However, parapatric speciation is greatly debated. The examples from chapter 22 are used in lecture. Most other theories involving a gradual change in sympatry are controversial. Assortative mating among homozygotes is a key component of this theory. This minimally requires two loci to evolve simulatneously, and thus we establish strong linkage disequilibrium for the niche polymorphism loci and the assortative mating loci.
More loci makes it unlikely.
The evolution of such divergence, in the face of gene flow is a tremendous challenge for the evolution of species by the process of sympatric speciation. This key fact places a tremendous genetic constraint on speciation by the process of reinforcement. Every generation segregation of chromosomes will lead to parental species that lack the correct discrimination alleles. Even if the genes fitness and mate discrimination are linked on the same chromosome, recombination during meiosis is a powerful enough force to dissolve away such associations and eliminate the possibility of sympatric speciation. Sympatric speciation is like the measles.