Speciation - What is the role of reinforcement in speciation?
Does reinforcement really exist?
It is extremely controversial as to whether reinforcement really exists. There will be a time window after secondary contact during which reinforcement must happen, for the following reasons:
Suppose there are alleles A and a at a locus, and AA and aa have higher fitness, while Aa is selected against. Natural selection then favors AA types that mate preferentially with other AA types; and aa types that mate only with other aa . The result could be speciation through reinforcement with one species having genotype AA and the other species aa .
However, before speciation is complete, the selection against heterozygotes will also act to remove the rarer allele and natural selection favors whichever genotype is commoner. There will be an evolutionary race between speciation and the loss of the allele.
The latter process will often be faster because it needs no new genetic variation, whereas speciation cannot happen without genetic variation for mating preferences. Moreover, recombination between the genes controlling the mating preference and the genes under selection will frustrate the evolution of reinforcement.