Question

Kerstin Johannesson and colleagues (1995) stud- ied two populations of a marine snail living in the intertidal zone on the shore of Ursholmen Island. Each year, the researchers determined the allele fre- quencies for the enzyme aspartate aminotransferase (don't worry about what this enzyme does). Their data are shown in the graphs in Figure 6.36. The 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 extermi- nated 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 explana- tion for the data in the graphs. In each part, be sure to name the evolutionary mechanism involved (se- lection, mutation, migration, or drift).

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 evolution- ary mechanism, and explain.
d. Predict what would happen to the allele frequen- cies if we followed these two populations for another 100 years (assuming there are no more toxic algal blooms). Explain your reasoning.

Answer 1

The allele frequency changes in marine snails after a toxic algae bloom can be explained by migration, selection, and genetic drift as the main evolutionary mechanisms. Migration brought new alleles from the splash zone to the intertidal zone, selection favored different alleles over time, and genetic drift caused random changes in allele frequencies.

Step-by-step explanation:

The study of Kerstin Johannesson and colleagues on marine snails and the allele frequencies of aspartate aminotransferase provides a practical example of evolutionary mechanisms at work. These mechanisms are selection, mutation, migration, and genetic drift.

a. The frequency of the Aat120 allele was higher in 1990 compared to 1987 likely due to migration. After the toxic algae bloom that killed the snails in the intertidal zone, the surviving snails from the splash zone recolonized the area, bringing their allele frequencies with them.

b. The allele frequency decline from 1990 through 1993 can be attributed to selection. The new environment of the intertidal zone may have favored different alleles over the Aat120 allele that was initially brought in by the splash zone snails.

c. The curves traced by the 1990-1993 data for the two populations are generally similar but not identical due to genetic drift and possibly some local selection. Each population is subject to its own random changes in allele frequencies and unique environmental pressures.

d. Predicting what would happen to the allele frequencies over another 100 years without further algal blooms involves selection and genetic drift. It would be expected that the allele frequencies would stabilize to reflect the alleles that confer an advantage in the local environment, with some variation introduced by random changes over time.