Microgeographic genetic differentiation in the intertidal isopod Jaera albifrons Leach. II. Temporal variation in allele frequencies

Abstract

Genetic differentiation in the intertidal isopod Jaera albifrons has been shown to occur on a highly localised scale on British rocky shores. Here we examine the temporal persistence of such genetic heterogeneity by monthly comparison of allozyme frequencies in 20 “rock-populations” in the South Wales (UK) area over a 2-yr period (December 1990–December 1992). Seasonal changes in Wright's F ST indicate persistent microgeographic genetic differentiation between the sites, with higher degrees of divergence occurring during the winter months. Large fluctuations in allele frequency in individual rock-populations were observed between months and deemed too large to be attributable to selective forces or random genetic drift. It is proposed that the instability of rock-populations results in different assemblages of animals being sampled each month. The temporal persistence of 25 rock-populations was estimated directly by examining the change in the density of animals over a 10-day period. Occasional marked and rapid changes indicate that localised extinctions and recolonizations are a relatively common occurrence in natural populations, and suggests that the previously proposed two-dimensional stepping stone model of population structure can display metapopulation tendencies. The significance of such a structure in producing population divergence is discussed.