Evolution of body shape in sympatric versus non-sympatric Tropheus populations of Lake Tanganyika

Allopatric speciation often yields ecologically equivalent sister species, so that theirsecondary admixis enforces competition. The shores of Lake Tanganyika harbor about 120distinct populations of the cichlid genus Tropheus, but only some are sympatric.When alone, Tropheus occupies a relatively broad depth zone, but in sympatry,fish segregate by depth. To assess the effects of competition, we studied the partialco-occurrence of Tropheus moorii ‘Kaiser'' and‘Kirschfleck'' with Tropheus polli. A previous study demonstrated viastandardized breeding experiments that some observed differences between Tropheus‘Kaiser'' living alone and in sympatry with T. polli have a geneticbasis despite large-scale phenotypic plasticity. Using geometric morphometrics and neutralgenetic markers, we now investigated whether sympatric populations differ consistently inbody shape from populations living alone and if the differences are adaptive. We foundsignificant differences in mean shape between non-sympatric and sympatric populations,whereas all sympatric populations of both color morphs clustered together in shape space.Sympatric populations had a relatively smaller head, smaller eyes and a more anteriorinsertion of the pectoral fin than non-sympatric populations. Genetically, however,non-sympatric and sympatric ‘Kaiser'' populations clustered together to theexclusion of ‘Kirschfleck''. Genetic distances, but not morphologicaldistances, were correlated with geographic distances. Within- and between-populationcovariance matrices for T. moorii populations deviated from proportionality. Itis thus likely that natural selection acts on both phenotypic plasticity and heritabletraits and that both factors contribute to the observed shape differences. The consistencyof the pattern in five populations suggests ecological character displacement.