Speciation models

In chapter three Coyne and Orr focus on allopatric and parapatric models of speciation. Allopatric speciation refers to the divergence of populations that are separated by some kind of physical barrier, so that there is no gene flow between populations. There are two models of allopatric speciation- vicariant speciation and peripatric speciation. The main difference between these two models is the relative sizes of the populations involved in speciation. In the vicariant model, a population is split into two or more large populations by a barrier or by extinction of intermediate populations. In the peripatric model, a small population becomes isolated from the main population either by physical barriers or when a new area is colonized by a few individuals.

Parapatric speciation refers to the divergence of populations between which there is some gene flow. One model of parapatric speciation is the clinal model. In this model a species is distributed continuously across a variable environment. Subpopulations adapt to local habitats, but adaptation is hindered by gene flow from nearby ecologically distinct populations. When populations have become differentiated enough, reinforcement may play a role in generating reproductive isolation. In the stepping stone model of parapatric speciation, there are discrete populations with restricted gene exchange. Since populations are discrete it is easier for selection or drift to result in reproductive isolation than in the clinal model.

Though white sands and dark soils lizards have not speciated, we can still think of which of these models is most applicable to divergence of these populations. The first question we should answer is how much gene flow is occuring between white sands and dark soils lizards. Previous research has shown that there is some gene flow, though the amount differs between the different species of lizards (Rosenblum 2006). This means that the parapatric models are more applicable than the allopatric models. Next question: are populations distributed continuously over a variable environment, or are there discrete populations? While the environment is definitely variable from white sands to dark soils, the amount of connectivity between populations is also different between different species of lizards. For example, Holbrookia maculata show the most genetic differentiation between populations, and also the largest color difference between white sands and dark soils populations. Sceloporus undulatus shows an intermediate level of phenotypic and genetic difference between populations, and Aspidoscelis inornata shows the least of both (Rosenblum 2006).

This evidence shows that Holbrookia populations are probably the most discrete, with Sceloporus populations occurring in a less discrete distribution, and Aspidoscelis distributed almost continuously. So maybe the stepping stone model is most likely for Holbrookia, clinal for Aspidoscelis, and some combination for Sceloporus? And if the models are different, does this mean that different types of reproductive isolation could be important for different species? Reinforcement is thought to be important for the clinal model, but in the stepping stone model premating isolation might evolve more easily as a byproduct of selection or drift. Hmmmmmm…