Relaxed trait covariance in interspecific cichlid hybrids predicts morphological diversity in adaptive radiations

The process of adaptive radiation involves multiple events of speciation in
short succession, associated with ecological diversification. Understanding
this process requires identifying the origins of heritable phenotypic variation
that allows adaptive radiation to progress. Hybridization is one source of
genetic and morphological variation that may spur adaptive radiation. We
experimentally explored the potential role of hybridization in facilitating the
onset of adaptive radiation. We generated first- and second-generation
hybrids of four species of African cichlid fish, extant relatives of the putative
ancestors of the adaptive radiations of Lakes Victoria and Malawi. We com-
pared patterns in hybrid morphological variation with the variation in the
lake radiations. We show that significant fractions of the interspecific mor-
phological variation and the major trajectories in morphospace that charac-
terize whole radiations can be generated in second-generation hybrids.
Furthermore, we show that covariation between traits is relaxed in second-
generation hybrids, which may facilitate adaptive diversification. These
results support the idea that hybridization can provide the heritable pheno-
typic diversity necessary to initiate adaptive radiation.