Evidence for speciation by reinforcement
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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.[3]: 354 [4][5] Differences in behavior or biology that inhibit formation of hybrid zygotes are termed prezygotic isolation. Reinforcement can be shown to be occurring (or to have occurred in the past) by measuring the strength of prezygotic isolation in a sympatric population in comparison to an allopatric population of the same species.[3]: 357 Comparative studies of this allow for determining large-scale patterns in nature across various taxa.[3]: 362 Mating patterns in hybrid zones can also be used to detect reinforcement.[6] Reproductive character displacement is seen as a result of reinforcement,[7] so many of the cases in nature express this pattern in sympatry. 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.[6] Studies of reinforcement in nature often prove difficult, as alternative explanations for the detected patterns can be asserted.[3]: 358 Nevertheless, empirical evidence exists for reinforcement occurring across various taxa[7] and its role in precipitating speciation is conclusive.[8]
Evidence from nature
Amphibians
The two frog species
Allopatric populations of
Three species of
The rainforests of northeast Queensland, Australia were separated into north and south
An alternative to detecting reproductive character displacement in populations that overlap in sympatry is measuring rates of hybridization in contact zones.
Birds
The Ficedula flycatchers exhibit a pattern that suggests premating isolation is being reinforced by sexual selection.
Crustaceans
Reproductive character displacement in body size was detected in sympatric populations of
Echinoderms
An example of gametic isolation involves the allopatric sea urchins (Arbacia) have minimal bindin differences (bindin is a protein involved in the process of sea urchin fertilization, used for species-specific recognition of the egg by the sperm) and have insufficient barriers to fertilization.[3]: 243 Comparison with the sympatric species Echinometra and Strongylocentrotus of the Indo-Pacific finds that they have significant differences in bindin proteins for fertilization and marked fertilization barriers.[24]
Laboratory matings of closely related sea urchin species Echinometra oblonga and E. sp. C (the species is unnamed, dubbed C) produce fertile and viable hybrids, but are unable to fertilize eggs of the parent species due to divergence of the alleles that code for bindin proteins: an example of post-zygotic isolation.[3]: 343–344 Populations in sympatry manifest this difference in bindin protein versus those in allopatry.[3]: 343–344 Selection actively acts against the formation of hybrids in both nature (as no documented cases of hybrids have been found) and in the laboratory.[25] Here, the evolution of female egg receptors is thought to pressure bindin evolution in a selective runaway process.[25] This example of reproductive character displacement is highly suggestive of being the result of—and has been cited as strong evidence for—reinforcement.[25][3]: 343–344
Fish
In British Columbia,
Fungi
A strong case of reinforcement occurring in fungi comes from studies of Neurospora.[28] In crosses between different species in the genera, sympatric pairs show low reproductive success, significantly lower than allopatric pairs.[28] This pattern is observed across small and large geographic scales, with distance correlating with reproductive success.[28] Further evidence of reinforcement in the species was the low fitness detected in the hybrids create from crosses, and that no hybrids have been found in nature, despite close proximity.[28]
Insects
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.[29]
Female mate discrimination is increased with intermediate migration rates between allopatric populations of Timema cristinae (genus Timema) compared to high rates of migration (where gene flow impedes selection) or low rates (where selection is not strong enough).[30][31]
Where the ranges of the
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.[34]
Males in sympatric populations of the damselflies
Fifteen species of sympatrically distributed
Drosophila
Mammals
The deer mice
Molluscs
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).[48] 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.[48] In areas where P. suturalis lives sympatrically with other sinistral and dextral Partula species, the opposite P. suturalis morph is typically present.[9] 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.[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.[48]
The
Plants
Plants are thought to provide suitable conditions for reinforcement to occur.[5] This is due to a number of factors such as the unpredictability of pollination, pollen vectors, hybridization, hybrid zones, among others.[5] The study of plants experiencing speciation by reinforcement has largely been overlooked by researchers;[3]: 364 however, there is evidence of its occurrence in them.[54]
