Fossil evidence for trait diversification in an adaptive radiation.

Adaptive radiation is an important process for the origin of functional and ecological biodiversity. Understanding how, when, and why adaptive radiations occur is a long-standing interest in evolutionary ecology. Although many adaptive radiations have been studied, few studies resolved the temporal sequence of events during adaptive radiation. Here, we assembled a continuous record of tooth fossils of Lake Victoria’s haplochromine cichlid fish, the most rapid and youngest of the classical adaptive radiations, from sediment cores extending from lake refilling ~ 17 thousand years ago to the present. We use these fossil records to reconstruct ecomorphological temporal patterns in the unfolding of this adaptive radiation. Our results reveal a rapid expansion in morphospace, from an undiverse ancestral condition, within the first three millennia after the onset of the modern lake. Comparison with modern cichlid teeth suggests that large-scale diversification across the food web emerged within these first three millennia. We detect a clear signal of an evolutionary trend from trophic generalists to specialists, but we also show that generalists persisted amid the growing radiation of specialists. Altogether, this pattern confirms the unusual evolutionary potential of the Lake Victoria hybrid lineage of haplochromine cichlids that seeded the radiation and the unusual speed with which the adaptive radiation occurred.