Evolutionary lability of a key innovation spurs rapid diversification

Rates of lineage diversification vary considerably across the tree of life, often as a result of evolutionary innovations^1-5. Although the ability to produce new traits can vary between clades and may drive ecological transitions^6-9, the impact of differences in the pace at which innovations evolve at macroevolutionary scales has been overlooked. Complex teeth are one innovation that contributed to the evolutionary success of major vertebrate lineages^10-12. Here we show that evolutionary lability of tooth complexity, but not complexity itself, spurs rapid diversification across ray-finned fishes. Speciation rates are five times higher when transitions between simple and complex teeth occur rapidly. We find that African cichlids are unique among all fishes; they are dominated by lineages that transition between simple and complex teeth at unparalleled rates. This innovation interacted with the ecological versatility of complex teeth to spur rapid adaptive radiations in lakes Malawi, Victoria and Barombi Mbo. The marked effect on diversification stems from the tight association of tooth complexity with microhabitat and diet. Our results show that phylogenetic variation in how innovations evolve can have a stronger effect on patterns of diversification than the innovation itself. Investigating the impact of innovations from this new perspective will probably implicate more traits in causing heterogeneous diversification rates across the tree of life.