MICROGEOGRAPHIC GENETIC DIFFERENTIATION IN THE INTERTIDAL ISOPOD JAERA ALBIFRONS LEACH .2. TEMPORAL VARIATION IN ALLELE FREQUENCIES

Genetic differentiation in the intertidal isopod Jaera albifrons has b een shown to occur on a highly localised scale on British rocky shores . Here we examine the temporal persistence of such genetic heterogenei ty by monthly comparison of allozyme frequencies in 20 ''rock-populati ons'' in the South Wales (UK) area over a 2-yr period (December 1990-D ecember 1992). Seasonal changes in Wright's F-ST indicate persistent m icrogeographic genetic differentiation between the sites, with higher degrees of divergence occurring during the winter months. Large fluctu ations in allele frequency in individual rock-populations were observe d 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 sa mpled each month. The temporal persistence of 25 rock-populations was estimated directly by examining the change in the density of animals o ver a 10-day period. Occasional marked and rapid changes indicate that localised extinctions and recolonizations are a relatively common occ urrence in natural populations, and suggests that the previously propo sed two-dimensional stepping stone model of population structure can d isplay metapopulation tendencies. The significance of such a structure in producing population divergence is discussed.