Reproductive isolation and ecological speciation

In chapter 8 of The Ecology of Adaptive Radiation, Schluter addresses the role of reproductive isolation in adaptive radiation, focusing on different ways that reproductive isolation between populations may arise. Mayr and Dobzhansky initially formulated the idea that pre- and postmating isolation could arise between populations evolving in distinct selection environments, with complete reproductive isolation resulting ultimately as a consequence of divergent natural selection.

There are two models of how divergent selection causes reproductive isolation and ecological speciation:

By-product speciation is the first model. In the case of by-product speciation, divergent natural selection on phenotypes causes reproductive isolation to evolve incidentally between populations in different environments. Reproductive isolation is considered a “by-product” because it is not directly favored by selection. Both postmating and premating isolation can occur through by-product speciation. Postmating isolation evolves when genetic changes favored in different populations in different environments are incompatible when recombined in hybrids (this is known as hybrid inviability). Premating isolation can occur if mating preferences are genetically correlated with morphological or physiological traits that are the targets of divergent selection.

The second model Schluter mentions is competitive speciation. When intermediate genotypes in a population are at a fitness disadvantage selection may favor a population split leading to reproductive isolation. Competitive speciation is considered to be reinforcement if a period of allopatry leads to incomplete pre- and postmating isolation, and then when the populations come back together hybrids are selected against because they are at a fitness disadvantage. The process is known as sympatric speciation when there is no allopatric phase, and speciation is initiated by disruptive selection in a population. These two versions of competitive speciation are distinct from the by-product model because reproductive isolation is directly favored by natural selection in the sympatric phase.

I am interested in investigating whether premating isolation has occurred in white sands and dark soils lizards. In the by-product model, premating isolation between populations can develop when populations are in different selection environments, and mate preferences are linked to traits that are affected by natural selection in these differing environments.

What would have to occur for premating isolation to evolve between white sands and dark soils populations? First of all, the populations would have to be diverging in different selection environments. Check. White sands populations differ dramatically from dark soils populations in skin color, among other things. Second of all, divergent natural selection would have to affect traits linked to mate preferences or recognition. Here’s where things get a little more ambiguous. All three lizard species in the area have social signaling patches that are used in inter- and intrasexual interactions. The colors of these patches are different between white sands and dark soils populations. We know that this difference in skin color is linked to differences in the Mc1r gene. Mutation of Mc1r in white sands lizards has led to adaptive blanched dorsal skin coloration, and this change has affected the coloration of the social signaling patches as well.

So yes, social signaling patch color, which is involved in social interactions, is diverging due to natural selection in different environments. But is social signaling patch color related to mate choice or recognition? Maybe mate choice is linked to chemical secretions or social interactions that are divergent between populations by chance, not because of divergent natural selection? Or, for that matter, is there even mate choice occurring in these lizards? Maybe they would just be game for mating with any conspecific they run into. These are questions I’m hoping to begin answering during my first field season this spring. Okay, this is getting long, so I’ll stop…but if anyone has any input on my ideas I would love to hear it!