The geographical and macro- and microhabitat distribution of and mate recognition among
color morphs of a complex of Lake Victoria cichlids were studied. The distribution of most
morphs was mutually exclusive, but two morphs overlapped extensively and differed only in
microhabitat requirements. Coloration differences between them were inherited, and wild
fishes were homozygous in this respect. Positive morph-assortative female mate choice was
demonstrated. Over the largest part of their common geographical range, intermediate
phenotypes are absent and the two morphs are to be considered two biological species.
However, in populations at one end of the range, intermediate phenotypes with continuously
varying male coloration make up the majority of the individuals. It is suggested that the
status of color morphs as phenotypes of one species or as separate biological species is a
function of environmental conditions that vary considerably between the localities. The data
support the hypothesis that polymorphisms can be a step preceding speciation in cichlid fish.