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17.2 Evolution as Genetic Change in
Populations
Lesson Objectives
Explain how natural selection affects single-gene and polygenic traits.
Describe genetic drift.
Explain how different factors affect genetic equilibrium.
Lesson Summary
How Natural Selection Works Natural selection on a single-gene trait can lead to
changes in allele frequencies and changes in phenotype frequencies. For polygenic traits,
populations often exhibit a range of phenotypes for a trait. When graphed, this range usually
forms a bell curve, with fewer individuals exhibiting the extreme phenotypes than those with
the average (in the case of beak size, the extremes may be tiny and large beaks). Natural
selection on polygenic traits can cause shifts to the bell curve depending upon which
phenotype is selected for.
Directional selection takes place when individuals at one end of the bell curve have
higher fitness than those near the middle or at the other end of the curve. For example,
when large seeds are plentiful, large-beaked birds in a population may be selected for.
Stabilizing selection takes place when individuals near the middle of the curve have
higher fitness than individuals at either end.
Disruptive selection takes place when individuals at the upper and lower ends of the
curve have higher fitness than individuals near the middle.
Genetic Drift In small populations, alleles can become more or less common simply by
chance. This kind of change in allele frequency is called genetic drift.
The bottleneck effect is a change in allele frequency following a dramatic reduction in
the size of a population.
The founder effect is a change in allele frequency that may occur when a few individuals
from a population migrate to and colonize a new habitat.
Evolution Versus Genetic Equilibrium If allele frequencies in a population do not
change, the population is in genetic equilibrium. Evolution is not taking place.
The Hardy-Weinberg Principle states that allele frequencies in a population should
remain constant unless one or more factors cause those frequencies to change. These
factors include: non-random mating, small population size, immigration or emigration,
mutations, and natural selection.
Populations are rarely in genetic equilibrium. Most of the time, evolution is occurring.
For example, many species exhibit non-random mating patterns. Sexual selection, or the
process in which an individual chooses its mate based on heritable traits (such as size or
strength), is a common practice for many organisms.
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How Natural Selection Works
1. If a trait made an organism less likely to survive and reproduce, what would happen to
the allele for that trait?
2. If a trait had no effect on an organism’s fitness, what would likely happen to the allele for
that trait?
Use the table showing the evolution of a population of mice to answer Questions 3–5.
Initial Population
Generation 10
Generation 20
Generation 30
90%
80%
70%
40%
10%
20%
30%
60%
3. Is the trait for fur color a single-gene trait or a polygenic trait? Explain your answer.
4. Describe how the relative frequency of fur color alleles is changing in this population and
propose one explanation for this change.
5. Suppose a mutation causes a white fur phenotype to emerge in the population. What
might happen to the mouse population after 40 generations?
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6. What effect does stabilizing selection have on variation in a population?
For Questions 7–9, match the type of selection with the correct situation.
Type of Selection
7. Directional
8. Stabilizing
9. Disruptive
Situation
A. Individuals at the upper and lower ends of the
curve have higher fitness than individuals near
the middle.
B. Individuals at one end of the curve have higher
fitness than individuals in the middle or at the
other end.
C. Individuals near the center of the curve have
higher fitness than individuals at either end.
10. THINK VISUALLY Draw the missing line in the graph on the right to show how
disruptive selection affects beak size.
Genetic Drift
For Questions 11–13, complete each statement by writing the correct word or words.
11. In small populations, random changes in
is called genetic drift.
12. A situation in which allele frequencies change as a result of the migration of a small
subgroup of a population is known as the
.
13. The
is a change in allele frequency following a dramatic
reduction in the size of a population.
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14. Complete the concept map.
Genetic Drift
can result from
Bottleneck Effect
caused by
caused by
Evolution Versus Genetic Equilibrium
15. What does the Hardy-Weinberg principle state?
16. What is genetic equilibrium?
17. List the five conditions that can disturb genetic equilibrium and cause evolution to occur.
18. Explain how sexual selection results in non-random mating.
Apply the Big idea
19. Suppose a population of insects live in a sandy habitat. Some of the insects have tan
bodies and some have green bodies. Over time, the habitat changes to a grass-filled
meadow. Use the ideas of natural selection to explain how and why the insect population
might change.
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