African cichlid fishes have
undergone outbursts of explosive speciation in several lakes, accompanied
by rapid radiations in coloration and ecology. Little is known about the
evolutionary forces that triggered these events but a hypothesis,
published by Wallace Dominey in 1984, has figured prominently. It states
that the evolution of colour patterns is driven by sexual selection and
that these colour patterns are important in interspecific mate choice, a
combination which holds the potential for rapid speciation. Here we
present phylogenetic analyses that describe major events in colour
evolution and test predictions yielded by Dominey's hypothesis. We
assembled information on stripe patterns and the presence or absence of
nuptial coloration from more than 700 cichlid species representing more
than 90 taxa for which molecular phylogenetic hypotheses were available.
We show that sexual selection is most likely the selection force that
made male nuptial coloration arise and evolve quickly. In contrast,
stripe patterns, though phylogenetically not conserved either, are
constrained ecologically. The evolution of vertical bar patterns is
associated with structurally complex habitats, such as rocky substrates
or vegetation. The evolution of a horizontal stripe is associated with
a piscivorous feeding mode. Horizontal stripes are also associated with
shoaling behaviour. Strength of sexual selection, measured in terms of
the mating system (weak in monogamous, strong in promiscuous species),
has no detectable effects on stripe pattern evolution. In promiscuous
species the frequency of difference between sister species in nuptial hue
is higher than in pair bonding and harem forming species, but the
frequency of difference in stripe pattern is lower. We argue that
differences between the two components of coloration in their exposure to
natural selection explain their very different evolutionary behaviour.
Finally, we suggest that habitat-mediated selection upon chromo-motor
flexibility, a special form of phenotypic plasticity found in the
river-dwelling outgroups of the lake-dwelling cichlids, explains the
rapid and recurrent ecology-associated radiation of stripe patterns in
lake environments, a new hypothesis that yields experimentally testable
predictions.