Kejaun Johnson
Genetics Joins Evolutionary Theory:
In genetic terms, evolution is any change in the relative frequency of alleles in the gene pool of
a population over time.
Researchers discovered that heritable traits are controlled by genes.
Changes in genes and chromosomes generate variation.
Genotype & Phenotype in Evolution:
An organism’s genotype is the particular combination of alleles it carries.
An individual’s genotype, together with environmental conditions, produces its phenotype.
Phenotype includes all physical, physiological, and behavioral characteristics of an organism.
Natural selection acts directly on phenotype, not genotype.
Some individuals have phenotypes that are better suited to their environment than others.
These individuals produce more offspring and pass on more copies of their genes to the next
generation.
Populations & Gene Pools:
A population is a group of individuals of the same species that mate and produce offspring.
A gene pool consists of all the genes, including all the different alleles for each gene that are
present in a population.
Researchers study gene pools by examining the relative frequency of an allele.
The relative frequency of an allele is the number of times a particular allele occurs in a gene
pool, compared with the number of times other alleles for the same gene occur.
Evolution is any change in the relative frequency of alleles in the gene pool of a population
over time.
Natural selection operates on individuals, but resulting changes in allele frequencies show up in
populations.
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Populations, rather than individuals, evolve.
Sources of Genetic Variation:
Three sources of genetic variation are mutatic,
genetic recombination during sexual reproproduction,
and lateral gene transfer.
Mutations:
Mutations that produce changes in phenotype may or may not affect fitness.
Some mutations may be lethal or may lower fitness; others may be beneficial.
Mutations matter in evolution only if they can be passed from generation to generation.
The mutation must occur in the germ line cells that produce either eggs or sperm.
How Natural Selection Works:
Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to
changes in phenotype frequencies.
Natural selection on polygenic traits can affect the distribution of phenotypes in…
Three Ways;
1) Directional selection
2) Stabilizing selection
3) Disruptive selection
Evolutionary fitness is the success in passing genes to the next generation.
Evolution adaptation is any genetically controlled trait that increases an individual’s ability
to pass along its alleles.
Kejaun Johnson
Natural Selection on Single-Gene Traits:
Natural selection for a single-gene trait can lead to changes in allele frequencies and then
to evolution.
Black lizards might be able to absorb sunlight.
Polygenic traits have a range of phenotypes that often form a bell curve.
The fitness of individuals may vary from one end of the curve to the other.
Natural selection can affect the range of phenotypes and hence the shape of the bell curve.
Directional Selection;
…occurs when individuals at one end of the curve have higher fitness than individuals in the
middle or at the other end.
Think of moving in only one direction.
The range of phenotypes shifts because some individuals are more successful at surviving and
reproducing than others.
For example, if only large seeds were available, birds with larger beaks would have an easier
time feeding and would be more successful in surviving and passing on genes.
Stabilizing Selection;
…occurs when individuals near the center of the curve have higher fitness than individuals at
either end.
This situation keeps the center of the curve at its current position, but it narrows the overall
graph.
For example, very small and very large babies are less likely to survive than average-sized
individuals have the greatest fitness.
Kejaun Johnson
Disruptive Selection;
…occurs when individuals at the upper and lower ends of the curve have higher fitness than
individuals near the middle.
Disruptive selection acts against individuals of an intermediate type and can create two
distinct phenotypes.
For example, in an area where medium-sized seeds are less common, birds with unusually
small or large beaks would have higher fitness.
Evolution Versus Genetic Equilibrium
The Founder Effect
The founder effect occurs when allele frequencies change as a result of the migration of a
small subgroup of a population
Genetic Bottlenecks
The bottleneck effect is a change in allele frequency following a dramatic reduction in the
size of a population.
For example, a disaster may kill many individuals in a population, and the surviving
population’s gene pool may contain different gene frequencies from the original gene pool.
A population is in genetic equilibrium if allele frequencies in the population remain the same.
If allele frequencies don’t change, the population will not evolve.
