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The genetic variation of the two largest Modiolus capax (Conrad, 1837) populations that occur on the west coast of the Gulf of California was studied by the allozyme analysis of eight polymorphisms of twelve isozyme loci. Genetic variation was examined in about 200 mussels collected from three levels of the intertidal zone and from three sublittoral demes. Two loci (LAP* and GPI*) displayed particularly high allelic diversity (18 and 19 alleles, respectively), many of which were rare (p < 0.1). Heterozygote deficiency was a common feature across all loci, probably explained by the Wahlund effect. The genetic structure of the population was analyzed using Wright's F-statistics, and its significance was assessed by permutation and numerical resampling methods. There was a lower degree of genetic divergence among intertidal levels (FST = 0.042) than among sublittoral demes (FST = 0.080), yet both values were significant. Comparisions between grouped genotype frequencies for each extreme locality (San Felipe vs La Paz) revealed a low but significant level of genetic divergence (FST = 0.049), which suggests genetic flow over vast geographic distances (approximately 1100 km), but slight local genetic divergence due to physical or biological factors acting over the larval dispersal and settlement.
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