Amphibians ,
Litoria ewingi The two frog species
Litoria ewingi and
L. verreauxii live in southern Australia with their two ranges overlapping. The species have very similar calls in allopatry, but express
clinal variation in sympatry, with notable distinctness in calls that generate female preference discrimination. Three species of
true frogs (
Lithobates sphenocephalus,
L. berlandieri, and
L. blairi) are temporally isolated in that their breeding seasons are spaced out in areas where they live in sympatry, but not where they live in allopatry. Selection against interspecific mating due to low hybrid fitness and low hybrid fertility has reinforced the observed character displacement of breeding times. About 6500 years ago, the rainforests reconnected, bringing the diverged, incipient populations of
Spicicalyx serrata into secondary contact. The species contact zones exhibit, "strong postzygotic selection against hybrids" and enhanced isolation from differences in mating call. An alternative to detecting reproductive character displacement in populations that overlap in sympatry is measuring rates of hybridization in contact zones.
Birds , one of
Darwin's finches of the
Galápagos The
Ficedula flycatchers exhibit a pattern that suggests premating isolation is being reinforced by sexual selection. The pied flycatcher (
Ficedula hypoleuca) has brown females, brown males, and black-and-white males. The related collard flycatcher (
Ficedula albicollis) has brown females and only black-and-white males. The two species exist in separate populations that overlap in a zone of sympatry. though there is also evidence indicating that such
character displacement is explained by heterospecific competition for territory rather than reinforcement. Mating choice tests of the species find that females of both species choose conspecific males in
sympatry, but heterospecific males in allopatry
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. Other birds such as the dark and light subspecies of the
western grebe show enhanced prezygotic isolation. It has been argued that reinforcement is extremely common in birds and has been documented in a wide range of bird species.
Crustaceans Reproductive character displacement in body size was detected in sympatric populations of
Orconectes rusticus and
O. sanbornii.
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. '', one of several species in the genus that produce fertile hybrids 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. Here, the evolution of female egg receptors is thought to pressure bindin evolution in a
selective runaway process. Female benthic morphs in sympatric populations actively discriminate against limnetic males, resulting in low rates of crossing (some gene flow has occurred between the morphs) and low fitness hybrids.
Fungi A strong case of reinforcement occurring in fungi comes from studies of
Neurospora. In crosses between different species in the genera, sympatric pairs show low reproductive success, significantly lower than allopatric pairs. 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). in the U.S. The yellow area corresponds to the sympatric overlap of
Magicicada neotredecim (blue) with
Magicicada tredecim (red). Where the ranges of the
cicada species
Magicicada tredecim and
M. neotredecim overlap (where they are sympatric), the pitch of
M. neotredecim male calling songs is roughly 1.7 kHz compared to 1.1 kHz for those of
M. tredecim, with corresponding female song pitch preference differences. In allopatric
M. neotredecim populations, the mating call pitch is 1.3–1.5 kHz. A similar pattern exists with
Allonemobius fasciatus and
A. socius, species of ground crickets in eastern North America. Males in sympatric populations of the damselflies
Calopteryx maculata and
C. aequabilis are able to discriminate between females of different species better than those in allopatric populations; with females of
C. aequabilis in sympatric populations exhibiting lighter wing colors compared to allopatric females—an illustration of reproductive character displacement. Fifteen species of sympatrically distributed
Agrodiaetus butterflies with pronounced differences in wing color pattern likely arose as a result of speciation by reinforcement.
Phylogenetic patterns indicate the differences arose in allopatry and were reinforced when the distributions came into secondary contact.
Drosophila '' fruit fly
Drosophila is one of the most studied species in speciation research.
Dobzhansky and Koller were the first to study isolation between
Drosophila species. or the enhanced isolation found in sympatric populations of
D. mojavensis and
D. arizonae in southwest America. Rare, sterile hybrids form between
D. pseudoobscura and
D. persimilis, with sympatric
D. pseudoobscura females discriminating against
D. persimilis males; more so than allopatric populations. Other
Drosophila research on reinforcement has been from laboratory experiments and is discussed below. On the east coast of Australia,
D. serrata shares a zone of sympatric overlap with the closely related species
D. birchii. The species exhibits reproductive character displacement, with sexual selection operating on the
hydrocarbons of the
flies cuticle. Reinforcement appears to be driving their speciation in nature, supported by simulated experimental laboratory populations.
Mammals The deer mice
Peromyscus leucopus and
P. gossypinus exhibit reproductive character displacement in mating preferences, with heterospecific matings taking place between the species.
Molluscs s, like those of
Partula can coil in
sinistral and dextral directions such as these,
Neptunea angulata (left) and
N. despecta (right) shells.
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. 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. with patterns of reproductive character displacement suggesting speciation by reinforcement.), 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. In the Texas wildflower
Phlox drummondii,
cis-regulatory mutations of genes that code for
anthocyanin pigmentation have caused genetic divergence of two populations. Hybrids (between
P. drummondii and
P. cuspidata) with
maladaptive, intermediate characteristics are under-pollinated; increasing reproductive isolation through reinforcement. Similarly, in
P. pilosa and
P. glaberrima, character displacement of petal color has been driven by selection, aided by pollen discrimination. 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. The
bishop pine is divided into two populations distinguished by
monoterpene,
stomata, and
alloenzyme differences; flowering time; and needle color: blue foliage in the northern population and green foliage in the southern populations in California. 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. •
Anthoxanthum odoratum •
Costus plants:
Costus allenii,
C. laevis, and
C. guanaiensis;
C. pulverulentus and
C. scaber • 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.
Allochrony may play a role. == Comparative studies ==