With all this discussion of the green anole’s genome, it seems like a good time to remind everyone of how Anolis carolinesis came to be the model organism that it is today. The simple answer, of course, is that A. carolinensis is the only species of anole endemic to the continental United States. As such, its always been the anole species most accessible to the broadest range of researchers. The deeper answer – and the focus of this post – concerns how A. carolinesis happened to become the continental United States’s only native anole in the first place.
Anolis carolinensis belongs to a clade of prolific over-water disperers known as the carolinensis subgroup, whose other members include eight morphologically similar species of trunk-crown ecomorph anoles scattered throughout the northern Caribbean, from the smallest and most isolated islands (Navassa) to the region’s largest island (Cuba) (see Fig. 1). Although classic island biogeographic theory would predict colonization from the mainland to the islands, A. carolinensis and its relatives buck this trend. Back in 2005, my colleagues and I used a phylogenetic analyses of the carolinensis subgroup with fairly comprehensive taxonomic sampling to show that A. carolinensis and all of the species endemic to small islands or island banks actually result from colonization from a Cuban source population (Glor et al. 2005). This can be seen in the mitochondrial and nuclear DNA phylogenies in Fig. 1, which are extracted from our paper; note that the green tips representing A. carolinensis are nested in the blue Cuban tips in both the mtDNA and nDNA derived trees.
Anolis carolinensis‘s Cuban origins are further solidified when we look at the species that are most closely related to the carolinensis subgroup (Fig. 2). The sister group to the carolinensis subgroup is a clade of three small, poorly-known montane trunk-crown anoles from Cuba (the isolepis subgroup, Fig. 2). These subgroups combine to form the carolinensis group, which is, in turn, assigned to the carolinensis series (Fig. 2, see Burnell & Hedges 1990 and Poe 2006 for precise definitions of the names used here). Simple phylogenetic reconstructions of biogeographic history strongly suggest that the ancestor of the carolinensis series and most of its closest relatives was a Cuban endemic (Rabosky & Glor 2010, Mahler et al. 2010). In the large clade including carolinensis that is comprised primarily of Cuban endemics, the only two non-Cuban Greater Antillean species are two twig species that diversified after dispersing from Cuba to Hispaniola (see right panel of Fig. 2, where green branches nested within red branches indicate dispersal from Cuba to Hispaniola).
Although detailed phylogenetic analyses firmly establish A. carolinensis‘s Cuban origins, we have only relatively crude estimates of when A. carolinensis arrived in mainland North America. Both an earlier allozyme analysis (Buth et al. 1995) and external calibrations applied to the mtDNA dataset (Glor et al. 2005) suggest that A. carolinensis likely arrived in North America prior to the Pleistocene and likely sometime in the Pliocene. Having only a very limited number of loci, however, we can’t say with certainty that some degree of overwater gene flow hasn’t occurred throughout this period. Sampling of additional individuals and loci are required to address the timing and dynamics of natural dispersal between Cuba and the mainland. This type of work is also required to determine the fate of Cuban populations of A. porcatus that appear to have been recently introduced into southern Florida by humans. The two species are very difficult to distinguish morphologically and some of the available molecular evidence suggests hybridization and gene flow between native A. carolinensis and invasive A. porcatus.
At this point it should be abundantly clear that there is plenty of room for additional work on the origins of A. carolinensis. Hopefully it won’t be long before the questions above, and others that remain unanswered, will be revisited with analyses exploiting the new tools provided by the genome of A. carolinensis. There are certainly lots of tissues waiting to be put to good use toward this end!