), 490, microevolution, 254, 270
525
495 microscope, 56, 86
joint, 428, 442
Lymph, 466 microsporocyte, 352
lymph, 473 microsporocytes, 339
K lymphocyte, 458, 473 migration, 255, 270
K-selected species, 510, 525 lysosome, 86 mimicry, 516, 525
karyogram, 164, 170 lysosomes, 66 mineral, 442
karyotype, 164, 170 Minerals, 413
keystone species, 522, 525 M mismatch repair, 208, 220
kidney, 442 Mitochondria, 68
macroevolution, 254, 270
kidneys, 406 mitochondria, 86
macromolecule, 24, 52
kinetic energy, 95, 113 mitosis, 138, 149
macromolecules, 9, 39
kinetochore, 140, 149 mitotic, 137, 138
macrophage, 457, 473
kingdom, 276, 288 mitotic phase, 149
madreporite, 381, 397
mitotic spindle, 149
major histocompatibility class
L (MHC) I, 473
model organism, 245
model organisms, 238
labia majora, 487, 495 major histocompatibility class
model system, 174, 194
labia minora, 487, 495 (MHC) I molecules, 458
modern synthesis, 254, 270
lagging strand, 205, 220 major histocompatibility class
mold, 320
lamprey, 396 (MHC) II molecule, 473
molds, 313
Lampreys, 386 mammal, 397
molecular systematics, 284, 288
lancelet, 396 Mammals, 392
molecule, 9, 24
Lancelets, 384 mammary gland, 397
Mollusca, 374, 397
large intestine, 411, 442 Mammary glands, 392
monocot, 352
larynx, 415, 442 mantle, 375, 397
monocots, 347
lateral, 387 mark and recapture, 501, 525
monocyte, 457, 473
lateral line, 397 marsupial, 397
monoecious, 363, 397
law of dominance, 179, 194 Marsupials, 392
monohybrid, 180, 194
law of independent assortment, mass number, 28, 52
monophyletic group, 285, 288
183, 194 mast cell, 473
monosaccharide, 52
law of segregation, 181, 194 Mast cells, 457
Monosaccharides, 40
leading strand, 205, 220 Matter, 28
monosomy, 165, 170
lichen, 319 matter, 52
monotreme, 397
Lichens, 317 maximum parsimony, 287, 288
monotremes, 392
life cycle, 170 medusa, 364, 397
mortality rate, 502, 525
life cycles, 154 megasporocyte, 339, 352
moss, 352
life science, 24 meiosis, 154, 170
mosses, 334
610 Index

