Natural Selection Question 9

A species of snail lives in the intertidal zone along the coast of New England. The dark-colored variety of the species is more common in northern New England, the light-colored variety is more common two hundred miles away in southern New England, and both varieties are commonly found together in central New England. Which of the following best explains the observed distribution pattern of the snails?

The founder effect suggests that dark-colored snails migrated from the southern regions to the north and established the populations found there.

Genetic drift at the shell-color locus caused the northern population to become homozygous for the dark-color allele.

The mutation rate is higher in the south, as the longer days expose the snails to more ultraviolet radiation than in the north.

Dark-colored snails absorb more solar energy and so survive more readily in the colder northern waters.