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Despite the growing recognition of an important role of ecological speciation in evolution, the mechanisms driving sympatric divergence within a single population are not fully understood. We documented a series of parallel divergence events and tried to assess general regularities of the diversification of the ancestral gene pool. For that we analysed variation at microsatellite loci in populations of a highly polymorphic complex species Salvelinus alpinus from 14 Transbaikalian lakes, 10 of them hosting two or three forms: dwarf, small, and large. Our results suggest sympatric or parapatric origin of forms in all 10 lakes (in one lake, two forms out of three). Gene diversity and allelic richness of form populations are positively correlated with lake size and negatively correlated with modal length of mature fish, so that the dwarf form typically has the highest, and the large form, the lowest indices of genetic variation. The latter effect might be caused by differences in life histories, thus reflecting adaptive divergence. Sympatric forms have differently segregated gene pools ([F.sub.ST] = 0.030-0.497, [R.sub.ST] = 0.011-0.440) and restricted (m = 0.002-0.042) typically asymmetric long-term gene flow. The level of reproductive isolation among forms assessed using putatively neutral microsatellite loci is correlated with their differentiation in morphology, including trophic- related gill raker number, and with lake depth (i.e., with segregation of diets and with habitat diversity), which corresponds to 'isolation-by-adaptation' pattern. Our data suggest that the advance in speciation stage apparently more depends upon ecological opportunities of lake ecosystems than upon their age. Malgre la reconnaissance accrue de l'importance de la speciation ecologique dans l'evolution, la comprehension des mecanismes a l'origine de la divergence sympatrique dans une population donnee demeure incomplete. Nous avons documente une serie d'evenements paralleles de divergence et tente d'evaluer les regularites generales de la diversification d'un fonds genetique ancestral. Pour ce faire, nous avons analyse les variations en des sites microsatellites dans des populations de l'espece complexe et tres polymorphique Salvelinus alpinus de 14 lacs transbaikaliens, dont 10 renfermant deux ou trois des formes suivantes: naine, petite et grande. Nos resultats indiqueraient une origine sympatrique ou parapatrique des formes dans ces 10 lacs (de deux formes sur trois dans l'un d'eux). La diversite genetique et la richesse allelique des populations de forme sont positivement correlees a la taille du lac et negativement correlees a la longueur modale des poissons matures, la forme naine presentant typiquement les indices les plus eleves de variation genetique et la grande forme, les indices les plus faibles. Ce second effet pourrait decouler de differences sur le plan des cycles biologiques, refletant ainsi une divergence adaptative. Les formes sympatriques presentent des fonds genetiques segreges differemment ([F.sub.ST] = 0,030-0,497, [R.sub.ST] = 0,011-0,440) et un flux genetique a long terme limite (m = 0,002-0,042) typiquement asymetrique. Le degre d'isolement reproductif entre les formes evalue a l'aide de microsatellites presumes neutres est correle a leur differenciation morphologique, dont le nombre de branchictenies associe au niveau trophique, et a la profondeur du lac (c'est-a-dire a la segregation des regimes alimentaires et a la diversite de l'habitat), ce qui correspond au motif <>. Nos donnees donnent a penser que le stade de speciation depend plus des possibilites ecologiques qu'offrent les ecosystemes lacustres que de l'age. [Traduit par la Redaction], Introduction Ecological speciation implies evolution of reproductive isolation between populations as a result of ecologically based divergent natural selection (Schluter 1996, 2000; Rundle and Nosil 2005; Hendry 2009). Sympatric adaptive [...]