Species diversity and speciation in the endemic amphipods of Lake Baikal: Molecular evidence

The amphipod diversity in the Siberian Lake Baikal is unique, with some 260 endemic species and 80 additional subspecies recognized so far. Three gene ral patterns of differentiation in molecular data, however, suggest that th is is still a gross underestimate of the actual number of species. Firstly, allozyme analyses regularly indicate a species-level distinction for taxa previously treated as subspecies there corroborated for Micruropus talitroi des,l eurypus, M. wahlii / platycercus and Eulimnogammarus verrucosus / oli gacanthus). Secondly, so far unrecognized (sibling) species are detected ev en sympatrically (e.g., in both the Micruropus complexes above). Thirdly,'c onspecific' samples from different parts of the lake, of several Pallasea s pp., regularly show diagnostic allozyme differences suggesting presence of vicariant sibling. species in the main geological subdivisions of the basin . Extrapolating the observations to the whole of the Baikalian amphipod fau na, a reasonable projection for the total number may be close to a thousand species. Molecular data suggest that the conventional Baikalian lineages are remarka bly old, whereas the vicariant new taxa may have arisen recently in the (ea rly) Pleistocene. These dual levels of diversity are paradoxical in view of the lake's history and the forces supposed to underlie the diversification and speciation processes. The well defined and specialized forms originate d in times when the climate and environments were grossly different from th e present: not as a response to the present kind of environments. On the ot her hand, the divergence that has arisen within the time frame of the envir onmentally modern Lake Baikal (a single basin and cool climate, < 2-3 Myr) appears to be related to geography rather than to adaptive features of morp hology and ecology. The patterns prompt a reconsideration of the role of ge ographical isolation in recent speciation within Lake Baikal.