Reinforcement: enhancement of prezygotic isolation in sympatry by natural selection – Two taxa diverge in allopatry; upon secondary geographic contact, hybridization occurs at some rate, yielding unfit hybrids. Because production of hybrids is maladaptive, individuals who only mate with their own taxa enjoy a fitness advantage.
Evidence: Reveals that sympatry can enhance prezygotic isolation, but does not indicate how often sympatry matters.
Prezygotic isolation is often stronger in sympatry than allopatry.
A publication bias exists since studies that support the existence of reinforcement are more exciting than those that do not. However, the bias does not counteract the probable existence of reinforcement; the evidence is still convincing.
Alternative theories exist and some of them are probable. A good way to tell if what you’re looking at is really reinforcement: Reinforcement should typically result in a larger change in females in sympatry than in males, since females suffer larger fitness costs from mating with the wrong male. Eggs and / or pregnancy is much more expensive than sperm. This is a reasonably robust signature of reinforcement.
