Anolis nitens (formerly A. chrysolepis). Photo from http://www.sciencephoto.com/images/download_lo_res.html?id=907650178
Today I had a chat with an evolutionary biologist who specializes on the evolution of tropical forests. We were discussing the effect of climate change on the Amazon, and he first made the point that dire warnings about the Amazon are possibly slightly overstated. Yes, the rainforest may be in big trouble, but it’s not as if the trees will fall down and the area turn into barren desert. Rather, there are many dry-adapted tree species in South America, and they will probably take over, replacing wet forest with dry forest. Conversation then turned to what is known as the Pleistocene Refugia Hypothesis (PRH), the idea that during the ice ages, the climate became drier because so much water was locked up in glaciers. As a result, so the theory goes, rainforests were fragmented as the forest was replaced in many places by other habitats. As a result, a formerly widespread species might become isolated into multiple, unconnected populations. The PRH suggests that these isolated populations often evolved into different species, and the famously high species richness of the Amazon may be a result of high rates of speciation resulting from a series of cycles of forest contraction and expansion. The PRH, however, has fallen on hard times for a variety of reasons, and I think it is safe to say that most workers in the field no longer favor it. Nonetheless, my colleague averred that the hypothesis has been too hastily discarded; in his mind, the idea has been caricatured, and more reasonable versions of the idea have merit and deserve more attention.
Core areas of A. chrysolepis distribution in the Amazon. Map H from Vanzolini and Williams (1970)
Of course, this conversation immediately turned my mind to one thought: anoles! In particular, it reminded me of two significant, yet in recent years little read, papers on Amazonian anoles. The first was a 1970 monograph in Arquivos de Zoologia by Paulo Vanzolini, famed Brazilian herpetologist and samba composer, and Ernest Williams on the pan-Amazonian anole then referred to as A. chrysolepis, now known as A. nitens, and soon to be divided into several species (stay tuned to these pages for more on that as the story develops). Based on detailed examination of geographic variation of this species throughout its range, the authors came to the conclusion that four core areas existed on the periphery of the species’ range, within which character variation was quite cohesive; between these areas, characters intergraded in complex and different ways. From this pattern, they argued that the core areas were sites in which populations had diverged in isolation at times of forest contraction, and that the confusing patterns of intergradations were the result of range expansion and intermixture as differentiated forms mingled as the forest advanced when the climate became wetter. Jürgen Haffer’s 1969 Science paper is usually credited as the source of the PRH, but clearly Vanzolini and Williams had the same idea, which I gather was developed independently because their massive monograph must have been many years in the making. Vanzolini and Williams, in a postscript, pointed out that Haffer had had much the same idea, and that the proposed refugial areas for anoles and birds were actually quite congruent. Though not as widely cited as Haffer’s paper (959 citations vs. 201, according to Google Scholar), V&W’s paper, and the proposed congruence in patterns, played an important early role in the development of the PRH and was part of the reason that the theory was initially so popular.
A much less well-known paper is V&W’s 1981 offering in Papéis Avulsos de Zoologia that proposed the Vanishing Refuge Theory (VRT). Evolutionary biologists have long been fascinated by the occurrence of closely related species that occupy different habitats, but whose geographic ranges abut where the two habitats come into contact (technically termed a “parapatric” distribution; picture where forest meets grassland, or where black lava meets desert sand). The question is: how could one ancestral species give rise to two species adapted to different habitats? In recent years, the idea of “ecological speciation”—which suggests that natural selection drives speciation—has become very popular, and many adherents suggest that selection can drive populations of a species to diverge into different species, even in the presence of ongoing genetic exchange between the populations. This contradicts traditional wisdom, at least among some, that isolation—and thus lack of genetic exchange—is necessary for speciation to occur (what is termed “allopatric speciation”).
V&W had an alternative explanation for the occurrence of parapatric species, one relying on allopatric speciation. Their model goes like this. Picture a forest adapted species, say a small, ground-dwelling anole. Imagine that the climate dries, and the forest fragments into multiple, isolated patches, the forest refugia discussed above. However, as the drying trend continues, some of the forests become progressively more open and less moist. This is bad news for most lizard populations, but one population is able to adapt to the drier conditions. Eventually, it becomes so dry that many forest patches disappear entirely, the “vanishing refuges.” Most lizard populations perish as well. But one population has been able to adapt rapidly enough, and persists in whatever type of open habitat replaces the forest. Subsequently, this species expands its range such that it comes into contact with other forest fragments, and thus with members of their ancestral species that still occupy the forest, thus creating a parapatric distribution, with the new species in the open habitat abutting the forest, and the ancestral species still in the forest. In other words, allopatric speciation has produced a parapatric distribution.
V&W cited a number of cases of Amazonian species that seemed to exhibit different stages of this process, from some populations that were in the most open parts of the edge of the forest, to other cases in which some populations ventured out of the forest to nearby fence posts, to some species in which some populations no longer were in forests at all. The population of Anolis chrysolepis in Cariri, Brazil, was one an example of a population of a forest species occurring in an open area without any forest.
I do not know whether this idea would hold up to modern scrutiny, but it seems worth looking into. Given modern niche modeling and other approaches, and the interest in connecting patterns of geographic/environmental distribution to underlying speciation processes, the VRT could be very important in reminding us of alternative possible scenarios for the evolution of new species. In addition, with great concern about how the rainforest and other habitats will change as climate warms, ideas like this may be very relevant to modern concerns.
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Cody Porter
Thank you for this post!
As a bird-enthusiast of northern latitudes, I’m accustomed to thinking about glacier refugia in the context of ice fragmenting the landscape and how this influenced speciation and present-day distributions. Admittedly, I’ve never even thought about the effects of glaciation on tropical forest fragmentation as a result of moisture loss.
Jonathan Losos
Yep, all that water in the glaciers had to come from somewhere!