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. Everybody gets it, but they soon get over it. Templeton's Cohesion Species Concept this is a completely different classification scheme from the Geographic modes of speciation.
Reproductive isolation is enhanced by large decrements in heterozygote fitness.
If Ne is small, the probability of peak shift is large, if Ne is large, the probability of peak shift is trivial. Genetic homeostasis and gene flow opposes divergence.