Natural Selection (College Board AP Biology)

Figure 1. Sampling sites of marine mussels at various locations (1-8) in Long Island Sound and the Atlantic Ocean

TABLE 1. PERCENT OF INDIVIDUALS POSSESSING lap⁹⁴ ALLELE

	Long Island Sound					Atlantic Ocean
Site	1	2	3	4	5	6	7	8
lap94 frequency (%)	13	16	25	37	55	59	59	59
Salinity						High

Leucine aminopeptidases (LAPs) are found in all living organisms and have been associated with the response of the marine mussel, Mytilus edulis, to changes in salinity. LAPs are enzymes that remove N-terminal amino acids from proteins and release the free amino acids into the cytosol. To investigate the evolution of LAPs in wild populations of M. edulis, researchers sampled adult mussels from several different locations along a part of the northeast coast of the United States, as shown in Figure 1. The researchers then determined the percent of individuals possessing a particular lap allele, lap⁹⁴, in mussels from each sample site (table 1).

On the axes provided, construct an appropriately labeled bar graph to illustrate the observed frequencies of the lap⁹⁴ allele in the study populations.

Based on the data, describe the most likely effect of salinity on the frequency of the lap⁹⁴ allele in the marine mussel populations in Long Island Sound. Predict the likely lap⁹⁴ allele frequency at a sampling site between site 1 and site 2 in Long Island Sound.

Describe the most likely effect of LAP⁹⁴ activity on the osmolarity of the cytosol. Describe the function of LAP⁹⁴ in maintaining water balance in the mussels living in the Atlantic Ocean.

Marine mussel larvae are evenly dispersed throughout the study area by water movement. As larvae mature, they attach to the rocks in the water. Explain the differences in lap⁹⁴ allele frequency among adult mussel populations at the sample sites despite the dispersal of larvae throughout the entire study area. Predict the likely effect on distribution of mussels in Long Island Sound if the lap⁹⁴ allele was found in all of the mussels in the population. Justify your prediction.

A researcher removed a plant nodule and identified several "cheater" rhizobacteria that do not produce IAA or fix nitrogen. Describe the evolutionary advantage of being a bacterial cheater in a population composed predominantly of noncheater bacteria. Plants can adjust the amount of carbon-containing molecules released into nodules in response to the amount of nitrogen fixed in the nodule. Predict the change in the bacterial population that would cause the plant to reduce the amount of carbon-containing molecules provided to the nodule.