Variation of allozyme loci in endemic gammarids of Lake Baikal

The outstandingly rich and highly endemic fauna of Lake Baikal includes more than 200 species of gammarids (Crustacea, Amphipoda), most of which occur only in Baikal. Allelic variation at 23 enzyme loci has been studied for the first time in four species of Baikalian gammarids. The levels of polymorphism are similar to those observed in other invertebrates, including gammarids: c. 20–25% of the loci are polymorphic with 2.53 to 3.3 alleles and mean heterozygosity of 0.048 to 0.105 per polymorphic locus. The genotype frequences in local populations in most cases markedly deviate from Hardy‐Weinberg equilibrium with a deficiency of heterozygotes. Populations were shown to be strongly subdivided even over a range of less than 100 m. On the larger scale genetic differences among the populations do not show any strong correlation with geographical separation. Genetic distances among species were found to be rather low: from 0.159 between closely related, but morphologically clearly distinct species of the genus Spinacanthus to 0.546 between the representatives of different genera. We discuss these patterns of population structure with respect to the problem of the origin of endemic faunas in ancient lakes. Our results suggest that speciation occurred rapidly and relatively recently. Genetic distances within and among species support models of speciation based on the founder effect or differentiation of a peripheral population rather than on divergence of large subpopulations. The rate of morphological evolution in Baikalian gammarids is very high compared with their biochemical evolution.