Lizards’ feet adapt rapidly following ecological changes

Evolution is an experiment played over millions of years, with endless failures, dead-ends, obstacles, impasses, and the occasional success. Species come and species go, wiped out by disaster, drought, famine, and sometimes-overzealous predators. Others simply miss out on a few too many mating opportunities, perhaps because populations of individuals are scattered too widely for potential couples to court.

But evolution doesn’t necessarily need millions of years to run its biological experiments. Russian geneticist Dmitri Belyaev showed that he could create friendly, domesticated foxes from their wild ancestors in just forty generations by applying a very strict selection criteria. That is, he only allowed the top 20% friendliest individuals from each generation to breed. Now, the results of a decades-long published last week in the journal Science shows that this sort of rapid evolution isn’t limited to laboratories: as long as the natural selection criteria are strict, it can occur in the wild as well. And that’s got important implications for native species as they contend with biological invasions from foreign species.

In 1995, a group of researchers led by the University of Tampa’s Todd Campbell turned to a set of man-made islands just off of Florida’s Atlantic coast, on which dwell the United States’ only native Anolis lizard species, Anolis carolinensis, or green anoles. On some of those islands, they released a similar group of lizards, Anolis sagrei, also called brown anoles. Nearly twenty years later, biologist Yoel Stuart returned to those islands as a graduate student in the laboratory of Harvard’s Jonathan Losos (Stuart is now a postdoc at the University of Texas at Austin).

Because the two species use their habitats in similar ways, the researchers expected them to come into conflict. When they’re left alone, green anoles perch on trees anywhere from ground level to the tree’s crown. But in spaces where green and brown anoles were already known to co-exist, the green anoles left the lower parts of the tree to the invaders, retreating to higher ground.

In May 1995, the researchers measured perch height for green anoles on six islands where they were the only lizards, save for the occasional nocturnal gecko. Then they created artificial invasions by releasing brown anoles onto three of them. As expected, the invaders drove the natives up into higher parts of the trees, and that was observable already by August of that year, just three months later. Each summer through 1998 the researchers returned to check up on their lizards, and the vertical divide held up. Green anoles stuck to the higher parts of the tree, and brown anoles perched closer to the ground.

Stuart returned to the islands in 2010. After confirming that the vertical divide had persisted all this time, he turned to the question of evolution. He suspected that the green anoles’ vertical shift would drive the evolution of larger toepads, and with more scales, compared with those from un-invaded islands. “Larger and better-developed toepads improve clinging ability,” he writes, “permitting anoles to better grasp unstable, narrow, and smooth arboreal perches.” And that’s exactly what they found.

In just twenty generations, A. carolinensis had evolved larger toepads with more scales, allowing it to better grasp the smooth tree trunks in the upper reaches of the trees in which it now lived.

“To put this shift in perspective, if human height were evolving as fast as these lizards’ toes, the height of an average American man would increase from about 5 foot 9 inches today to about 6 foot 4 inches within 20 generations – an increase that would make the average U.S. male the height of an NBA shooting guard,” said Stuart in an official statement. The extra seven inches might not seem like much, all things considered, but it would reflect an extremely rapid increase over just twenty generations.

To rule out any other possible explanations, Stuart and his colleagues first verified that each island’s population of green anoles was genetically distinct, meaning that large-toed individuals had not simply migrated between islands. That meant that larger toepads evolved independently on each individual island. Second, they verified that the vegetation on each island was identical; there was nothing unique to the trees on the invaded islands that would have promoted the evolution of larger toepads independently of the presence of the invasive brown anoles.

Finally, they verified that the changes in toepad size were genetic, not environmental, by moving a bunch of pregnant green anoles from invaded and un-invaded islands into identical laboratory conditions. Despite the equivalent environment, baby lizards from the invaded islands continued to have larger toepads than their counterparts from un-invaded ones. Finally, the chances that the pattern observed could have occurred by random chance were just one in 462, an extremely unlikely possibility. “Our data suggest strongly that interactions with A. sagrei have led to evolution of adaptive toepad divergence in A. carolinensis,” they conclude.

The results aren’t just interesting to evolutionary biologists, nor are the implications limited to evolutionary theory. Rather, this sort of rapid evolution – once thought by evolutionary biologists to be unlikely, at least in the wild – lends important insight into the ways in which native species might be able to rapidly adapt both in the presence of invaders and following human-induced environmental changes, at least if the local environment contains enough alternative micro-habitats to allow both species to coexist, each in their own niche.

“The extent to which the costs of invasions can be mitigated by evolutionary response remains to be determined, but studies such as this demonstrate the ongoing relevance of evolutionary biology to contemporary environmental issues,” says Stuart. – Jason G. Goldman | 29 October 2014

Source: Stuart Y.E., T. S. Campbell, P. A. Hohenlohe, R. G. Reynolds, L. J. Revell & J. B. Losos (2014). Rapid evolution of a native species following invasion by a congener, Science, 346 (6208) 463-466. DOI: http://dx.doi.org/10.1126/science.1257008

Header image: Native green anoles (left) have evolved better gripping feet in response to an invasion of brown anoles (right) on islands in Florida. Credit: Todd Campbell and Adam Algar.

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