mRNA, 210, 220 nucleotide excision repair, 208, paper, 37

MRSA, 320 220 parasite, 320, 519, 525
mutation, 209, 220 nucleotides, 49 parasites, 305
mutualism, 519, 525 nucleus, 28, 52, 65, 86 parasympathetic nervous
mycelium, 312, 320 system, 439, 442
Mycorrhiza, 316 O parathyroid gland, 442
mycorrhiza, 320 parathyroid glands, 423
occipital lobe, 436, 442
mycoses, 315 parietal lobe, 436, 442
oceanic zone, 556, 564
mycosis, 320 Parthenogenesis, 480
octet rule, 31, 52
myelin sheath, 433, 442 parthenogenesis, 496
oil, 52
myofibril, 442 passive immune, 461
oils, 44
myofibrils, 430 passive immunity, 473
Okazaki fragments, 205, 220
myofilament, 442 Passive transport, 77
oncogene, 150
myofilaments, 431 passive transport, 86
oncogenes, 143
Myxini, 385, 397 pathogen, 296, 320
one-child policy, 513, 525
pectoral girdle, 428, 442
N oogenesis, 488, 495
peer-reviewed article, 24
open circulatory system, 442
Peer-reviewed articles, 23
nacre, 376, 397 Open circulatory systems, 417
pelagic realm, 555, 564
nasal cavity, 415, 442 Opisthokonta, 306, 320
pellicle, 320
natural killer (NK) cell, 458, 473 oral cavity, 409, 442
pellicles, 305
natural science, 24 order, 276, 288
pelvic girdle, 428, 442
natural sciences, 18 organ, 24
penis, 485, 496
Natural selection, 251 organ system, 10, 24
pepsin, 410, 442
natural selection, 270 organelle, 24, 86
peptidoglycan, 295, 320
nematocyst, 397 organelles, 10, 60
periodic table of elements, 29,
nematocysts, 363 organism, 24
52
Nematoda, 370, 397 Organisms, 10
peripheral nervous system
nephron, 442 organogenesis, 484, 496
(PNS), 437, 442
nephrons, 407 Organs, 10
peristalsis, 408, 442
neritic zone, 556, 564 origin, 145, 150
permafrost, 553, 564
Net primary productivity, 535 osculum, 362, 397
peroxisome, 86
net primary productivity, 564 osmolarity, 79, 86
Peroxisomes, 68
neuron, 442 Osmoregulation, 406
petal, 352
neurons, 432 osmoregulation, 442
Petals, 344
neutron, 52 Osmosis, 79
Petromyzontidae, 386, 397
Neutrons, 28 osmosis, 86
pH scale, 37, 52
neutrophil, 458, 473 osmotic balance, 406, 442
Phagocytosis, 83
nitrogenous base, 200, 220 Osteichthyes, 387, 397
phagocytosis, 86
non-renewable resource, 541, ostracoderm, 397
Pharmacogenomics, 240
564 ostracoderms, 385
pharmacogenomics, 245
noncompetitive inhibition, 100, ovarian cycle, 491, 496
pharyngeal slit, 397
114 ovary, 344, 352
Pharyngeal slits, 382
nondisjunction, 164, 170 oviduct, 496
pharynx, 415, 442
nonpolar covalent bond, 52 oviducts, 487
phase, 137
Nonpolar covalent bonds, 32 oviparity, 482, 496
phenotype, 178, 194
nontemplate strand, 211, 220 ovoviparity, 482, 496
phloem, 334, 352
nonvascular plant, 352 ovulation, 492, 496
phosphate group, 200, 220
nonvascular plants, 331 oxidative phosphorylation, 105,
phospholipid, 52
notochord, 382, 397 114
Phospholipids, 45
nuclear envelope, 65, 86
photic zone, 555, 564
nucleic acid, 52 P photoautotroph, 132, 564
nucleic acids, 49
P, 175, 194 photoautotrophs, 118, 535
nucleolus, 65, 86
pancreas, 411, 423, 442 photon, 124, 132
nucleotide, 52
pandemic, 320 photosystem, 124, 132
pandemics, 297 phototroph, 320

