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Lesson Overview
17.2 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
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.
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. 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.
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 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.
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
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.
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?
In small populations, individuals that carry a particular allele may leave more
descendants than other individuals, just by chance. Over time, a series of chance
occurrences can cause an allele to become more or less common in a population.
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.
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, and
(5) no natural selection.
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.
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, such as the seals
shown.
Large population size helps maintain genetic equilibrium.
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
All members of the population must have an equal opportunity to
produce offspring. 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. Such nonrandom mating means that alleles for those traits are under selection
pressure.
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.
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.