It is not sufficiently understood why some lineages of cichlid fishes have proliferated
in the Great Lakes of East Africa much more than anywhere else in the world, and much faster
than other cichlid lineages or any other group of freshwater fish. Recent field and
experimental work on Lake Victoria haplochromines suggests that mate choice-mediated
disruptive sexual selection on coloration, that can cause speciation even in the absence
of geographical isolation, may explain it. We summarize the evidence and propose a hypothesis
for the genetics of coloration that may help understand the phenomenon. By defining colour
patterns by hue and arrangement of hues on the body, we could assign almost all observed
phenotypes of Lake Victoria cichlids to one of three female ("plain", "orange blotched",
"black and white") and three male ("blue", "red-ventrum", "red-dorsum") colour patterns.
These patterns diagnose species but frequently co-occur also as morphs within the same
population, where they are associated with variation in mate preferences, and appear to
be transient stages in speciation. Particularly the male patterns occur in almost every
genus of the species flock. We propose that the patterns and their association into
polymorphisms express an ancestral trait that is retained across speciation. Our model
for male colour pattern assumes two structural loci. When both are switched off, the
body is blue. When switched on by a cascade of polymorphic regulatory genes, one
expresses a yellow to red ventrum, the other one a yellow to red dorsum. The expression
of colour variation initiates speciation. The blue daughter species will inherit the
variation at the regulatory genes that can, without new mutational events, purely by
recombination, again expose the colour polymorphism, starting the process anew. Very
similar colour patterns also dominate among the Mbuna of Lake Malawi. In contrast,
similar colour polymorphisms do not exist in the lineages that have not proliferated
in the Great Lakes. The colour pattern polymorphism may be an ancient trait in the
lineage (or lineages) that gave rise to the two large haplochromine radiations. We
propose two tests of our hypothesis.