This OpenStax book is available for free at http://cnx.org/content/col11487/1.9

Index 611

phototrophs, 292 prokaryotic cell, 59, 86 resistance, 531

phylogenetic tree, 14, 24, 279, prometaphase, 139, 150 resistance (ecological), 564
288 promoter, 210, 221 restriction enzyme, 245
phylogeny, 276, 288 prophase, 139, 150 restriction enzymes, 229
phylum, 276, 288 Prosimians, 393 reverse genetics, 232, 245
physical map, 245 prosimians, 398 Rhizaria, 306, 320
Physical maps, 236 prostate gland, 486, 496 ribonucleic acid (RNA), 49, 52
physical science, 24 protein, 52 ribosome, 86
physical sciences, 18 protein signature, 243, 245 Ribosomes, 68
pigment, 120, 132 Proteins, 45 RNA polymerase, 211, 221
pinocytosis, 83, 86 proteomics, 243, 245 rooted, 279, 288
pioneer species, 524, 526 proto-oncogene, 150 rough endoplasmic reticulum
pistil, 344, 352 proto-oncogenes, 143 (RER), 65, 86
pituitary gland, 422, 443 proton, 28, 52 rRNA, 213, 221
placenta, 493, 496 protostome, 398
planktivore, 564 Protostomes, 360 S
planktivores, 558 pseudocoelomate, 398
S phase, 138, 150
plasma membrane, 63, 86 pseudocoelomates, 360
S-shaped curve, 505
plasmid, 228, 245 pseudopeptidoglycan, 296, 320
S-shaped growth curve, 526
plasmodesma, 86 pulmonary circulation, 417, 443
salamander, 398
Plasmodesmata, 71 Punnett square, 180, 194
salamanders, 388
plastid, 303, 320
Q salivary gland, 443
pneumatic, 391
salivary glands, 409
pneumatic bone, 397
quadrat, 501, 526 saprobe, 320
polar covalent bond, 32, 52
quiescent, 150 saprobes, 310
Polymerase chain reaction
sarcolemma, 430, 443
(PCR), 227
polymerase chain reaction
R sarcomere, 431, 443
Sarcopterygii, 387, 398
(PCR), 245 r-selected species, 510, 526
saturated fatty acid, 52
polyp, 364, 397 radial symmetry, 358, 398
Saturated fatty acids, 44
polypeptide, 46, 52 radioactive isotope, 52
savanna, 564
polyploid, 167, 170 radioactive isotopes, 29
Savannas, 549
polysaccharide, 41, 52 radula, 374, 398
Science, 17
population, 12, 24 receptor-mediated endocytosis,
science, 19, 25
population density, 500, 526 83, 86
scientific law, 25
population genetics, 254, 270 Recessive, 177
scientific laws, 18
population size, 500, 526 recessive, 195
scientific method, 18, 25
Porifera, 361, 397 reciprocal cross, 177, 195
scientific theory, 18, 25
post-anal tail, 383, 397 recombinant, 158, 170
scrotum, 485, 496
post-transcriptional, 217, 220 recombinant DNA, 230, 245
sebaceous gland, 398
post-translational, 217, 220 recombinant protein, 245
Sebaceous glands, 392
potential energy, 95, 114 recombinant proteins, 230
secondary consumer, 564
primary bronchi, 415 recombination, 191, 195
Secondary consumers, 531
primary bronchus, 443 rectum, 411, 443
secondary immune response,
primary consumer, 564 reduction division, 162, 170
465, 473
primary consumers, 531 Relative species abundance,
secondary plant compound, 590
primary immune response, 464, 521
secondary plant compounds,
473 relative species abundance, 526
572
primary succession, 523, 526 renal artery, 407, 443
secondary succession, 523, 526
Primates, 393, 397 renal vein, 407, 443
selectively permeable, 77, 86
primer, 205, 221 replication fork, 221
Semen, 485
producer, 564 replication forks, 205
semen, 496
producers, 531 Reproductive cloning, 230
semiconservative replication,
progesterone, 491, 496 reproductive cloning, 245
205, 221
prokaryote, 24 resilience, 531
seminal vesicle, 496
Prokaryotes, 10 resilience (ecological), 564
612 Index

