Demographic and temporal structure of an allele frequency cline in the mussel Mytilus edulis

Abstract

In eastern Long Island Sound there is a steep allele-frequency cline at the Lap locus in the mussel Mytilus edulis. The frequency of the Lap 94 allele decreases from 0.55 in oceanic populations to 0.12 in Sound populations over a distance of 30 km. I have examined the size-frequency distribution and the dependence of Lap 94 frequency on demographic structure in these clinal populations from 1980 to 1983. Clinal populations exhibit large temporal and demographic variation in the frequency of Lap 94. Recruitment occurs in the late summer and is composed of larvae originating from oceanic populations. Following recruitment, selection is directed against the Lap 94 allele and is most intense in the fall among individuals <20 mm in shell length. Larger size classes appear relatively immune to selective forces. The duration of selection is coincident with a period of reduced shell growth.