For years, evolutionary biologists have debated the predictability of evolution. Stephen Jay Gould famously said that if the tape of life could be rewound to the same starting point, it would replay with a very different outcome. On the other hand, many evolutionary biologists have pointed to the ubiquity of adaptive convergent evolution–when species facing similar environmental pressures evolve in highly similar ways–as evidence that evolution is deterministic.
A particularly interesting example of repeated convergent evolution occurs when two groups evolving in different places diversify to produce similar sets of descendant species. The existence of such “replicated adaptive radiations” would seem to be strong evidence for evolutionary determinism. For example, the cichlid fishes of the African Rift lakes have become an iconic textbook example, illustrated by diagrams matching up pairs of species from two lakes that are morphologically convergent (see figure on right). The problem is that there are hundreds of fish species in each lake—the example illustrates a number of cases of convergent evolution between the lakes, but does not demonstrate that the lake radiations themselves are overall more similar than one might expect; another possibility is that there are some cases of convergence embedded in a larger pool of non-convergent evolution.
Another classic case of replicated adaptive radiation is the lizards of the genus Anolis, which have diversified mostly independently across the four islands of the Greater Antilles (Cuba, Hispaniola, Jamaica, and Puerto Rico). As with the cichlids, sets of similar species have evolved convergently on each island; for example, short-legged, camouflaged species have evolved on each island to adapt to living on twigs, and green species with large toepads have evolved to live in the canopy. But also as with the cichlids, some types are ecologically and morphologically unique to a single island; for example, Hispaniola has an anatomically distinctive leaf litter-dwelling species that has no counterpart on the other islands.
Given the considerable amount of research on the evolution of these lizards, including a well worked out evolutionary tree, they would seem to be a perfect group in which to investigate whether entire radiations are truly replicated. Luke Mahler of the University of California at Davis and colleagues (including me!) have done just that in a paper published in this week’s issue of Science. By taking anatomical measurements on the species on the four islands, the paper demonstrates that, although the set of species that have evolved on each island is not identical, they are very similar, much more so than would be expected by chance. In other words, despite evolutionarily radiating independently on each island, the outcome has been deterministic—species diversifying independently in similar environments do, indeed, produce highly similar evolutionary outcomes.
But why does this occur? Years ago, the great paleontologist George Gaylord Simpson proposed what is now called the “macroevolutionary landscape.” Analogous to a topology map, Simpson suggested that there are “adaptive peaks,” particular combinations of features that natural selection would favor. A corollary of this idea is that similar environments should have similar adaptive peaks, and thus should lead to evolutionarily similar radiations. The idea, though intuitive and widely-discussed, has never previously been tested in the context of replicated adaptive radiations.
Using a powerful new analytical method, Mahler et al. showed that, indeed, the adaptive landscapes have been highly similar for the diversifying island groups. Not only does this study demonstrate that similar evolutionary diversification is the result of similar underlying natural selection pressures, but, given the antiquity of these lizards (which began to radiate more than 40 million years ago), it also reveals that evolutionary pressures can remain relatively constant over long periods of time. Ironically, even while disproving the idea that replays of the same evolutionary tape will not produce similar outcomes, this study finds support for another of Stephen Jay Gould’s ideas, the existence of long term evolutionary stasis.