In a recent Anole Annals post, Luke Mahler mentioned the pioneering work of Webster and Burns on variation in the Hispaniolan trunk anole, A. brevirostris. This paper presents one of the most compelling cases for the occurrence of reinforcement—the phenomenon in which natural selection leads to the evolution of increased reproductive isolation when two hybridizing species come into sympatry. Surprisingly, however, this example is not well known; indeed, Google Scholar reports only 29 citations, only two of these post-1991. This is too bad, because it is a wonderful example and deserves to be more widely known. For this reason, I present a slightly modified description of variation in these lizards taken from Lizards in an Evolutionary Tree:
Three members of the A. brevirostris species complex, nearly indistinguishable in appearance, occur contiguously along the western coast of Haiti. The southernmost of these species is A. brevirostris itself, which has a light-colored, pale dewlap. By contrast, the northernmost species, A. websteri, has a vivid, orange dewlap. Most interesting, however, is the species sandwiched in between the other two, A. caudalis, whose dewlap color varies geographically: at the southern border of its range, near A. brevirostris, its dewlap is bright orange, and at the northern edge of its range, where it comes into contact with A. websteri, its dewlap is white. Interior populations exhibit variability in dewlap color with change occurring at least somewhat clinally from one end of the range to the other. Display behavior also differs among all three species, with the behavior of A. caudalis being the most distinct from the other two species. The most parsimonious explanation for these differences—particularly the geographic variation in dewlap color in A. caudalis—is that they evolved to prevent hybridization between closely related species. Indeed, electrophoretic analyses by Webster and Burns confirm that levels of gene flow are high among populations within each species, but extremely low or non-existent between species, including adjacent heterospecific populations.
Luke and I got the samples to reinvestigate this contact zone when we were in Haiti. We’re hoping to do so with a large set of nuclear loci. I was struck by the fact that the distribution of dewlap color and patterns is almost exactly the same as it was in Webster’s time (both here and elsewhere in Haiti).