For generations, biologists have cited a sleek Mexican salamander with minuscule legs as an amazing example of how animals can adapt to a burrowing lifestyle. Now, the creature turns out to be an equally amazing example of something else: the evolution of similar body features in unrelated species, or convergent evolution. A genetic study has shown the salamander, Lineatriton lineolus, to be not one but two species that have come to look almost identical. "This is one of the best cases of convergence I've ever seen," says herpetologist Jonathan Campbell of the University of Texas, Arlington.
Several groups of burrowing salamanders have evolved extra-long bodies and tiny legs. Almost all of them got their lanky frames by developing extra vertebrae in the trunk region--as many as 18 to 22 in all, instead of the standard 14. Only Lineatriton lineolus has stretched itself thin by lengthening each of its 14 vertebrae as it develops.
To study interrelationships among a number of tropical salamander species, David Wake of the University of California, Berkeley, and his colleague Gabriela Parra-Olea, who's now at Harvard, sequenced three mitochondrial genes from each of them. The data for Lineatriton were so surprising that the researchers repeated their work again and again. They got the same result every time: Lineatriton consists of two distinct groups, each of which is most closely related to very different-looking species in the genus Pseudoeurycea. "I could not believe this result," Wake says. "I've been studying adaptive diversification for years, and to get a surprise like this at such a late point is exciting." The results, which will appear in the 3 July Proceedings of the National Academy of Sciences, may persuade herpetologists to revise the salamanders' taxonomy.
That's what's being proposed by Campbell and his colleagues, who have studied the animals independently. Campbell says he can distinguish the different lineages from slight differences in their anatomy. But he had always assumed they were very close relatives, not distant cousins. "This paper really puts a new twist on things," Campbell says. "It amazes me that two things evolved independently that look so much the same."