Many sets of closely related species exhibit a geographic distribution in which species only come into contact at their range border, with one species replacing another across the geographic landscape. Such a “parapatric” distribution could be explained in many ways, such as:
1. The species are adapted to different environments, and their distributions reflect geographic differences in environmental conditions;
2. The environment does not change geographically, but the species are so ecologically similar that neither is able to displace the other from its current range;
3. The species are not reproductively isolated; when they come into contact, they interbreed, thus preventing coexistence;
4. The species are newly-arisen, and have not yet expanded their ranges into sympatry, or one species has not yet displaced the other completely.
A case in point are the large green anoles of Hispaniola, Anolis chlorocyanus and A. coelestinus. Except for their dewlap, these two trunk-crown species are nearly identical in morphology, and they also occupy similar structural habitats. Yet, A. coelestinus occurs only in the southern peninsula, whereas A. chlorocyanus occurs throughout the rest of the island. The range of these two species is separated by a narrow strip of land which is extremely hot and dry, and in which green anoles are only found in human habitations and in mesic conditions near rivers (this strip is called the Plaine du cul-de-sac in Haiti and the Valle de Neiba in the Dominican Republic; it demarcates what has been called “Mertens’ Line,” named after the famous German herpetologist and indicating that it is a biogeographic boundary for many Hispaniolan herps). Although this stripe of desert demarcates the meeting point of the two species, it is not an absolute barrier, as the species do trickle across and are known to occur in sympatry.
A little background: the low-lying desert strip periodically has been underwater at periods of high sea-level. In addition, genetic data suggest that the two species have been evolving separately for many million years. Hence, a reasonable hypothesis is that species evolved in allopatry in the past and have reached an equilibrium in their geographic distributions, ruling out possibility #4 above. So, the question remains: is it that the two species are adapted to different environmental conditions, and the range border marks the spot where the environment changes? Or, have the two species come to a stalemate, coexistence precluded by ecological or reproductive issues?
Using state-of-the-art environmental niche modeling methods, Glor and Warren in a recent paper in Evolution (featured on the cover of the issue) confirmed that the Plaine du Cul-de-Sac (to use the Haitian name) is an area outside the environmental conditions occupied by the species elsewhere in their ranges. Moreover, although the environmental conditions that characterize the range of the two species are not identical, they are very similar, so much so that it seems likely that each species could occur in much of the range occupied by the other. Finally, using a novel null model technique of their own invention, they showed that the environmental differences in the ranges of the two species are no greater than would be expected by chance. That is, the differences between the two species (calculated by comparing the environmental conditions in the ranges of the two species) are no greater than what you would get if you randomly drew a line across Hispaniola at any place and then compared the environments on either side of that line.
Taken together, these points suggest that the species evolved in allopatry at times of higher sea-levels and have expanded their ranges to the point where they meet, if barely, at the Plaine du Cul-du-Sac. Further advance is blocked, mostly likely by their ecological similarity, a phenomenon termed “niche incumbency” or the “priority effect.” It is possible that reproductive issues may play a role, as the species are known to hybridize despite having different patterned dewlaps. Further ecological and behavioral data investigating how the species interact where they come together would be of great interest.