Mate choice may play an important role in animal speciation. The haplochromine cichlids
of Lake Victoria are suitable to test this hypothesis. Diversity in ecology, coloration
and anatomy evolved in these fish faster than postzygotic barriers to gene flow, and little
is known about how this diversity is maintained. It was tested whether recognizable forms
are selection-maintained morphs or reproductively isolated species by investigating in the
field reproductive timing, location of spawning sites, and mate choice behaviour. There
was a large interspecific overlap in timing of breeding and location of spawning sites,
which was largest in members of the same genus. Behavioural mate choice of such closely
related taxa was highly assortative, such that it is likely that they are sexually isolated
species and that direct mate choice is the major force that directs gene flow and maintains
form diversity. The results differ from what is known about recent radiations of other
lacustrine fish groups where speciation seems to be driven by diverging microhabitat
preferences or diverging timing of reproduction, but are in agreement with predictions
from models of speciation by diverging mate preferences.