In the Texas wildflower
The
Similar patterns of both character displacement in sympatric populations of species have been documented in:[9][3]: 361
- Agrostis tenuis[60]
- Anthoxanthum odoratum[60]
- Gilia[61]
- Costus plants: Costus allenii, C. laevis, and C. guanaiensis;[62][63] C. pulverulentus and C. scaber[64]
- A unique case of post-zygotic instead of prezygotic isolation has been observed in both Gossypium and Gilia, suggesting that in plants, post-zygotic isolation's role in reinforcement may play a larger role.[3]: 361
- Sympatric populations of Juncus effusus (common rush) exhibits genetic differentiation of plants that flower at different times preventing hybridization.[65] Allochrony may play a role.[66]
Comparative studies
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Assortive mating is expected to increase among sympatric populations experiencing reinforcement.[8] This fact allows for the direct comparison of the strength of prezygotic isolation in sympatry and allopatry between different experiments and studies.[3]: 362 Jerry Coyne and H. Allen Orr surveyed 171 species pairs, collecting data on their geographic mode, genetic distance, and strength of both prezygotic and post-zygotic isolation; finding that prezygotic isolation was significantly stronger in sympatric pairs, correlating with the ages of the species.[3]: 362 Additionally, the strength of post-zygotic isolation was not different between sympatric and allopatric pairs.[8]
This finding lends support the predictions of speciation by reinforcement and correlates well with another later study by Daniel J. Howard.[3]: 363 In his study, 48 studies with observed reproductive character displacement (including plants, insects, crustaceans, molluscs, fish, amphibians, reptiles, birds, and mammals) were analyzed.[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] In addition to Howard's comparative study, he guarded against the potential for positive-result publication bias by surveying 37 studies of hybrid zones. A prediction of reinforcement is that assortive mating should be common in hybrid zones; a prediction that was confirmed in 19 of the 37 cases.[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.[68] One study in the plants Glycine and Silene; however, did not find enhanced isolation.[69]
Laboratory experiments
Laboratory studies that explicitly test for reinforcement are limited.[3]: 357 In general, two types of experiments have been conducted: using artificial selection to mimic natural selection that eliminates the hybrids (often called "destroy-the-hybrids"), and using disruptive selection to select for a trait (regardless of its function in sexual reproduction).[3]: 355–357 Many experiments using the destroy-the-hybrids technique are generally cited as supportive of reinforcement; however, some researchers such as Coyne and Orr and William R. Rice and Ellen E. Hostert contend that they do not truly model reinforcement, as gene flow is completely restricted between two populations.[70][3]: 356 The table below summarizes some of the laboratory experiments that are often cited as testing reinforcement in some form.
Species | Experimental design | Result | Year |
---|---|---|---|
D. paulistorum | Destroyed hybrids | Pre-zygotic isolation | 1976[71] |
D.pseudoobscura & | Destroyed hybrids | Pre-zygotic isolation; reproductive character displacement | 1950[72] |
D. melanogaster | Destroyed hybrids | Pre-zygotic isolation; reproductive character displacement | 1974[73] |
D. melanogaster | Destroyed hybrids | Pre-zygotic isolation; reproductive character displacement | 1956[74] |
D. melanogaster | Destroyed hybrids | No pre-zygotic isolation detected | 1970[75] |
D. melanogaster | Destroyed hybrids | Pre-zygotic isolation | 1953[76] |
D. melanogaster | Destroyed hybrids | Pre-zygotic isolation | 1974[77] |
D. melanogaster | Allopatric populations in secondary contact | N/A | 1982[78] |
D. melanogaster | N/A | 1991[79] | |
D. melanogaster | No pre-zygotic isolation detected | 1966[80][81] | |
D. melanogaster | Allowed gene flow between populations | No pre-zygotic isolation detected | 1969[82] |
D. melanogaster | N/A | No pre-zygotic isolation detected | 1984[83] |
D. melanogaster | Destroyed some hybrids | No pre-zygotic isolation detected | 1983[84] |
D. melanogaster | Disruptive selection | Pre-zygotic isolation; assortive mating; all later replications of the experiment failed | 1962[85] |
D. melanogaster | N/A | N/A | 1997[86] |
D. melanogaster | Destroyed hybrids | Pre-zygotic isolation | 1971[87] |
D. melanogaster | Destroyed hybrids | Pre-zygotic isolation | 1973[88] |
D. melanogaster | Destroyed hybrids | Pre-zygotic isolation | 1979[89] |
Zea mays
|
Destroyed hybrids | Pre-zygotic isolation; reproductive character displacement | 1969[90] |
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