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image of Crustaceana

The amphipod diversity in the Siberian Lake Baikal is unique, with some 260 endemic species and 80 additional subspecies recognized so far. Three general patterns of differentiation in molecular data, however, suggest that this 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 (here corroborated for Micruropus talitroides / eurypus, M. wahlii / platycercus and Eulimnogammarus verrucosus / oligacanthus). Secondly, so far unrecognized (sibling) species are detected even sympatrically (e.g., in both the Micruropus complexes above). Thirdly, 'conspecific' samples from different parts of the lake, of several Pallasea spp., 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 fauna, a reasonable projection for the total number may be close to a thousand species. Molecular data suggest that the conventional Baikalian lineages are remarkably old, whereas the vicariant new taxa may have arisen recently in the (early) 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 originated in times when the climate and environments were grossly different from the present: not as a response to the present kind of environments. On the other hand, the divergence that has arisen within the time frame of the environmentally modern Lake Baikal (a single basin and cool climate, < 2-3 Myr) appears to be related to geography rather than to adaptive features of morphology and ecology. The patterns prompt a reconsideration of the role of geographical isolation in recent speciation within Lake Baikal.

La diversite des amphipodes du lac Baikal est unique, avec quelque 260 especes endemiques et 80 sous-especes supplementaires reconnues jusqu'a present. Cependant, trois schemas generaux de differenciation provenant des donnees moleculaires, suggerent que ceci est encore une sousestimation flagrante du nombre des especes actuelles. Premierement, les analyses d'alloenzymes indiquent regulierement une separation de niveau specifique pour des taxa auparavant traites comme des sous-especes (ici confirme pour Micruropus talitroides eurypus, M. wahlii platycercus et Eulimnogammarus verrucosus oligacanthus). Deuxiemement, des especes (jumelles) non reconnues jusqu'ici sont detectees meme sympatriquement (par exemple, dans les deux complexes de Micruropus ci-dessus). Troisiemement, des echantillons ''conspecifiques'' de differentes regions du lac, de plusieurs Pallasea spp., montrent regulierement des differences diagnostiques d'alloenzymes suggerant la presence d'especes jumelles vicariantes dans les principales subdivisions g eologiques du bassin. En extrapolant les observations a l'ensemble de la faune d'amphipodes du Baikal, une projection raisonnable du nombre total peut etre proche d'un millier d'especes. Les donnees moleculaires suggerent que les lignees ''baikaliennes'' conventionnelles sont remarquablement anciennes, tandis que les nouveaux taxa vicariants peuvent avoir apparu recemment au Pleistocene. Ces niveaux de diversite doubles sont paradoxaux par rapport a l'histoire du lac et aux forces supposees sous-tendre les processus de diversification et de sp eciation. Les formes bien definies et specialisees sont apparues en des temps ou le climat et les environnements etaient tres differents de ceux du present: non comme une reponse au type actuel d'environnement. D'autre part, la divergence survenue au cours de la periode environnementale moderne du lac Baikal (un bassin unique et un climat frais, < 2-3 millions d'annees) semble en relation avec la geographie plutot qu'a des caracteres adaptatifs de morphologie et d'ecologie. Les modeles incitent a reconsideration du role de l'isolement geographique dans la speciation recente dans le lac Baikal.


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