The patterns of constrained evolution suggest a non-adaptive radiation.
There is more to evolution than adaptation. This message was conveyed by Stephen Jay Gould and Richard Lewontin in their 1979 paper with the wonderful title “The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme.” In this paper, they argued that evolutionary thought has been dominated by the idea that organisms can be broken up into separate traits that are driven to an optimum by natural selection. Researchers would tell an “evolutionary story” to describe the most likely trajectory for a particular adaptation. Gould and Lewontin criticized this approach and proposed an alternative perspective that focuses on non-adaptive processes. Organisms should be analyzed as integrated wholes, with a bauplan that is constrained by phylogenetic history, developmental pathways, and general architecture. Some traits are not the optimal outcome of natural selection, but rather the byproduct of constrained, non-adaptive processes.
A similar discussion can be applied to the evolution of species-rich groups, such as island radiations. An often-heard explanation is that an ancestral population arrived on the island and diversified into several species that each adapted to a particular ecological niche. A well-studied case that immediately comes to mind is the Darwin’s Finches, a textbook example of an adaptive radiation. But this reasoning cannot automatically be applied to other radiations on islands on or the mainland. There might also be examples of non-adaptive radiations.
A recent study in the Journal of Biogeography took a closer look at the White-eyes (genus Zosterops). These small songbirds have been called “the Great Speciator” because they have diversified into more than 100 species in the last two million years. But are they also an example of an adaptive radiation? To answer this question, Julia Day and her colleagues performed a morphological analysis of 120 Afrotropical species.
The analyses revealed a striking difference between mainland and island species. On the mainland, morphological evolution seems to be constrained, leading to convergence on certain phenotypes. In particular, White-eyes repeatedly evolve into highland or lowland forms. This pattern suggests that mainland White-eyes are “stuck” in an adaptive landscape with two optima. This constrained evolution can be due to the general morphology of these birds which does not allow for the evolutionary exploration of other phenotypes, or the lack of available niches due to competition with other species.
The situation on islands is slightly different. Here, different White-eye species have evolved novel phenotypes. The authors suspect that the evolution of different morphologies in island species might be due to less interspecific competition, allowing the birds to explore new ecological niches. However, the expansion of morphospace is still limited around the general bauplan of a typical White-eye, indicating that certain phylogenetic or developmental constraints might be at play here. Based on these patterns, the researchers concluded that “Given the apparent lack of ecological diversification, and limited insular diversification in Zosterops, the general pattern observed in this group may be explained by geographical speciation involving non-adaptive radiation.”
Day, J. J., Martins, F. C., Tobias, J. A., & Murrell, D. J. (2020). Contrasting trajectories of morphological diversification on continents and islands in the Afrotropical white‐eye radiation. Journal of Biogeography, 47(10), 2235-2247.
Featured image: Cape white-eye (Zosterops pallidus) © Lip Kee | Wikimedia Commons