For a number of years, Roger Thorpe and colleagues have been studying patterns of geographic variation in Anolis roquet on the island of Martinique. This species is famous–along with A. marmoratus on Guadeloupe to the north–for the tremendous amount of phenotypic variation that occurs on a relatively large island, so great that Skip Lazell described six subspecies of A. roquet. The photo above illustrates how different looking these populations can be.
Martinique is an unusual island, unique in the Caribbean as far as I’m aware, in that it is an amalgam of several different islands that were distinct for millions of years before being united by a volcanic eruption that poured out lava that connected them. Previous work has shown that there is still a clear genetic signature of this historic isolation, with different lineages occupying their ancient homelands. In addition, Martinique harbors considerable environmental heterogeneity, from sealevel to the 1400 meter peak of Mount Pelée. Much of the mountainous area is cloaked in rainforest, whereas in the rainshadow of the mountains, the environment is quite dry.
This situation has allowed Thorpe and colleagues to ask: which drives divergence more, historic isolation (i.e., allopatry) or the divergent selection pressures that occur in different environments? To examine this question, they have sampled along transects that either cross the boundaries where two lineages meet or that cross environmental transition zones within a single lineage. These transects are exhibited in the figure above–the white lines are the separation among the lineages, the background color represents the environment, and the red lines are the transects (note that the transects cross the lineage boundary at one end, but those sites were excluded from the analysis). Across these transects, the authors measured genetic and morphological differentiation, the latter by examining body patterning and the color of the dewlap and body, as well as limb dimensions and scalation.
The results reported in their most recent paper show that both isolation and environmental differences can lead to divergence, though more predictably so for the latter. In only one of six cross-lineage transect (#1) was there consistent evidence for genetic and morphological differentiation. By contrast, both cross-environment transects showed strong differentiation.
Moreover, examination of patterns of differentiation reveal that morphology and genetics differentiate in tandem, and for the cross environment transects, primarily where the environment changes, as illustrated below.
It is notable that all five morphological character types show substantial differentiation in parallel with the genetic changes. The one exception is scalation (solid red line), which in the across-lineage transect does not change in step with the others. The authors suggest that, of the traits measured, variation in scalation is probably the most clearly related to natural selection, whereas the other characters may be affected by both natural and sexual selection, though the relative importance varies among characters.