Question

Researchers studied genetic variation in the marine mussel Mytilus edulis around Long Island, New York. They measured the frequency of a particular allele (lap94) for an enzyme involved in regulating the mussel’s internal saltwater balance. The researchers presented their data of sampled sites within Long Island Sound, where the salinity is highly variable, and along the coast of the open ocean, where salinity is constant.

Considering natural selection, describe the expected allelic frequencies of the mussel populations found at the sites labeled A - D on the map.
Responses
A Allelic frequency will vary proportionately; highest incidence of the allele should be found at site D.Allelic frequency will vary proportionately; highest incidence of the allele should be found at site D.
B All the mussel populations, sites A - D, are expected to have the same frequency of the lap94 allele.All the mussel populations, sites A - D, are expected to have the same frequency of the lap94 allele.
C Allelic frequency will only vary at sites C and D, closest to the end of the island.Allelic frequency will only vary at sites C and D, closest to the end of the island.
D The mussel population at site A will not have the allele; the mussel population at site D will have the allele.

Answer 1

A.

Because lap94 affects salt-balance ability, natural selection will favor it where salinity is variable (Long Island Sound) and not where salinity is constant (open ocean). That produces a cline in allele frequency with the highest incidence at the site deepest in the variable-salinity habitat (site D) and progressively lower frequencies toward the open-ocean sites. This is more realistic than uniform frequencies (B), variation only at two sites (C), or an all-or-none presence (D).