According to the Hardy-Weinburg principle, five conditions are required to maintain genetic
equilibrium:
1) The Population MUST be very large
2) There can be NO mutations
3) There MUST be Random Mating
4) There can be NO movement Into or Out of the population
5) NO natural selection
Kejaun Johnson
Isolating Mechanisms
When populations become reproductively isolated, they can evolve into two separate species.
Reproductive isolation can develop in a variety of ways including:
1) Behavioral Isolation
2) Geographic Isolation
3) Temporal Isolation
Speciation:
The Formation of a new species
A species is a population whose members can interbreed and produce fertile offspring.
Reproductive isolation occurs when a population splits into two groups and the two
populations no longer interbreed.
When populations become reproductively isolated, they can evolve into two separate species.
Behavioral Isolation
Behavioral Isolation occurs when two populations that are capable of interbreeding develop
differences in courtship rituals or other behaviors.
1) Birds of Paradise -want to see a dance.
2) Certain Breeds of frogs -want to hear a specific croak.
Geographic Isolation
Geographic isolation occurs when two populations are separated by geographic barriers such
as rivers, mountains or bodies of water.
Separate gene pools formed, and genetic changes in one group were not passed on to the other.
Temporal Isolation
Temporal isolation happens when two or more species reproduce at different times
For example, three species of orchid live in the same rain forest
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Each species has flowers that last only one day and must be pollinated on that day to produce
seeds
Because the species bloom on different days, they cannot pollinate each other
Frogs mating at different times of the month
Classification:
Assigning Scientific Names
Keep (Kingdom)
Ponds (Phylum)
Clean (Class)
Or (Order)
Frogs (Family)
Get (Genus)
Sick (Species)
The first step in understanding and studying diversity is to describe and name each species.
By using a scientific name, biologists can be sure that they are discussing the same organism.
Why Classify?
In binomial nomenclature, each species is assigned a two-part scientific name.
The goal of systematics is to organize living things into groups that have biological meaning.
Binomial Nomenclature
Carolus Linnaeus
It is written in italics.
The first word begins with a capital letter. (Genus)
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The second word is lowercase. (Species)
The first part of the name—Ursus—is the genus to which the organism belongs.
A genus is a group of similar species.
The second part of a scientific name—maritimus for polar bears—is unique to each species and
is often a description of the organism's habitat or of an important trait.~
Classifying Species into Larger Groups
In addition to naming organisms, biologists try to organize, or classify, living and fossil species
into larger groups that have biological meaning.
Biologists often refer to these groups as taxa (Singular; Taxon)
The science of naming and grouping organisms is called systematics
Linnaean Classification System
Linnaeus grouped species according to anatomical similarities and differences.
Linnaeus's original classification system would expand to include seven hierarchical taxa:
–kingdom, phylum, class, order, family, genus, species
Keep Ponds Clean Or Frogs Get Sick
Problems With Traditional Classification
Modern classification schemes look beyond overall similarities and differences and group
organisms based on evolutionary relationships
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Evolutionary Classification
The goal of phylogenetic systematics, or evolutionary classification is to group species into
larger categories that reflect lines of evolutionary descent, rather than overall similarities and
differences.
Clades
A clade is a group of species that includes a single common ancestor and all descendants of
that ancestor—living and extinct
A monophyletic group must include all species that are descended from a common ancestor.
Cladograms
A cladogram links groups of organisms by showing how evolutionary lines, or lineages,
branched off from common ancestors.
Building Cladograms
The bottom, or "root” of the tree represents the common ancestor shared by all organisms
on the cladogram.
A cladogram's branching patterns indicate degrees of relatedness among organisms.
Derived Characters
In contrast to Linnaean classification, cladistic analysis focuses on certain kinds of characters,
called derived characters, when assigning organisms into clades.
A derived character is a trait that arose in the most recent common ancestor of a particular
lineage and was passed along to its descendants.
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Reading Cladograms
The lowest node represents the last common ancestor of all four-limbed animals.
The forks show the order in which various groups branched off over the course of evolution.
The positions of the derived characters on the cladogram reflect the order in which those
characteristics arose in this lineage.
Each derived character defines a clade.
