Convergent evolution is a central topic in comparative biology because it is often taken as evidence of adaptation by natural selection. Similar attributes in unrelated groups reflect adaptative responses to similar environmental pressures, even though the initial ancestors states were different. Convergent evolution has been documented in many cases, including classic examples involving morphological, ecological and behavioral similarities between placental and marsupial ‘wolves’, the similar wing shape and size of bats and birds, or the morphological similarity between the cacti of the Americas and spurges and milkweeds of Africa. Because the central prediction of convergence is similar organismal structure and function in similar environmental contexts, it is crucial to evaluate the similarity of the environment just as carefully as organismal similarity.
Most convergence studies have focused on biological attributes as opposed to environmental ones, including morphology, community structure, physiology and species diversity. All these studies have provided useful information to understand convergence, but because they included only qualitative descriptions of the environment or coarse climatic measurements, they have left untested the crucial assumption of similar environmental pressures. Newly available climate data allow examination of the environmental aspect of convergence predictions. Convergence in climatic conditions can be interpreted as overlap in environmental space; in contrast, divergence can be interpreted as different areas in environmental space.
To test the hypothesis of convergence in environmental requirements quantitatively, a recent paper in Annals of Botany examines the classic example of apparent convergent evolution between the succulent plants of the American arid regions, the cacti, and their distantly related African analogues, the milkweeds (Apocynaceae), spurges (Euphorbiaceae) and ice-plants (Aizoaceae). This case has illustrated convergent evolution in countless publications for over 100 years. Comparing environmental similarity using niche modelling tools, randomization tests of niche similarity and multivariate analyses for 19 bioclimatic variables, the authors find that although the sites selected have ‘similar’ but unrelated life forms, almost all the results highlight more climate differences than similarities between the hotspots. A coarser perspective shows that the sites are similar as drylands with relatively moderate drought and mild temperatures, but the results highlight the potentially objective nature of ascribing ‘similarity’ in such studies.
To converge or not to converge in environmental space: testing for similar environments between analogous succulent plants of North America and Africa. Annals of Botany (2013) 111 (6): 1125-1138. doi: 10.1093/aob/mct078
Convergent evolution is invoked to explain similarity between unrelated organisms in similar environments, but most evaluations of convergence analyse similarity of organismal attributes rather than of the environment. This study focuses on the globular succulent plants of the Americas, the cacti, and their counterparts in Africa in the ice-plant, spurge and milkweed families. Though often held up as paragons of convergent morphological evolution, the environmental similarity of these plants has remained largely unexamined from a quantitative perspective. Five hotspots (centres of high species diversity of globular succulents) were selected, two in Mexico and three in South Africa. Their environments were compared using niche modelling tools, randomization tests of niche similarity and multivariate analyses to test for environmental similarity. Although the sites selected have ‘similar’ but unrelated life forms, almost all our results highlighted more climate differences than similarities between the hotspots. Interprediction of niches within and between continents, a niche equivalence test, and MANOVA results showed significant differences. In contrast, a niche similarity test showed that the comparisons of Cuatrociénegas–Richtersveld, Huizache–Knersvlakte and Huizache–Richtersveld were similar. Differences in rainfall and temperature regimes and the potential effect of edaphic factors may be involved in the differences between the hotspots. In addition, differences in structure, morphology and physiology of the globular succulents may coincide with some of the climatic dissimilarities; i.e. given convergence as the evolution of similar morphologies under similar conditions, then it may be that differing environments diagnose inconspicuous morphological differences. Moreover, although fine-scale differences between sites were found, a coarser perspective shows that these sites are clearly similar as drylands with relatively moderate drought and mild temperatures, illustrating how all studies of convergence must address the issue of how similar two entities must be before they are considered convergent.