In a provocative new Bioessays article, Olson and Arroyo-Santos attempt to kill the term "adaptive radation." Olson and Arroyo-Santos are certainly correct when they suggest that interest in adaptive radiation is paradoxical: although it is almost universally regarded as an important (if not the most important) mode of biological diversification, evolutionary biologists have argued for decades about how to diagnose and define it. Olson and Arroyo-Santos suggest that this century-old debate stems fundamentally from the fact that "adaptive radiation cannot be winnowed to any core meaning, because there is no phenomenon in nature to which the term corresponds, simply arbitrary divisions of continua."
The problem with this argument is that it focuses exclusively on the one feature of adaptive radiation that is controversial - namely, whether extraordinary diversification is intrinsic to adaptive radiation. While I agree with Olson and Arroyo-Santos's suggestion that efforts to define adaptive radiation on the basis of the extraordinary levels of diversity are hopelessly ambiguous and arbitrary, I disagree with the fundamental premise that extraordinary diversification, in any form, is intrinsic to adaptive radiation. If we focus on the shared features of modern definitions of adaptive radiation, we find that it can be universally defined as a response to natural selection and ecological opportunity involving divergence of species and associated adaptive features. In this sense, adaptive radiation is analogous to the ‘principle of divergence’ that Darwin introduced in the Origin by suggesting that “the more diversified the descendants from any one species become in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers.” Perhaps we should replace 'adaptive radiation' with 'principle of divergence,' but abandoning efforts to label the profoundly important evolutionary phenomenon that underlies these terms seems like a bad idea to me.