50 years of inordinate fondness

When theologians inquired whether anything could be concluded about the creator from studying the creation, evolutionary biologist J.B.S. Haldane allegedly replied “an inordinate fondness for beetles”, as there are more kinds of beetles than any other insect. We may similarly conclude that the creator is not particularly fond of the two species of colugos (that are sister to more than 350 primate species (Bininda-Emonds et al. 2007)⁠), or the osprey, which is the single sister species to more than a hundred hawks and eagles (Sibley and Ahlquist 1990; Sibley and Monroe 1990)⁠. Of course, few evolutionary biologists would endorse the view that numbers of species reflect the interest of a divine creator, but there is no consensus on an alternative explanation. We can distinguish three broad categories of explanations for distributions of species over taxa: (i) random chance, (ii) ecological opportunity, and (iii) key adaptations of species. All three explanations have garnered substantial support, but none accurately explains distributions of species over taxa by itself. Here we explain how these different explanations can be conceptually combined in a theory that yields accurate predictions of distributions of species over taxa.