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Lesson Overview
17.2 Evolution as Genetic
Change in Populations
Lesson Overview
Evolution as Genetic Change in Populations
THINK ABOUT IT
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
How does natural selection affect single-gene and polygenic traits?
Lesson Overview
Evolution as Genetic Change in Populations
How Natural Selection Works
How does natural selection affect single-gene and polygenic traits?
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.
Lesson Overview
Evolution as Genetic Change in Populations
How Natural Selection Works
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.
Lesson Overview
Evolution as Genetic Change in Populations
Natural Selection on Single-Gene Traits:
The example of Lizard Color
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
Single-Gene Traits: 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 Single-Gene Traits
Single-Gene Traits: The allele for black color might become more
common.
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.
Lesson Overview
Evolution as Genetic Change in Populations
Directional Selection
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.
Lesson Overview
Evolution as Genetic Change in Populations
Stabilizing Selection
For example, very small and very large babies are
less likely to survive than average-sized individuals.
The fitness of these smaller or larger babies is
therefore lower than that of more average-sized
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.
Lesson Overview
Evolution as Genetic Change in Populations
Disruptive Selection
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
What is 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.
Lesson Overview
Evolution as Genetic Change in Populations
The Founder Effect
Two groups from a large, diverse population could produce
new populations that differ from the original group.
Lesson Overview
Evolution as Genetic Change in Populations
Evolution Versus Genetic Equilibrium
What conditions are required to maintain genetic
equilibrium?
According to the Hardy-Weinberg 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.
Lesson Overview
Evolution as Genetic Change in Populations
Evolution Versus Genetic Equilibrium
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.
Lesson Overview
Evolution as Genetic Change in Populations
The Hardy-Weinberg Principle
The Hardy-Weinberg principle describes the
conditions under which evolution does not
occur.
The Hardy-Weinberg principle states that allele frequencies in a population remain
constant unless one or more factors cause those frequencies to change.
Lesson Overview
Evolution as Genetic Change in Populations
Large Population
Genetic drift can cause changes in allele
frequencies in small populations.
Genetic drift has less effect on large
populations.
Large population size helps maintain
genetic equilibrium.
Lesson Overview
Evolution as Genetic Change in Populations
No Mutations
If mutations occur, new alleles may be
introduced into the gene pool, and allele
frequencies will change.
Lesson Overview
Evolution as Genetic Change in Populations
Random Mating
Individuals must mate with other members
of the population at random.
In natural populations, however, mating is
not random. Female peacocks, for example,
choose mates on the basis of physical
characteristics such as brightly patterned tail
feathers.
Lesson Overview
Evolution as Genetic Change in Populations
No Movement Into or Out of the Population
Individuals who join a population may
introduce new alleles into the gene pool.
Individuals who leave may remove alleles
from the gene pool.
Thus, for no alleles to flow into or out of the
gene pool, there must be no movement of
individuals into or out of a population.
Lesson Overview
Evolution as Genetic Change in Populations
No Natural Selection
All genotypes in the population must have
equal probabilities of surviving and
reproducing. No phenotype can have a
selective advantage over another.
Lesson Overview
Evolution as Genetic Change in Populations
Sexual Reproduction and Allele Frequency
Meiosis and fertilization do not change the relative frequency of alleles in a
population.
The shuffling of genes during sexual
reproduction produces many different gene
combinations but does not alter the relative
frequencies of alleles in a population.