That’s no viper

If there’s one thing we like here at AA, it’s convergent evolution (e.g., 1,2), so we’re always delighted to learn of new examples. Thus, we were delighted to read the recent report on convergence between Australian and North American snake faunas, written by Grundler and Rabosky and now available online at Proceedings of the Royal Society.

Australia is famous for being the only continent on which venomous snakes outnumber non-venomous ones. That is the result of radiation of a single clade of elapid snakes, known as the oxyuranines. It has long been noted that some oxyuranines seem convergent on counterparts elsewhere, such as the death adder, which looks and acts much like a heavy-bodied viper (photo above).

Grundler and Rabosky set out to test this idea of convergence more quantitatively, specifically asking whether the Australian radiation as a whole was more statistically similar to North American snakes than one might expect by chance. That is, does the convergence extend beyond a few pairs of species to encompass the entire radiation–is the radiation-wide degree of convergence greater than one would expect by chance? Alert readers will recall that this is the same question that Mahler et al. recently asked about Greater Antillean anoles, and Grundler and Rabosky used some of the new techniques presented in Mahler et al.

And the results, in a nutshell, are positive. Not only are there many cases of convergence (see figure below), but the overall amount of convergence is statistically significant. This can be seen in two analyses. First, for Australian snakes, the nearest morphological neighbor in North America is more similar than one would expect by chance. Second, Australian elapids have diverged to occupy 10 phenotypic peaks in morphological space, and seven of these are occupied by North American snakes as well.

grundler

The paper has three interesting twists:

1. Despite the great convergence in morphology, North American and Australian snakes are not convergent in diet. In particular, the Aussies eat a lot of lizards and snakes, and insect-eating is much more prevalent in North America.

2. There has been speculation in some quarters that replicate adaptive radiation is an island phenomenon, but this study shows that it can occur between mainland faunas as well.

3. Moreover, the convergence spans multiple lineages. Although the oxyuranines are a single radiation in Australia, their counterparts in North America belong to five different colubroid lineages (not that this analysis was restricted to colubroids, which include the great majority of snakes).

I think it’s safe to conclude that snakes aren’t quite as cool as anoles, but they’re getting there.

Jonathan Losos
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