Question

The graphs show changes in allele frequencies in a population at three points in time as a drought becomes more severe.


Allele Frequency at Time 1.
Allele z.
25%
Allele X.
45%
Allele Y.
30%
Allele Frequency at Time 2.
Allele z.
30%
Allele X.
34%
Allele Y.
36%
Allele Frequency
at Time 3.
Allele z.
42%
Allele X.
28%
Allele Y.
30%

Which statement best describes how natural selection affects alleles?
A
Natural selection always affects how the same allele influences an organism's reproductive success.
B
Natural selection affects alleles randomly, leading to small changes in allele frequencies.
C
Natural selection affects allele frequencies in individuals, not in populations.
D
Natural selection affects allele frequencies differently depending on how helpful an allele is to an organism's fitness for its environment.

Answer 1

D

Natural selection changes allele frequencies depending on how much a given allele improves survival and reproduction in the current environment. The graph shows allele z rising (beneficial in drought) while X falls and Y changes less—not random and not acting only on individuals.