Transcript Lesson Overview

17.1
Genes
and
Variation
Lesson Overview
Genes and Variation
Genetics Joins Evolutionary Theory
Darwin developed his theory of evolution without knowing how
heritable traits passed from one generation to the next or where
heritable variation came from.
Researchers discovered that heritable traits are controlled by
genes.
Changes in genes and chromosomes generate variation.
For example, all of these children received their genes from the
same parents, but they all look different.
Lesson Overview
Genes and Variation
Genotype and Phenotype in Evolution
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.
Lesson Overview
Genes and Variation
Populations and 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.
Lesson Overview
Genes and Variation
For example, this diagram shows the gene pool for
fur color in a population of mice.
Lesson Overview
Genes and Variation
Populations and Gene Pools
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. Populations, rather than individuals,
evolve.
Lesson Overview
Genes and Variation
Sources of Genetic Variation
Three sources of genetic variation are:
• mutation,
• genetic recombination during
sexual reproduction, and
• lateral gene transfer.
Lesson Overview
Genes and Variation
Mutations
Mutations are any change in the DNA sequence.
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.
Lesson Overview
Genes and Variation
Genetic Recombination in Sexual Reproduction
Most heritable differences are due to genetic
recombination during sexual reproduction. This
occurs during Meiosis when each chromosome in
a pair moves independently. Genetics
recombination also occurs during crossing-over in
meiosis.
Lesson Overview
Genes and Variation
Lateral Gene Transfer
Lateral gene transfer occurs when organisms pass
genes from one individual to another that is not its
offspring.
It can occur between organisms of the same species
or organisms of different species.
Lateral gene transfer can increase genetic variation
in a species that picks up the “new” genes.
Lesson Overview
Genes and Variation
Single-Gene and Polygenic Traits
What determines the number of phenotypes for a given trait?
Lesson Overview
Genes and Variation
Single-Gene Traits
The number of phenotypes produced for a trait depends on
how many genes control the trait.
A single-gene trait is a trait controlled by only one gene.
Single-gene traits may have just two or three distinct
phenotypes.
The most common form of the allele can be dominant or
recessive.
Lesson Overview
Genes and Variation
Polygenic Traits
Polygenic traits are traits controlled by two or more
genes.
Each gene of a polygenic trait often has two or more
alleles.
A single polygenic trait often has many possible
genotypes and even more different phenotypes.
Lesson Overview
Genes and Variation
Polygenic Traits
Human height, which varies from very short to very
tall, is an example of a polygenic trait.
The bell-shaped curve in the graph is typical of
polygenic traits.
17.2
Evolution as
Genetic
Change in
Populations
Lesson Overview
Evolution as Genetic Change in Populations
• Insect populations often contain a few
individuals that are resistant to a
particular pesticide. Those insects pass
on their resistance to their offspring and
soon the pesticide-resistant offspring
dominate the population. The
relationship between natural selection
and genetics explains how pesticide
resistance develops.
Lesson Overview
Evolution as Genetic Change in Populations
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 distributions
of phenotypes in three ways: directional selection, stabilizing
selection, or disruptive selection.
Evolutionary fitness is the success in passing genes to the next
generation.
Evolutionary adaptation is any genetically controlled trait that
increases an individual’s ability to pass along its alleles.
Lesson Overview
Evolution as Genetic Change in Populations
Natural Selection on Single-Gene Traits
Natural selection for a single-gene trait can lead to changes in allele
frequencies and then to evolution.
For example, a mutation in one gene that determines body color in lizards
can affect their lifespan. So if the normal color for lizards is brown, a
mutation may produce red and black forms.
Lesson Overview
Evolution as Genetic Change in Populations
Natural Selection on Single-Gene Traits
Natural selection for a single-gene trait can lead to changes in allele
frequencies and then to evolution.
For example, a mutation in one gene that determines body color in lizards
can affect their lifespan. So if the normal color for lizards is brown, a
mutation may produce red and black forms.
Lesson Overview
Evolution as Genetic Change in Populations
Natural Selection on Single-Gene Traits
If red lizards are more visible to predators, they might be less
likely to survive and reproduce. Therefore the allele for red
coloring might not become common.
Lesson Overview
Evolution as Genetic Change in Populations
Natural Selection on Single-Gene Traits
Black lizards might be able to absorb sunlight. Higher body
temperatures may allow the lizards to move faster, escape
predators, and reproduce.
Lesson Overview
Evolution as Genetic Change in Populations
Natural Selection on Polygenic Traits
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.
Lesson Overview
Evolution as Genetic Change in Populations
Directional Selection
Directional selection occurs when individuals at one end of
the curve have higher fitness than individuals in the middle or
at the other end. 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.
Lesson Overview
Evolution as Genetic Change in Populations
Stabilizing Selection
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 averagesized individuals. The fitness
of these smaller or larger
babies is therefore lower
than that of more averagesized individuals.
Lesson Overview
Evolution as Genetic Change in Populations
Disruptive Selection
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. Therefore, the
population might split into
two groups—one with
smaller beaks and one with
larger beaks.
Lesson Overview
Evolution as Genetic Change in Populations
Genetic Drift
Genetic drift occurs in small populations when an
allele becomes more or less common simply by
chance. Genetic drift is a random change in allele
frequency.
Lesson Overview
Evolution as Genetic Change in Populations
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.
Lesson Overview
Evolution as Genetic Change in Populations
The Founder Effect
The founder effect occurs when allele frequencies change as
a result of the migration of a small subgroup of a population.
Two groups from a large, diverse population could produce
new populations that differ from the original group.
17.3
The
Process
of
Speciation
Lesson Overview
The Process of Speciation
Factors such as natural selection and
genetic drift can change the relative
frequencies of alleles in a population, but
this alone does not lead to development of
a new species.
How does one species
become two?
Lesson Overview
The Process of Speciation
Isolating Mechanisms
Speciation is the formation of a new species.
A species is a population whose members can
interbreed and produce fertile offspring.
Lesson Overview
The Process of Speciation
Isolating Mechanisms
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.
Lesson Overview
The Process of Speciation
Behavioral Isolation
Behavioral isolation occurs when two populations that are
capable of interbreeding develop differences in courtship
rituals or other behaviors.
The eastern meadowlark (left) and western meadowlark (right) have overlapping
ranges. They have different mating songs.
Lesson Overview
The Process of Speciation
Geographic Isolation
Geographic isolation occurs when two populations are
separated by geographic barriers such as rivers, mountains,
or bodies of water.
For example, the Kaibab
squirrel is a subspecies of
the Abert’s squirrel that
formed when a small
population became isolated
on the north rim of the
Grand Canyon. Separate
gene pools formed, and
genetic changes in one
group were not passed on to
the other.
Kaibab
Squirrel
Abert’s
Squirrel
Lesson Overview
The Process of Speciation
Temporal Isolation
Temporal isolation happens when two or more
species reproduce at different times.
Cicadas breed
every 13 years
Cicadas breed
every 17 years