Evolution Meeting 2013: Phylogenetic Analysis Of The Last Stage Of Adaptive Radiation In Puerto Rican Anoles

Yesterday, Matt McElroy presented a phylogenetic analysis of the Puerto Rican radiation of anoles. The work was focused around the “stages of radiation” hypothesis that states that divergence occurs along different niche axes at different points in time. In the case of anoles, it has long been argued that the last stage in radiation is divergence of ecomorphologically similar species into different climatic niches.

McElroy constructed a phylogeny for 180 individuals of eight species, encompassing the geographic distribution of these species (most of which occur island-wide). Ten genes were sequenced, nine nuclear and one mitochondrial. The resulting phylogeny was well-resolved and in agreement with previous phylogenetic hypotheses, indicating that ecomorphs evolved relatively early in the radiation and that closely related sister taxa pairs are usually members of the same ecomorph, but differ in climate–the one exception–which always has struck me as odd, but apparently is correct–is the sister taxon relationship between the deep rainforest trunk ground species A. gundlachi and the xeric grass-bush species, A. poncensis.

The time of divergence was estimated for each of the four sister taxa pairs, indicating that there were three phases of radiation. The deepest split, pegged at 15 mya, was between the two trunk-crown species, A. evermanni and A. stratulus. At 10 mya, two pairs split simultaneously, the aforementioned one above and the two trunk-ground species, A. cooki and A. cristatellus. Both of these pairs include one species that occurs in the xeric southwestern portion of Puerto Rico, perhaps not a coincidence? Finally, 5 mya, the two grass-bush species, A. pulchellus and A. krugi diverged. These latter two species have recently been shown to hybridize, and McElroy’s data confirms that this is the only one of the four pairs in which hybridization occurs, perhaps due to their recency of divergence?

This is a fabulous example of detailed phylogenetic work spanning both interspecific comparisons and including the extensive degree of phylogeographic divergence that occurs within many anole species. More work of this sort is needed on anoles on the other three islands of the Greater Antilles. The monophyletic Jamaican radiation would be a good starting point.

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Jonathan Losos

Professor of Biology and Director of the Living Earth Collaborative at Washington University in Saint Louis. I've spent my entire professional career studying anoles and have discovered that the more I learn about anoles, the more I realize I don't know.

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