Monday, July 22, 2013

"Lizards rewind the evolutionary clock but end up the same every time"

Interesting, from Ars Technica:
There’s no doubt that we’ve come a long way in our understanding of evolution since Darwin's time. However, there's still a lot we don’t understand about the processes of natural selection, adaptation, and speciation. One question in particular still looms over the field: how predictable is evolution?

On one side, biologists such as Stephen Gould have long argued that evolution is “utterly unpredictable and quite unrepeatable” due to all sorts of historical accidents and perturbations. "Wind back the tape of life," he wrote, and "the chance becomes vanishingly small" that evolution would proceed in precisely the same way, creating the same set of species. However, in recent years, more and more research has suggested that evolution may actually be quite predictable. New research in the journal Science strengthens this argument by identifying a group of islands where Gould's tape has been wound back multiple times, and evolution has created a near-carbon copy almost every time.

The Greater Antilles are chock full of small lizards called anoles; there are anoles that specialize on twigs, on grasses, and on various parts of trees. Lizards on each island exploit these same niches, and across the entire island chain, anoles that use a particular niche look nearly identical. Grass specialists in Cuba are dead-ringers for those in Jamaica, and anoles that specialize on tree trunks in Puerto Rico could be mistaken for those in Hispaniola or Cuba. The kicker is that each of these islands was colonized separately by just one species of anole. In the forty million years since then, anoles on each island have independently adapted in near-identical ways, right down to the shapes of their tails and the stripes on their sides.

Using various modeling techniques, a group of researchers have shown that this is not a lucky coincidence; instead, evolution may be surprisingly deterministic.

The scientists measured various attributes of 100 lizard species across the Greater Antilles, including measurements of their body size, tail length, limb length, and how many sticky toe-pads they have. After plugging these values into two different models—a Bayesian model and a four-dimensional principal components analysis—the results were clear: species on each island were more similar to those on other islands than would be expected by chance.

But has the whole radiation really repeated itself on each island? To tackle this question, the researchers modeled the “adaptive landscape” of each island, or the various configurations of traits that determine each species’ fitness. They found that there were specific combinations of traits that increased the anoles’ fitness, called “adaptive peaks,” and that anoles on each island tend to converge on these peaks. In these very similar habitats, there are a limited number of configurations that spell success for anoles, producing repeated sets of near-identical lizards.

Over the last 40 million years, there have been ample opportunities for lizards on the four islands to take different evolutionary paths: genetic drift, species invasions, and small climatic differences could have caused divergence. But despite these historical fluctuations, the anoles of the Greater Antilles have evolved in near-parallel ways provided they were occupying the same niche, suggesting that evolution may be more robust to these perturbations than Gould (and others) expected.

(This isn't to say that all the anole species were identical. On the larger islands, Cuba and Hispañola, there are some distinctive habitats that anole species have occupied that aren't shared by the other islands. Those habitats drove distinctive adaptations.)

This work is a remarkable example of convergent evolution on a very grand scale and is a pretty compelling evidence that in at least some cases, evolution can be remarkably deterministic. The Greater Antillean anoles are likely not an isolated case; the authors suggest that the diversification of African cichlids may offer similar evidence of evolution’s predictability. While random genetic mutations may be the fodder for natural selection over the long run, some aspects of evolution may not be very random after all.

HT Nick.

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