Name
Class
Date
17.2 Evolution as Genetic Change in Populations
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?
Genetic Drift
Complete each statement by writing the correct word or words.
3. In small populations, random changes in
is called genetic drift.
4. A situation in which allele frequencies change as a result of the migration of a small subgroup of a population
is known as the
.
5. The
size of a population.
is a change in allele frequency following a dramatic reduction in the
For Questions 6–8, match the type of selection with the correct situation.
Type of Selection
6. Disruptive
7. Directional
8. Stabilizing
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
17.2 Evolution as Genetic Change in Populations
Use the table showing the evolution of a population of mice to answer Questions 9–10.
Initial Population
90%
10%
Generation 10
Generation 20
Generation 30
80%
20%
70%
40%
30%
60%
9. Describe how the relative frequency of fur color alleles is changing in this population and propose one
explanation for this change.
10. Suppose a mutation causes a white fur phenotype to emerge in the population. What might happen to the
mouse population after 40 generations?
17.2 Evolution as Genetic Change in Populations
11. Draw the missing line in the graph on the right to show how disruptive selection affects beak size.
12. Complete the concept map.
Genetic Drift
can result from
Bottleneck Effect
caused by
caused by
Evolution Versus Genetic Equilibrium
13. What does the Hardy-Weinberg principle state?
14. What is genetic equilibrium?
15. List the five conditions that can disturb genetic equilibrium and cause evolution to occur.
17.2 Evolution as Genetic Change in Populations
How Natural Selection Works
Follow the directions.
16. Label the graph that shows stabilizing selection.
original
population
Birth Weight
Number of Birds in Population
Percentage of Human
Population
17. Label the graph that shows disruptive selection.
original
population
Beak Size
Answer the questions.
18. With which type of selection do organisms in the middle of the curve have the highest fitness?
19. In disruptive selection, organisms on which part of the curve have the lowest fitness?
20. How does the curve change in stabilizing selection?
A. The curve becomes shorter and wider.
B. The curve becomes taller and narrower.
C. The curve moves to the right.
21. The plants in an area have either very small or very large seeds. Birds with small beaks can eat small seeds,
and birds with large beaks can eat large seeds. Birds with smaller beaks and birds with larger beaks become
more common than birds with medium beaks. What type of selection is this?
A. stabilizing selection
B. disruptive selection
C. directional selection
Evolution Versus Genetic Equilibrium
Read the chart below. Use phrases in the word box to identify each condition that leads to genetic
equilibrium. The first row has been completed for you.
large population
random mating
no natural selection
no mutations
no movement
The Hardy-Weinberg Principle
large population
The more individuals in the population, the
smaller the effect of genetic drift.
No changes to genes means new alleles are not
introduced into the population’s gene pool.
Each individual in a population has the same
chance of passing on its alleles.
No new alleles are introduced into the
population’s gene pool by new individuals.
No phenotype can have a selective advantage
over another—all individuals have equal fitness.
Read the descriptions below. Use what you know about the Hardy-Weinberg principle to identify what is
happening.
22. Genetic drift strongly affects the population.
A. small population
B. random mating
C. no movement into or out of the population
D. no natural selection
23. Lions with a darker fur color have the same chance to reproduce as lions with a lighter fur color.
A. large population
C. random mating
B. no mutations
D. no natural selection
24. Fitness is basically the same among individuals in the population.
A. large population
C. random mating
B. no mutations
D. no natural selection