Kerstin Johannesson and colleagues (1995) studied two populations of a marine snail living...

Kerstin Johannesson and colleagues (1995) studied two populations of a marine snail living in the intertidal zone on the shore of Ursholmen Island. Each year, the researchers determined the allele frequencies for the enzyme aspartate aminotransferase (don’t worry about what this enzyme does). Their data are shown in the graphs in FigureThe first year of the study was 1987. In 1988, a bloom of toxic algae (tan bars) killed all of the snails in the intertidal zone across the entire island. That is why there are no data for 1988 and 1989. Although the snails living in the intertidal zone were exterminated by the bloom, snails of the same species living in the splash zone just above the intertidal survived unscathed. By 1990, the intertidal zone had been recolonized by splash-zone snails. Your challenge in this question is to develop a coherent explanation for the data in the graphs. In each part, be sure to name the evolutionary mechanism involved (selection, mutation, migration, or drift).

Figure Changes over time in the frequency of an allele in two intertidal populations of a marine snail From Johannesson et al. (1995).

a. Why was the frequency of the Aat120 allele higher in both populations in 1990 than it was in 1987? Name the evolutionary mechanism, and explain.

b. Why did the allele frequency decline in both populations from 1990 through 1993? Name the evolutionary mechanism, and explain.

c. Why are the curves traced by the 1990–1993 data for the two populations generally similar but not exactly identical? Name the evolutionary mechanism, and explain.

d. Predict what would happen to the allele frequencies if we followed these two populations for another 100 years (assuming there are no more toxic algal blooms). Explain your reasoning.