Sexually active male haplochromine cichlid fishes possess pronounced yellow ovoid spots on the anal fro, which mimic eggs of the female and have therefore been called egg dummies (Wickler, 1962b).
It is thought that divergence in egg dummy characteristics can considerably reduce gene flow and in this way may trigger off reproductive isolation. Two ways in which egg dummy divergence can develop are described. Both mechanisms may have been operating, at the same time and in the same area, in different species, or possibly even in a single species.
Firstly, egg dummy divergence can develop in species with accurately mimetic egg dummies if selection pressures on egg characteristics (size, shape) differ in temporarily isolated demes. As soon as egg characteristics diverge, egg dummy characteristics follow. This type of egg dummy divergence leads to reproductive isolation by mimicry. Mimetic isolation is proposed as a term to describe the mechanism.
Secondly, egg dummy divergence can develop if selection pressures on the egg dummy characteristics are different in two temporarily isolated demes. Upon secondary contact, this divergence may work as a sexual isolation mechanism, since female mate choice is important in haplochromines. Egg dummy divergence is likely to develop rapidly and, if this happens repeatedly, explosive speciation may result. Apparently, explosive speciation has occurred in haplochromines. Neither of the two models has been properly tested. Some preliminary evidence was found that the second model is valid, but so far there is no evidence in favour of the first model.