Evolutionary biologists, from Charles Darwin onwards, have believed that isolation plays a key role in the origin of species.

Now a study of cave-dwelling salamanders that have evolved into separate species from their surface-dwelling kin despite regularly interbreeding suggests that isolation is not necessary for speciation.

Cave-dwelling species were thought to be classic examples of how isolation is necessary for speciation. Cave populations isolated underground gradually evolve to lose newly useless structures such as eyes and pigmentation.

But when Matthew Niemiller, an evolutionary biologist at the University of Tennessee in Knoxville, and his colleagues saw hints of hybridisation between Tennessee cave salamanders (Gyrinophilus palleucus) and the surface-dwelling spring salamander (G. porphyriticus), they decided to take a closer look.


Regular mixing

The researchers sequenced DNA samples from 109 cave and spring salamanders from 43 locations throughout Tennessee.

They then plugged the data into a sophisticated computer model that compared possible evolutionary histories for the salamanders and calculated which scenario provided the most likely explanation for the genetic patterns observed.

The results suggested that the cave salamanders could not have evolved in isolation from the surface species. In fact, the most likely history was one in which spring salamanders regularly interbred with cave salamanders even as the two species were diverging about 2 million years ago.

Natural selection in the cave salamander for traits helpful for life in caves – such as sharper non-visual senses and a permanently aquatic lifestyle – must have been strong enough to override this gene flow between the surface and cave animals, says Niemiller.

Applying the same analysis to other cases where species were thought to have evolved in isolation – both within caves and without – is likely to yield more examples, he speculates.

“It’s very difficult to show that two divergent forms have experienced gene flow,” he says. “It might be more prevalent than we currently recognise.”

Common occurrence?

Andrew Hendry, an evolutionary biologist at McGill University’s Redpath Museum in Montreal, Canada, agrees that speciation occurs in the face of gene flow much more often than many biologists are willing to admit. Niemiller’s study provides further proof, particularly because of the timing of the gene flow.

“What is interesting is that the migration events cluster fairly early on,” he says. “This suggests gene flow was initially fairly high, and then as they adapt to the new environment, you get the buildup of barriers due to adaptation, and then gene flow goes down.”

However, says Hendry, the computer model at the heart of Niemiller’s analysis is still relatively new, he says, and it takes so much computer time to run it that theoreticians have not yet learned all of its limitations.

As a result, Niemiller’s study may do little to convince confirmed isolationists, he says.

Journal reference: Molecular Ecology (DOI: 10.1111/j.1365-294X.2008.03750.x)

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