Unlike the extensive within-island speciation that anoles have undergone in the Greater Antilles, we have no evidence that the same has occurred in the Lesser Antilles. Rather, Lesser Antillean islands that contain two species are thought to be the result of dispersal events rather than in situ cladogenesis. Despite such low species diversity, however, phenotypic diversity on many of these islands certainly is not lacking. Some Lesser Antillean anoles exhibit spectacular geographic variation in head, body and dewlap colouration and pattern, as well as body size and scalation, that appears to be adaptive to different environments. So, while this variation has not led to complete speciation in any Lesser Antillean anole, is there some evidence that these phenotypically divergent populations are at some stage of the speciation process? Also, how does phenotypic divergence occur on these smaller islands when there seems to be little opportunity for geographical isolation?
AA contributor, Martha Muñoz and colleagues tackle these very questions in a recent paper in Molecular Ecology. Muñoz et al. focus on the stunning phenotypic diversity of the Anolis marmoratus complex on Guadeloupe, which has been categorised into 12 subspecies. On Grande Terre, in particular, two subspecies can be found: A. m. speciosus inhabits mesic habitats in the southwest and A. m. inornatus inhabits the xeric lowlands of the north and east. Males share a yellow-orange coloured dewlap but differ in head, body and eye ring colouration, while females and juveniles of the two subspecies are similarly drab in colour.
To investigate colour divergence and speciation in these two phenotypically divergent subspecies, Muñoz et al. use microsatellite markers and phenotypic and habitat reflectance data sampled across two transects. Their results show that first, change in colouration coincides with the mesic-xeric ecotone on Grande Terre. Curiously, the divergent colouration may actually be due to two different selective forces. The green body, and blue head and eye ring of A. m. speciosus stands out from the background suggesting that colouration in this subspecies may be driven by sexual selection. On the other hand, the duller colouration of A. m. inornatus is similar to the background suggesting that selection is driving crypsis in this subspecies.
Second, Muñoz et al. find that there has been a history of continual migration between the two subspecies and large effective populations sizes. This suggests that divergence in colouration has occurred in the face of gene flow (i.e. parapatric divergence), rather than the alternative model of divergence in geographic isolation followed by secondary contact.
Lastly, the pattern of gene flow throughout their history does not appear to have stopped. Muñoz et al. find that contemporary rates of gene flow are high despite clear differentiation in colour, suggesting that strong divergent natural selection is maintaining colour differentiation between the two subspecies while gene flow is homogenizing neutral regions of the genome (presumably where the microsatellite loci are).
This study is aligned with studies by Thorpe’s group on A. oculatus on Dominica and A. roquet on Martinique, which show that speciation has not reached the point of completion in Lesser Antillean anoles despite extensive adaptive variation in phenotype. However, Muñoz et al.’s results shine light on the possibility of parapatric divergence in anoles. So, if parapatric divergence is possible, what then is inhibiting speciation in the Lesser Antilles? Why does there appear to be a threshold island size of 3000km2 for within-island anole speciation to occur, as Losos & Schluter’s (2000) study proposed? A comparative approach and more fine-scaled studies on both the Lesser and Greater Antilles will certainly help move us towards elucidating the conditions required for anole speciation to reach the point of completion.