s form in the zone of contact 4.
Genome recombination results in speciation of the two populations, with an additional
hybrid species. All three species are separated by
intrinsic reproductive barriers One of the strongest forms of
reproductive isolation in nature is sexual isolation: traits in organisms involving mating. This pattern has led to the idea that, because selection acts so strongly on mating traits, it may be involved in the process of speciation.). It necessitates two forces of evolution that act on
mate choice:
natural selection and
gene flow. Selection acts as the main driver of reinforcement as it selects against hybrid
genotypes that are of low-
fitness, regardless if individual preferences have no effect on survival and reproduction. A common signature of reinforcement's occurrence in nature is that of
reproductive character displacement; characteristics of a population diverge in sympatry but not allopatry. Further, gene flow can diminish the isolation found in sympatric populations. In instances of
peripatric speciation, reinforcement is unlikely to complete speciation in the case that the peripherally isolated population comes into secondary contact with the main population.
Genetics The underlying genetics of reinforcement can be understood by an ideal model of two haploid populations experiencing an increase in
linkage disequilibrium. Here, selection rejects low
fitness Bc or bC
allele combinations while favoring combinations of BC alleles (in the first subpopulation) and bc alleles (in the second subpopulation). The third locus A or a (the assortive mating alleles) have an effect on mating pattern but is not under direct selection. If selection at B and C cause changes in the frequency of allele A, assortive mating is promoted, resulting in reinforcement. Both selection and assortive mating are necessary, that is, that matings of A \times A and a \times a are more common than matings of a \times A and A \times a. A restriction of migration between populations can further increase the chance of reinforcement, as it decreases the probability of the differing genotypes to exchange. with some relying on one locus per trait and others on
polygenic traits.
Population structures The structure and migration patterns of a population can affect the process of speciation by reinforcement. It has been shown to occur under an island model, harboring conditions with infrequent migrations occurring in one direction, . In direct selection, the frequency of the selected allele is favored to the extreme. In cases where an allele is indirectly selected, its frequency increases due to a different linked allele experiencing selection (
linkage disequilibrium). Any traits that promote isolation may be subjected to reinforcement such as mating signals (
e.g. courtship display),
signal responses, the
location of breeding grounds, the timing of mating (
e.g. seasonal breeding such as in
allochronic speciation), or even egg receptivity. Many of these examples are described below. == Evidence ==