Chapter 25, Problem 3A

The Eda allele that causes reduced armor originated about 2 MYA in marine stickleback fish and persists with a frequency of about 1% in marine environments. The frequency is much higher in freshwater populations. Apply your understanding of natural selection to determine the most likely reason for the difference in Eda allele frequency.

a. The Eda allele causing reduced armor has evolved repeatedly in marine and freshwater environments.

b. Fish have many predators in the marine environment and fish with reduced armor have reduced fitness.

c. There is negative selection for armor in freshwater because building armor is energetically expensive.

d. Both b and c are valid.