Answer Key
Biology 11: Types of Natural Selection
Vocabulary Review: Define the following terms:
1. Stabilizing Selection: Natural selection that favors the intermediate phenotype. (Average
individuals of the population are favored.)
2. Directional Selection: Natural selection that favors one of the extreme variations of a trait.
This selection results in a shift to one extreme.
3. Disruptive Selection: Natural selection that favors both extremes of the phenotype. The
intermediate phenotype is eliminated. This type of selection results in favoring both of the
extreme of a variation.
Multiple Choice: Circle the correct answer for each of the questions below.
4. In which of the following is variation reduced in the population:
a. Stabilizing Selection
b. Disruptive Selection
c. Directional Selection
d. None of the above.
5. In which of the following are individuals of intermediate phenotypes are lost from the
population?
a.
b.
c.
d.
Stabilizing Selection
Disruptive Selection
Directional Selection
None of the above
Question 7 to 10: Which type of selection does each of the following scenarios describe? Why?
7. For birds and parasitoids, females that lay close to the Lack optimum number of eggs have the
most surviving offspring. Those that lay fewer or more eggs have lower relative fitness.
Stabilizing – the intermediate phenotype is favored
8. A population of Madagascar hissing cockroaches lives in a woodpile. It suffers heavy
predation from lizards. Because their heads are small, the lizards are unable to eat the very
largest adult cockroaches, and instead prey upon small and medium sized adults. What type of
selection do the lizards impose on the roaches? Why do you think that?
Directional – one of the extreme is favored (largest cockroaches escape predation)
9. If a cow develops a preference for eating white four o’clock flowers and ignoring pink and
red four o’clock flowers, what type of selection is being demonstrated?
Directional - One extreme is favored, the other is selected against – white eaten, red are not.
10. Starlings produce an average of five eggs in each clutch. If there are more than five, the parents
cannot adequately feed the young. If there are fewer than five, predators may destroy the entire clutch.
Stabilizing – intermediate clutch size is favored.
11. Label the three types of selection illustrated below
a. Disruptive
b. Directional
c. Stabilizing
12. Describe what is happening in figures 1-3. Is the population of mice different in figure 3
than in figure 1? Explain why. Which mice in this population have the highest fitness?
What type of selection is occurring? Use a graph to illustrate your answer.
The bird is preying on mice. The mice that
are well camouflaged (dark) are able to
escape predation. Since they are better
adapted to their environment, they are said
to be more fit, and thus increase in
frequency. However the white mice are not
as well adapted and are therefore being
preyed upon and decreasing in frequency.
The type of selection is Directional, for one of
the extremes is being favored (dark mice) and one is being selected against
(white mice)
See directional selection graph above.