Different traits dominate evolution at early and late stages of adaptive radiation

Adaptive radiation (AR), a process of rapid speciation and ecomorphological diversification, played an important role in generating past and contemporary global biodiversity. An unsolved question is what maintains high rates of speciation during AR, a phenomenon we call the "speciation paradox." One possible explanation for resolving this paradox is a sequential trait evolution, i.e., a series of ecological diversifications, which enables evolving lineages to fully and more effectively exploit the ecological space. We tested this hypothesis using the highly diverse subterranean amphipod genus Niphargus. Niphargus shows distinct signatures of adaptive radiation both at the genus level and at the level of four larger clades. Our analysis revealed the decoupled evolution of habitat-related traits and trophic-biology-related traits. Moreover, on a genus level, we found evidence that AR commences with a tight association between speciation rates and the dynamics of habitat-related traits. At a later stage, speciation dynamics become associated with the diversification of trophic-biology-related traits. This suggests that the dependence of macroevolutionary rates in this group switches among niche axes before saturation, resulting in prolonged high speciation rates during AR.