A Few Words on Speciation
John Wilkins has been writing about modes of speciation (who would've thunk it?), using Sergey Gavrilets's recent book, Fitness Landscapes and the Origin of Species, as a framework. I have not read the Gavrilets book, but I am familiar with a lot of the literature on speciation -- mostly that dealing with studies of the genetics of speciation in natural populations. Wilkins has been writing about how we define modes of speciation (allopatric, sympatric, chromosomal, etc), and I like how he has framed the issue. Rather than individual types of speciation, Wilkins shows how there is overlap between the different modes and certain modes tend to occur with other modes. Both posts are worth reading, and I'm hoping he keeps them coming.
While I don't study speciation, what I have read in the literature has gotten me thinking about allopatric and sympatric speciation. I prefer to define sympatry and allopatry based on gene flow rather than geographic range (note that these are not mutually exclusive, as gene flow depends on geographical isolation). In this framework two sympatric population are essentially one single population with complete migration between them (m=0.5). Conversely, two allopatric populations are reproductively isolated (either pre-zygotically or post-zygotically) such that m=0. If 0<m<0.5, the populations are parapatric.
In studying speciation in natural populations, we can sample speciating populations at different points in the process. For example, two populations that recently became geographically isolated give a snap-shot of the early stages of speciation. Two populations that are partially reproductively isolated (due to mediocre mate discrimination, incomplete post-zygotic isolation, etc.) show the intermediate stages of speciation. Finally, two populations that have nearly complete reproductive isolation due to genetic factors provide insights into the later stages of speciation. From studying species pairs at different stages, we can increase our understanding of the entire speciation process.
This brings me to the current paradigms regarding sympatric speciation. Ernst Mayr advocated (and most biologists seem to agree) that allopatric speciation (now defined by geography) is the norm and sympatric speciation is very rare. There are some examples of geographical sympatric speciation (the Rhagoletis system is the most accepted), but they are few and far between. Even the sympatric nature of the Rhagoletis system has been called into question -- traditionally by the argument that host shifts due not really constitute sympatric speciation, and more recently by molecular studies that show the genetic potential for speciation originated allopatrically (defined geographically). Unless we can observe the speciation process from beginning to end (I know, I'm sounding like a creationist) we have no way of knowing that speciation was entirely sympatric.
Biologists can only measure the current (and recent) gene flow between populations. It is difficult to impossible to determine whether two populations ever were geographically isolated at some point in the past. The recent work on Rhagoletis, as well as the best model we have for chromosomal speciation with gene flow suggests that even sympatric speciation requires geographic isolation (or some other reproductively isolation) to allow for genetic differentiation between the populations. When these populations come back into contact, reinforcement favors the evolution of pre-zygotic and post-zygotic isolating barriers due to decreased fitness of the hybrids. If this is the case, we need to redefine the dichotomy, moving away from sympatric and allopatric speciation. Instead, the two modes should be “entirely allopatric” and “allopatry with reinforcement”. This new framework provides two plausible modes of speciation (rather than the extremely unlikely entirely sympatric speciation), but the mechanisms of speciation would still differ between the two modes so the dichotomy is biologically meaningful.