I just finished attending a workshop in Kesthely, Hungary on Niche evolution and speciation – two of my favorite topics. Sadly there was no Anolis news to report from any of the excellent talks, but the work I presented is related to one of the anole projects I’m planning for my postdoc.

Speciational evolution is, as the name implies, evolutionary change that occurs rapidly when one species is being split into two; this means the amount of evolution in a lineage should depend on the number of times speciation has happened in its history. This contrasts with the standard Brownian motion model of gradual evolution where the amount of evolution depends on the length of time that has passed.

Speciational evolution might occur for a number of reasons (for example, due to genetic drift in small geographically isolated populations), but when one trait shows speciational evolution and another does not, we may be able to infer something about the process of speciation. For example, speciation may involve divergence in habitat (the ‘beta-niche’), or in traits that affect local resource use within a habitat (the ‘alpha-niche’).

We can estimate the contribution of speciational and gradual evolution to the total evolutionary rate of a trait by essentially stretching a phylogenetic tree to give more importance to time (branch lengths) or speciation (nodes in the tree). We can then use likelihood to find which combination of gradual and speciational evolution maximizes the fit of the trait data to the tree. During my Ph.D. I extended methods to infer speciational vs. gradual evolution (many of them developed by Folmer Bokma), and applied them to my Sebastes rockfish study system. I found that the alpha-niche (diet and trophic morphology) seems to evolve gradually, but that about half of the evolutionary rate of the beta-niche (depth habitat) could be attributed to change at speciation (link to paper).

Anyways, back to anoles, and the question of whether this phylogenetic approach can tell us anything about speciation in our favorite lizards. At a first pass, I’ll define the anoles’ alpha-niche as their structural microhabitat, as these traits (related to ecomorph categories) are generally differentiated among species at small spatial scales. We think competition for food is important for the evolution of the ecomorphs, so it could also be involved in speciation itself; temporary geographic isolation may start the speciation process, but alpha-niche divergence could play a big role in finishing it. Alternatively, speciation might tend to occur when species diverge in the beta-niche: their larger-scale use of habitats that differ in temperature and precipitation. To my knowledge there aren’t any morphological traits that are strong predictors of the beta-niche (let me know if you think I’m wrong), but there are enough direct measures of thermal habitat (e.g. field body temperatures) that those can be used instead.

To apply the methods to anoles I’ll have to make some improvements to account for the fact that speciation can occur within an island or through dispersal between islands, and to accommodate particular features of these traits. For example the model can be modified to acknowledge that microhabitat generally diverges early in the process of anole radiation, whereas thermal habitat may or may not. I’m hoping that this kind of work will be a useful complement to other approaches – such as genetic studies of population structure – to tell us something about where all these anole species are coming from.

Travis Ingram
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