seminal vesicles, 486 Starch, 41 Temperate forests, 552

seminiferous tubule, 496 starch, 53 temperate grassland, 565
seminiferous tubules, 485 start codon, 214, 221 Temperate grasslands, 551
sensory-somatic nervous stereoscopic vision, 393, 398 Temperature, 35
system, 437, 443 steroid, 53 temperature, 53
sepal, 352 steroids, 45 template strand, 211, 221
sepals, 344 stigma, 344, 352 temporal lobe, 436, 443
septum, 145, 150, 313, 320 stoma, 132 tertiary consumer, 565
Sertoli cell, 496 stomach, 410, 443 Tertiary consumers, 531
Sertoli cells, 485 stomata, 120 test cross, 181, 195
set point, 404, 443 stop codon, 221 testes, 485, 496
sex determination, 481, 496 stop codons, 214 Testosterone, 490
sexual reproduction, 478, 496 Strobili, 335 testosterone, 496
shared ancestral character, 286, strobili, 352 Testudines, 391, 398
288 stroma, 121, 132 tetrad, 171
shared derived character, 286, stromatolite, 293, 320 tetrads, 158
288 style, 344, 352 Tetrapod, 383
sister taxa, 279, 288 subduction, 541, 564 tetrapod, 398
Skeletal muscle tissue, 430 substrate, 114 thalamus, 437, 443
skeletal muscle tissue, 443 substrates, 98 thallus, 312, 320
skull, 427, 443 subtropical desert, 564 Thermodynamics, 93
small intestine, 410, 443 Subtropical deserts, 549 thermodynamics, 114
smooth endoplasmic reticulum sudoriferous gland, 398 thoracic cage, 428, 444
(SER), 66, 86 Sudoriferous glands, 392 threshold of excitation, 432, 444
Smooth muscle tissue, 430 superior vena cava, 417, 443 thylakoid, 132
smooth muscle tissue, 443 surface tension, 36, 53 thylakoids, 120
solute, 79, 86 survivorship curve, 503, 526 thymus, 424, 444
solvent, 36, 53 swim bladder, 387, 398 thyroid gland, 423, 444
somatic cell, 157, 170 sympathetic nervous system, tight junction, 72, 87
source water, 561, 564 438, 443 tissue, 25
speciation, 262, 270 Sympatric speciation, 262 tissues, 10
species, 276, 288 sympatric speciation, 270 Tonicity, 79
species distribution pattern, 501, synapse, 443 tonicity, 87
526 synapses, 432 trachea, 398, 415, 444
Species richness, 520 synapsis, 158, 170 tracheae, 371
species richness, 526 synaptic cleft, 435, 443 tragedy of the commons, 578,
species-area relationship, 584, syngamy, 327, 352 590
590 Synovial joints, 428 trait, 176, 195
spermatogenesis, 488, 496 synovial joints, 443 trans-fat, 44, 53
Sphenodontia, 391, 398 systematics, 276, 288 transcription bubble, 210, 221
spicule, 398 systemic circulation, 417, 443 transduction, 296, 320
spicules, 362 systole, 418, 443 transformation, 296, 320
spinal cord, 443 transgenic, 232, 245
spindle, 138 T Transgenic, 235
spiracle, 398 translocation, 171
T cell, 473
spiracles, 371 translocations, 164
T cells, 460
splicing, 212, 221 tricuspid valve, 417, 444
tadpole, 389, 398
spongocoel, 362, 398 triglyceride, 53
taxon, 276, 288
sporangia, 327 triglycerides, 43
Taxonomy, 276
sporangium, 352 triploblast, 398
taxonomy, 288
sporophyll, 352 triploblasts, 359
telomerase, 206, 221
sporophylls, 335 trisomy, 165, 171
telomere, 221
sporophyte, 157, 170, 327, 352 tRNA, 221
telomeres, 206
Squamata, 391, 398 tRNAs, 213
telophase, 140, 150
stamen, 352 trophic level, 531, 565
temperate forest, 564
stamens, 344 trophoblast, 483, 496

This OpenStax book is available for free at http://cnx.org/content/col11487/1.9

Index 613

tropical rainforest, 565 Whole genome sequencing, 238

Tropical rainforests, 548 whole genome sequencing, 245
tumor suppressor gene, 150 wild type, 187, 195
Tumor suppressor genes, 144
tunicate, 398 X
tunicates, 383
X inactivation, 166, 171
U X-linked, 188, 195
Xylem, 334
unified cell theory, 59, 87 xylem, 352
unsaturated fatty acid, 44, 53
up-regulation, 422, 444 Y
ureter, 407, 444
yeast, 320
urethra, 407, 444
yeasts, 312
urinary bladder, 407, 444
Urochordata, 383, 398
Urodela, 388, 398
Z
uterus, 487, 496 zero population growth, 505,
526
V zona pellucida, 483, 496
Zygomycota, 314, 320
vaccine, 455, 473
vacuole, 87
vacuoles, 67
vagina, 487, 496
van der Waals interaction, 53
van der Waals interactions, 33
variable, 20, 25
variation, 252, 270
vascular plant, 352
Vascular plants, 331
vein, 444
Veins, 420
ventricle, 417, 444
vertebral column, 382, 398, 428,
444
vesicle, 87
Vesicles, 67
vestigial structure, 270
vestigial structures, 259
vicariance, 263, 270
viral envelope, 451, 473
virion, 451, 473
vitamin, 444
Vitamins, 413
viviparity, 482, 496

W
water vascular system, 380, 398
wavelength, 123, 132
wetland, 565
Wetlands, 562
whisk fern, 352
whisk ferns, 336
white blood cell, 457, 473
white-nose syndrome, 580, 590