The macro patterns of adaptive radiations
A fundamental challenge to our understanding of biodiversity is to explain why some groups of species undergo adaptive radiations, diversifying extensively into many and varied species, whereas others do not. Both extrinsic environmental factors (for example, resource availability, climate) and intrinsic lineage-specific traits (for example, behavioural or morphological traits, genetic architecture) influence diversification, but few studies have addressed how such factors interact. Radiations of cichlid fishes in the African Great Lakes provide some of the most dramatic cases of species diversification. However, most cichlid lineages in African lakes have not undergone adaptive radiations. We have compiled data on cichlid colonization and diversification in 46 African lakes, along with lake environmental features and information about the traits of colonizing cichlid lineages, to investigate why adaptive radiation does and does not occur. We find that extrinsic environmental factors related to ecological opportunity and intrinsic lineage-specific traits related to sexual selection both strongly influence whether cichlids radiate.
Cichlids are more likely to radiate in deep lakes, in regions with more incident solar radiation and in lakes where there has been more time for diversification. Among the suite of intrinsic traits that we investigate, sexual dichromatism, a surrogate for the intensity of sexual selection, is consistently positively associated with diversification. Thus, for cichlids, it is the coincidence between ecological opportunity and sexual selection that best predicts whether adaptive radiation will occur. These findings suggest in order to predict adaptive radiation, species traits and environmental factors have to be considered jointly. We are currently extending this approach to other fish radiations. Adaptive radiation’s hallmark is exceptional species richness within geographically restricted clades. Although understanding the factors influencing geographical patterns of species richness is a fundamental goal of ecological research, adaptive radiations have rarely been studied from macroecological perspectives.
Decades of ecological research have uncovered strong and predictable relationships between ecological variables (e.g. area, productivity) and species richness. These same variables may influence processes of species origination and the occurrence and richness of adaptive radiations. For example, habitat size may influence in situ speciation and adaptive radiation.
Adaptive radiations are often characterized by slowdowns in diversification rate through time, assumed to derive from niche-filling processes. If adaptive radiations have reached this slowdown phase, there should be clear relationships between species richness and environmental variables related to resource availability, if these variables limit species richness. However, rarely have such environmental variables been identified for adaptive radiations. We investigate these hypotheses using our data set for 48 African lakes. Recently we discovered that within-lake adaptive radiation strongly modifies the species-area relationship for African cichlid fishes. The total number of species derived from in situ speciation increases with lake size, resulting in faunas orders of magnitude higher in species richness than faunas assembled by immigration alone. We found that diversity of clades representing within-lake radiations show responses to lake area, depth and energy consistent with limitation by these factors, suggesting that ecological factors influence the species richness of radiating clades within these ecosystems.