Abstract
Enzyme gene variation was studied in 22 Finnish Gerris lacustris (L.) (Heteroptera) populations. Four populations represent the northern distribution margin of the species, and these populations are relatively isolated from each other. The other populations come from an area where the species is widespread and abundant, and gene flow between the populations may occur. Average heterozygosity showed little dependence on the degree of isolation of the population site. Isolation correlated clearly with population differentiation, measured with the apportionment of gene diversity into between- and within-population components in different areas, with the degree of gene differentiation, and with the standardized variances of allele frequencies. On the basis of genetic distances, populations near each other tended to be more similar than populations which are far apart. Random genetic drift is a sufficient explanation for the overall pattern of variation, but the results do not rule out the possibility of spatially differing selection pressures.
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