page_title: Evidence for speciation by reinforcement
section_title: Molluscs
hierarchical_section_title: Evidence for speciation by reinforcement / Evidence from nature / Molluscs
caption_attribution_description: English: Example of left- and right coiling in gastropods: figure at left side: Neptunea angulata (=contraria auct.) as left coiled; figure at right side: Neptunea despecta as right coiled species. Both are fossil from Pliocene deposits near Antwerp (Belgium), North Sea Basin.
context_page_description: Reinforcement is a process within speciation where natural selection increases the reproductive isolation between two populations of species by reducing the production of hybrids. 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. Differences in behavior or biology that inhibit formation of hybrid zygotes are termed prezygotic isolation. Reinforcement can be shown to be occurring by measuring the strength of prezygotic isolation in a sympatric population in comparison to an allopatric population of the same species. Comparative studies of this allow for determining large-scale patterns in nature across various taxa. Mating patterns in hybrid zones can also be used to detect reinforcement. Reproductive character displacement is seen as a result of reinforcement, so many of the cases in nature express this pattern in sympatry. Reinforcement's prevalence is unknown, but the patterns of reproductive character displacement are found across numerous taxa, and is considered to be a common occurrence in nature.
context_section_description: Partula suturalis is polymorphic for shell chirality in that it has two forms: sinistral (left-handed) and dextral (right-handed) shells, unlike other monomorphic species on the island of Mo'orea which have only one form (with the exception of P. otaheitana). This polymorphic trait has a direct effect on mate choice and mating behavior; as shown in laboratory mating tests that opposite-coil pairs mate much less often. In areas where P. suturalis lives sympatrically with other sinistral and dextral Partula species, the opposite P. suturalis morph is typically present. Butlin succinctly describes one example of this unique pattern:
  P. suturalis is sympatric with the dextral P. aurantia and sinistral P. olympia, whose ranges abut but do not overlap; P. suturalis is sinestral in the range of P. aurantia and dextral in the range of P. olympia and does not normally hybridize with either species. However, where their ranges meet there is a sharp transition in the coil of P. suturalis and in this transition zone it hybridizes with both P. aurantia and P. olympia.
  The reversal in chirality to sinistrality must have evolved as an isolating mechanism, with patterns of reproductive character displacement suggesting speciation by reinforcement.
  Satsuma largillierti lives on the western half of Okinawa Island while Satsuma eucosmia lives on the eastern half. Both populations overlap in sympatry along the middle of the island, where the penis length of the species differs significantly in sympatry (a case of reproductive character displacement), but not in allopatry. 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.
  The abalone genus Haliotis has 19 species that occur in sympatry and one that occurs in allopatry. Of the sympatric species, they all contain sperm lysin that drives gamete isolation, but the allopatric species does not. A similar pattern of sperm lysin differentiation is found in the mussel species Mytilus galloprovincalis and M. trossulus and has likely occurred within the last 200 years due to human-mediated distribution by ships.
caption: Some gastropod shells, like those of Partula can coil in sinistral and dextral directions such as these, Neptunea angulata (left) and N. despecta (right) shells.
exif: {}