Reinforcement

In my final blog about the book Speciation, I will reflect on Coyne and Orr’s discussion of reinforcement. In case you don’t remember from earlier posts, reinforcement is the enhancement of prezygotic isolation between populations in sympatry by natural selection. If populations experience divergent evolution while in allopatry, and then come into secondary contact and hybridize, the hybrids may be less fit than offspring produced by individuals from the same population. If the low fitness of hybrid offspring leads to natural selection on individuals not to hybridize (or to mate with others with similar phenotypic characteristics) then this selection is called reinforcement. Reinforcement leads to the evolution of adaptations that increase the premating isolation of the populations.

Reinforcement is distinct from reproductive character displacement. This term was coined by Butlin to describe situations in which taxa come into secondary contact and produce hybrids with are completely unfit. In this case gene flow is not possible, as the hybrid will not be able to produce surviving offspring. This means the two populations are already good species, and so any further enhancement of prezygotic isolation would have nothing to do with speciation and should not be considered reinforcement. In contrast, in the case of reinforcement the hybrids are only partially unfit, so gene flow is possible, and the speciation process is not complete. Enhancement of prezygotic isolation in this case is relevant to speciation.

As Felsentein (1981) explained, it should be theoretically difficult for reinforcement to evolve. Reinforcement requires that individuals assortatively mate with others that share similar phenotypic traits. This means that there must be strong linkage disequilibrium between alleles underlying the characters that have undergone divergent evolution between the two populations, and the alleles underlying mating preferences. Felsenstein pointed out that although this linkage may be generated by selection, recombination often destroys it. Unless the two loci are tightly linked, it should be difficult to overcome recombination.

One way to look for evidence of reinforcement is to quantify the level of prezygotic isolation between populations in sympatry and in allopatry. The prediction would be that prezygotic isolation would be higher between the populations in sympatry. This is because there would only by hybridization between the two populations they come into contact, and so in sympatry there would be an opportunity for natural selection to increase the strength of adaptations leading to prezygotic isolation, where in allopatry there would not.

Could you use this kind of a prediction to look for reinforcement in white sands lizards? Or is reinforcement even a relevant concept to apply to this system? Upon first examination I thought it would make sense that reinforcement could be going on at the ecotone. After all, you have white populations of lizards in white sands, dark populations in the various surrounding desert habitats, and intermediately colored lizards at the ecotone. So it seemed to me that these intermediately colored individuals could be hybrids of dark and light populations that had diverged in allopatry and come back into secondary contact. Turns out it is likely that this isn’t the case. For one thing, there really aren’t any light populations and dark populations located close enough together to suggest that frequent hybridization of parental populations at the ecotone is common. In addition, the genetic basis for dark and blanched coloration in Aspidoscelis and Sceloporus is a simple dominant recessive relationship. In Aspidoscelis the mutation in Mc1r causing blanched coloration is recessive, while in Sceloporus it is dominant (Rosenblum et al 2009). So for example, if you had a mating between a dark (heterozygous or homozygous dominant) Aspidoscelis and light (homozygous recessive) Aspidoscelis, you might get either a light colored hybrid or a dark colored hybrid, but you shouldn’t see an intermediately colored individual. So, though it is interesting to contemplate where these intermediately colored individuals are coming from, their existence may not be relevant in the case of our discussion of reinforcement. It seems from the evidence we have currently that reinforcement of prezygotic isolation is not an important mechanism of speciation in the white sands system.