Phenotypic rate and state are decoupled in response to river-to-lake transitions in cichlid fishes

Geographic access to isolated ecosystems is an important catalyst of adaptive radiation. Cichlid fishes repeatedly colonized rift, crater, and volcanic lakes from surrounding rivers. We test the lake effect on the phenotypic rate and state across 253 cichlid species. The rate of evolution was consistently higher (similar to 10-fold) in lakes, and consistent across different dimensions of the phenotype. Rate shifts tended to occur coincident with or immediately following river-to-lake transitions, generally resulting in 2- to 5-fold faster rates than in the founding riverine lineage. By contrast, river- and lake-dwelling cichlids exhibit considerable overlap in phenotypes, generally with less disparity in lakes, but often different evolutionary optima. Taken together, these results suggest that lake radiations rapidly expand into niches largely already represented by ancestral riverine lineages, albeit in different frequencies. Lakes may provide ecological opportunity via ecological release (e.g., from predators/competitors) but need not be coupled with access to novel ecological niches.

Automated summary

Geographic access to isolated ecosystems plays a crucial role in adaptive radiation, as demonstrated by the colonization of various lakes by cichlid fishes. Our study examined the phenotypic rate and state across 253 cichlid species in lakes, showing consistently higher rates of evolution compared to riverine lineages. While overlap in phenotypes was observed between river and lake-dwelling cichlids, there were often different evolutionary optima in lakes. These findings suggest that lake radiations mostly occupy niches already present in ancestral riverine lineages rather than accessing novel ecological niches.