Despite all of the research on anole evolution conducted in the last 40 years, one important question still eludes us: how does speciation in anoles occur? This, of course, is of fundamental importance, because the great species richness of these lizards implies that speciation has run rampant in this group. So, we’d like to know why.
We don’t know much about speciation in anoles, but we do know a little. First, it is thought that the dewlap plays an important role. Sympatric anole species almost never have identical dewlaps, and experimental and observational evidence suggests that anoles use their dewlaps for species-recognition. Hence, understanding anole speciation may, to a significant extent, reduce to understanding the factors that cause populations to evolve differences in their dewlaps.
A different perspective on anole speciation relates to the classic question of whether allopatry is necessary or whether, as suggested by many recent studies, natural selection driving differentiation—whether in allopatry or not—is a more important stimulus to genetic differentiation. Recent work in the Lesser Antilles by Thorpe and colleagues has argued that environmental differences are the primary drivers of genetic differentiation within anoles, a result also suggested by Leal and Fleishman’s studies on A. cristatellus in Puerto Rico.
In this light, perhaps the most enigmatic anole is Anolis distichus of Hispaniola. Unlike the vast majority of anoles, distichus exhibits an extraordinary amount of variation in dewlap color, not only among populations, but sometimes within as well, so much so that Albert Schwartz and others named 16 subspecies. How did such diversity arise? And what does it signify? Are there actually many genetically distinct species all currently masquerading under the name distichus, and are a variety of different stages of the speciation process evident in different parts of its range?
In a recent paper, Ng and Glor reported fascinating results of a study directed at addressing these questions. They focused their study in two areas, where over relatively short distances, populations transition from one dewlap color to the other. By sampling along transects across these two transitional areas, they documented how dewlap color changed, and whether such changes were correlated with genetic and environmental changes.
In the first transect, that spans an area in the Samana Peninsula that was underwater until recently (photos on bottom left), dewlap color and patterning transitions somewhat gradually over the length of the transect zone. Moreover, although there appears to be relatively little exchange of the mitochondrial genome through the transect, differentiation of the nuclear genome is relatively slight, suggesting the occurrence of ongoing gene flow. In other words, in this case it appears that populations differentiated genetically and in dewlap color in allopatry, but when the two populations have come back into contact, genetic interchange is occurring. It is not clear that this represents an early stage in the speciation process; very possibly, what we are seeing is two somewhat distinct populations whose differentiation has been halted, if not reversed, by secondary contact.
By contrast, in the second transect (bottom right), the transition from one dewlap color to another is much sharper, and occurs in the vicinity of an environmental transition from xeric to humid. Moreover, levels of gene flow, as measured by exchange of nuclear genes, are quite low. These are highly differentiated forms that by some definitions would count as distinct species. Nonetheless, there’s a quirk here, as well, as the mitochondrial genes of one population have migrated into the other population.
Two conclusions from this study: first, based on limited sample, populations with different dewlap colors that occur in different environments experience little gene flow, whereas those in similar environments are exchanging genes at a much higher rate. This agrees with the suggestion of many ecological speciationists, including some anole work, that adaptation to environmental differences is the most important factor in driving genetic differentiation. The results also support some other recent work suggesting that dewlap color is not the end-all and be-all in determining reproductive isolation—populations with moderately distinct dewlaps, when they come into contact, interbreed successfully, at least some times.
The most exciting aspect of this project, however, is that it is just the beginning. Anolis distichus exhibits a riot of dewlap color variation throughout Hispaniola, and there are many more areas where populations with different dewlaps come into contact; as a result, further work can replicate this study many times over and thus establish general patterns of what determines genetic differentiation, and perhaps even speciation, in this fascinating group.