Andropadus virens. Jiggins and Mallet surveyed a range of literature documenting every phase of parapatric speciation in nature positing that it is both possible and likely (in the studied species discussed). A study of tropical cave snails (Georissa saulae) found that cave-dwelling population descended from the above-ground population, likely speciating in parapatry. Partula snails on the island of Mo'orea have parapatrically speciated in situ after a single or a few colonization events, with some species expressing patterns of ring species. In the Tennessee cave salamander, timing of migration was used to infer the differences in gene flow between cave-dwelling and surface-dwelling continuous populations. Concentrated gene flow and mean migration time results inferred a heterogenetic distribution and continuous parapatric speciation between populations. Researchers studying Ephedra, a genus of gymnosperms in North American, found evidence of parapatric niche divergence for the sister species pairs E. californica and E. trifurca. One study of Caucasian rock lizards suggested that habitat differences may be more important in the development of reproductive isolation than isolation time. Darevskia rudis, D. valentini and D. portschinskii all hybridize with each other in their hybrid zone; however, hybridization is stronger between D. portschinskii and D. rudis, which separated earlier but live in similar habitats than between D. valentini and two other species, which separated later but live in climatically different habitats. Marine organisms It is widely thought that parapatric speciation is far more common in oceanic species due to the low probability of the presence of full geographic barriers (required in allopatry). Numerous studies conducted have documented parapatric speciation in marine organisms. Bernd Kramer and colleagues found evidence of parapatric speciation in Mormyrid fish (Pollimyrus castelnaui); whereas Rocha and Bowen contend that parapatric speciation is the primary mode among coral-reef fish. Evidence for a clinal model of parapatric speciation was found to occur in Salpidae. Nancy Knowlton found numerous examples of parapatry in a large survey of marine organisms. See also History of speciation Evidence for speciation by reinforcement References Further reading Quantitative speciation research Biogeography Ecology Evolutionary biology Speciation