Evidence for speciation by reinforcement
Reinforcement is a process within speciation where natural selection increases the reproductive isolation between two populations of species by reducing the production of hybrids.
[1][2] Evidence for speciation by reinforcement has been gathered since the 1990s, and along with data from comparative studies and laboratory experiments, has overcome many of the objections to the theory.
Reinforcement's prevalence is unknown,[4] but the patterns of reproductive character displacement are found across numerous taxa (vertebrates, invertebrates, plants, and fungi), and is considered to be a common occurrence in nature.
[9] Similar patterns of reproductive character displacement involving acoustic displays have been found in Hyla cinerea and H. gratiosa, with greater female preference for conspecific males in areas of sympatry.
The species contact zones exhibit, "strong postzygotic selection against hybrids" and enhanced isolation from differences in mating call.
[13] An alternative to detecting reproductive character displacement in populations that overlap in sympatry is measuring rates of hybridization in contact zones.
[14][9] A similar pattern was detected in the sympatric spadefoot toads Spea multiplicata and S. bombifrons have hybridized with decreasing frequency over a 27-year period (about 13 generations).
[19] Geospiza fuliginosa and G. difficilis males on the Galápagos Islands show a noted preference for conspecific females where they meet in sympatry, but not in allopatry.
[28] Ethological isolation has been observed between some mosquito species in the Southeast Asian Aedes albopictus group, suggesting—from laboratory experiments of mating trials—that selection against hybrids is occurring, in the presence of reproductive character displacement.
[32] The song differences of Laupala crickets on the Hawaiian Islands appear to exhibit patterns consistent with character displacement in sympatric populations.
[33] A similar pattern exists with Allonemobius fasciatus and A. socius, species of ground crickets in eastern North America.
[35][36] Fifteen species of sympatrically distributed Agrodiaetus butterflies with pronounced differences in wing color pattern likely arose as a result of speciation by reinforcement.
[9]The reversal in chirality to sinistrality must have evolved as an isolating mechanism,[49] with patterns of reproductive character displacement suggesting speciation by reinforcement.
[51] A similar pattern in snails is found with Lymnaea peregra and L. ovata in the Swiss lake Seealpsee; with mating signal acting as the sympatrically displaced trait.
[53][3]: 343 A similar pattern of sperm lysin differentiation is found in the mussel species Mytilus galloprovincialis and M. trossulus and has likely occurred within the last 200 years due to human-mediated distribution by ships.
[54] In the Texas wildflower Phlox drummondii, cis-regulatory mutations of genes that code for anthocyanin pigmentation have caused genetic divergence of two populations.
[56] Similarly, in P. pilosa and P. glaberrima, character displacement of petal color has been driven by selection, aided by pollen discrimination.
[57] Displacement in flower size has also been observed in the nightshade species Solanum grayi and S. lymholtzianum in sympatry as well as S. rostratum and S. citrullifolium.
[59] A small region exists where the species meet in a cline—sustained by selection due to a flowering time divergence, thought to represent reinforcement taking place.
[6] The cases met several criteria such as the trait in question serving as a reproductive barrier and if there existed clear patterns of sympatry versus allopatry.
[6] Out of the 48 candidates, 69 percent (33 cases) found enhanced isolation in sympatry, suggesting that the pattern predicted by reinforcement is common in nature.
[6] A survey of the rates of speciation in fish and their associated hybrid zones found similar patterns in sympatry, supporting the occurrence of reinforcement.
