African Cichlid Lake Radiations Recapitulate Riverine Axial Morphologies Through Repeated Exploration of Morphospace

Abstract

Macroevolutionary theory predicts that adaptive radiations are characterised by early bursts of trait evolution followed by rate slowdowns as ecological niches are filled. However, these dynamics are rarely detected within individual clades. Here, we test whether this discrepancy arises from limited temporal sampling by comparing replicated adaptive radiations of African cichlids spanning distinct evolutionary timescales. We show that the net rates of vertebral count evolution are inversely correlated with radiation age, consistent with early burst dynamics. However, we do not detect this pattern within the individual systems. We also find evidence of changes to the underlying processes that generate disparity across the timescale of adaptive radiation, signalled by increasing importance of phylogenetic signal with radiation age. This suggests that early phenotypic expansion is both rapid and phylogenetically unstructured, but that stabilising dynamics emerge on longer timescales, amid declining rates. Despite rapid phenotypic diversification, lacustrine lineages occupy only a subset of the broader morphospace of riverine taxa, suggesting that diversification over short timescales is constrained and may largely resample the variation already present in older lineages. Our results provide strong evidence for shifting evolutionary dynamics through time in adaptive radiations, and that the apparent rarity of early-burst signals in comparative analyses may partly reflect limited statistical power due to narrow temporal sampling.