American Society of Naturalists

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“Speciation rate is independent of the rate of evolution of morphological size, shape, and absolute morphological specialization in a large clade of birds”

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Nicholas Crouch and Robert Ricklefs (Apr 2019)

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Biodiversity may be limited by division of resources and not just ecological opportunity

A basic issue in evolutionary biology is the relationship between the rate of species formation and the rate of morphological diversification. We expect these rates to be correlated because conditions that favor an increase in one commonly accelerates the other. For example, a population that colonizes an island lacking competing species may diversify into new ecological roles while splitting into multiple species. Alternatively, rates of speciation and ecological diversification might not be correlated if descendant species become increasingly ecologically specialized in response to growing competition in diverse communities. In this study, researchers gathered morphological data from over 2000 species from 11 orders of predominantly arboreal birds to test whether speciation and morphological diversification are generally correlated. They found that the size, shape, and morphological specialization are unrelated to rate of diversification. They suggest that this is because most of the variation in the group arose early in their evolutionary history as the group diversified following the end-Cretaceous mass extinction. Subsequent species formation has predominantly been via geographic isolation and sexual selection, process that need not generate new ecologically-related morphological variation. This study contributes to our understanding of biodiversity. Instead of being constrained only by the ecological resources available to support species, diversity may also be limited by the ability or propensity of species to divide up resources.


Whether ecological differences between species evolve in parallel with lineage diversification is a fundamental issue in evolutionary biology. These processes might be connected if conditions that favor the proliferation of species, such as release from competitors, facilitate the evolution of novel ecological relationships. Despite this, phylogenetic studies do not consistently identify such a connection. Conversely, if higher diversity caused species to become increasingly specialized ecologically, lineage diversification might become dissociated from ecological diversification. In this analysis, we ask whether the rate of lineage diversification in a large clade of birds is correlated with morphological specialization and with rates of morphological evolution. We find that morphological variation is related to species richness within clades, but that rates of morphological evolution are decoupled from the rate of lineage diversification. Additionally, morphological specialization within lineages is independent of the rate at which lineages diversify, with the results apparently robust against false negative inference. This dissociation is likely a consequence of the major ecomorphological differences between avian clades arising early in their evolutionary history, with comparatively little variation added subsequently, while avian diversification has been driven predominantly by geographic isolation and sexual selection. Accordingly, biodiversity appears to limited by the extent to which taxa can subdivide exploited regions of ecological space, and not just overall ecological opportunity.