The Theory of Evolution
The Theory of Evolution by Natural
Selection
• The idea that life evolves was first proposed by a
Roman named Lucretius who lived about 1,900
years before the modern theory of evolution.
• The theory of evolution has developed through
decades of scientific observation and
experimentation. The modern theory began to
take shape as a result of the work of Charles
Darwin.
• In 1831, Darwin was asked to voyage on the
HMS Beagle as an unofficial naturalist. What he
found would challenge the traditional belief that
species are unchanging.
What Darwin Saw During His
Voyage
• Galapagos Islands – Darwin found signs
that species evolve. He was struck by the
fact that the flora and fauna of the islands
and those of the nearby coast of South
America resembled each other.
• Darwin proposed that the ancestors of
Galapagos species must have migrated to
the islands from S.America long ago and
changed after they arrived.
Darwin Sought a Reasonable
Explanation for Evolution
• The key that unlocked Darwin’s thinking was an
essay written in 1798 by Thomas Malthus.
• Malthus pointed out that the human population
grew faster than food supply (human pop. grew
exponentially while food increased arithmeticly) .
If human reproduction was left unchecked,
people would cover the world.
• We don’t because of things like disease, war,
and famine that will slow population growth.
• Population – all the individuals of a species that
live together in one place at one time. Ex. The
population of deer in Vancleave, MS.
Natural Selection
• Darwin realized Malthus’s ideas apply to all species.
Every organism has the potential to produce many
offspring during its lifetime. In most cases, only a
limited number of those offspring survive to
reproduce.
• Individuals that have physical or behavioral traits that
better suit their environment are more likely to survive
and reproduce than those that do not have such
traits. –Charles Darwin
• Natural selection – process by which populations
change in response to their environment.
• Darwin suggested that organisms differ from place to
place because their habitats present different
challenges to, and opportunities for survival and
reproduction.
• Adaptation – the changing of a species that results
in its being better suited to its environment
Darwin’s Ideas Have Been Updated
• Best suited to survive and do well in their
environment = most offspring with the traits of
the better suited parents. Certain forms of a trait
become more common in a pop. because more
individuals in the population carry the alleles for
those forms. Natural selection causes the
frequency of certain alleles to increase or
decrease over time.
• Environments differ from place to place, so
populations of the same species living in
different places tend to evolve in different
directions. Isolation – condition in which two
pop.s of the same species cannot breed with
one another due to changing.
Extinction Leads to Species
Replacement
• Over time, events such as climate
changes and natural disasters result in
some species becoming extinct, which
means that they disappear permanently.
Species that are better suited for the new
conditions will replace the others. For
example, dinosaurs were (in a sense)
replaced by humans.
Evidence of Evolution
Fossils Provide an Objective
Record of Evolution
• Darwin’s theory is almost universally
accepted. Based on a large body of
supporting evidence, most scientists
agree on the following major points:
• Earth is about 4.5 billion years old.
• Organisms have inhabited Earth for most
of its history.
• All organisms living today evolved from
earlier, simpler life-forms.
• However, the fossil record, (and so the record of
evolution) is not complete. Many species have lived in
environments where fossils do not form. Even if an
organism does live in good fossil-making
environments, the chances are slim that its dead body
will be buried in sediments before it decays.
• Although the fossil record will never be complete, it
presents strong evidence that evolution has taken
place. Paleontologists, scientists who study fossils,
can determine a fossil’s age fairly accurately using
radiometric dating.
• Radiometric dating enables paleontologists to
arrange fossils in sequence from oldest to youngest.
When this is done, orderly patterns of evolution can be
seen.
Biological Molecules Contain a
Record of Evolution
• If species change over time, then the genes that determine their
characteristics should have also changed (new taller giraffes,
no stretched necks).
• Common Ancestor – species from which two or more species
have diverged.
• Concerning Proteins: Species that share a recent common
ancestor (ex. Humans and gorillas) have few different amino
acid sequences (order of a.a. to make protiens) where species
that share a common ancestor more distantly (ex. Humans and
frogs) have many amino acid sequence differences.
• Concerning Nucleic Acids: Scientists can directly estimate the #
of nucleotide changes that have happened since two species
diverged from a c. ancestor by comparing the exact nucletide
sequence of genes. Using this, they can make a phylogenetic
tree to show how organisms are related through evolution (very
strong evidence)
Anatomy and Development
Suggest Common Ancestry
• Comparisons of the anatomy (structures) of different types
of organisms often reveal basic similarities in body
structures, even though their functions may be very
different.
• Vestigial structures – some structures (like bones) are
present in an organism, but reduced in size and either have
no use or have a less important function (ex. Flightless
wings on a cormorant or the appendix on humans)
• As different vertebrates evolved,
particular sets of bones evolved
differently, but similar bone structure
can be seen. Homologous structures
are structures that share a common
ancestry – a similar structure in two
organisms can be found in their common
ancestor.
Does Evolution Occur in Spurts?
• The model of evolution in which gradual
change over a long period of time leads to
species formation is called gradualism.
• But Gould and Eldredge from America
(hurrah us!) hypothesize that evolution
occurs in spurts. This model of evolution,
in which periods of rapid change in
species are separated by periods of little
or no change, is called punctuated
equilibrium.
Examples of Evolution
Industrial Melanism
• A well studied ex. Of natural selection in action is
industrial melanism, the darkening of populations of
organisms over time in response to industrial pollution.
• The best-known case involves the European peppered
moth, Biston betularia. Among the members of this
species, there are two color variations. Light and dark.
The dark was very rare until about the 1850s when
industrialization was occurring in major cities. 100 years
later, almost all of the peppered moths near industrial
centers were dark.
How? Think…
Industry = factories
Factories = pollution or darkish grime
Grime covers tree trunks, so white moths get eaten and dark moths
survive. Survival of the fittest.
The Formation of New Species
Begins with Small Changes
• Species formation occurs in a series of
stages. Evolution continuously molds and
shapes a species to improve its “fit” to its
environment.
• The accumulation of differences between
groups is called divergence.
• Within populations, divergence leads to
the formation of new species. Biologists
call the process by which new species
form speciation.
Forming Ecological Races
• A species often lives in several
different kinds of environments. If
their environment is different
enough, local pops can become
very different. Over time,
populations of the same species
that differ genetically bc of
adaptations to different living
conditions become what biologists
call ecological races. They are
not different enough to be
different species, but they may not
be able to interbreed successfully.
Maintaining a New Species
• Reproductive Isolation – inability of
formerly interbreeding groups to mate or
produce fertile offspring. Could be
geographically isolated or may reproduce
at different times.
