Rapid parallel morphological and mechanical diversification of South American Pike Cichlids (Crenicichla)

Explosive bouts of diversification are one of the most conspicuous
features of the tree of life. When such bursts are repeated in similar
environments it suggests some degree of predictability in the evolutionary
process. We assess parallel adaptive radiation of South American pike
cichlids (Crenicichla) using phylogenomics and phylogenetic comparative
methods. We find that species flocks in the Uruguay and Iguazú River
basins rapidly diversified into the same set of ecomorphs that reflect
feeding ecology. Both adaptive radiations involve expansion of functional
morphology, resulting in unique phenotypes in terms of jaw shape, size,
and protrusion. Yet, form and function were decoupled such that most
ecomorphs share similar mechanical properties or the jaws (i.e., jaw
motions during feeding). Prey mobility explained six to nine-fold
differences in the rate of morphological evolution, but had no effect on
the rate of mechanical evolution. We find no evidence of gene flow between
species flocks or with surrounding coastal lineages that may explain their
rapid diversification. When compared to cichlids of the East African Great
Lakes and other prominent adaptive radiations, pike cichlids share many
themes, including rapid morphological expansion, specialization along the
benthic-to-pelagic habitat and soft-to-hard prey axes, and the evolution
of conspicuous functional innovations. Yet, decoupled evolution of form
and function and the absence of hybridization as a catalyzing force are
departures from patterns observed in other adaptive radiations.