Evolution of diet breadth in scale insects is consistent with specialization by drift.

At least half of metazoan species are herbivorous insects. Why are they so diverse? Most herbivorous insects feed on few of the plant species in their environment, and adaptive host specialization is evoked frequently in explanations of their diversification. Nevertheless, it is possible that the narrow host range observed in many herbivorous insects is the non-adaptive result of genetic drift. Here, we perform a comparative phylogenetic analyses of scale insects, a group for which host specialization may be especially non-adaptive. We infer a positive relationship between host range and diversification rate, and a strong asymmetry in cladogenetic changes in diet breadth. These results are consonant with a model of pervasive non-adaptive host specialization in which small, drift- and extinction-prone populations are frequently isolated from persistent and polyphagous source populations. They also contrast with the negative relationship between diet breadth and taxonomic diversification that has been estimated in butterflies. This disparity likely stems from differences in the average costs and benefits of host specificity and host generalism in scale insects and butterflies, but our results indicate the potential for non-adaptive processes to be important for taxonomic diversification across herbivorous insects.

See more of: Ten-Minute Papers, SysEB Section: Insect Evolution
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