Sympatry

In biology, two related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter one another. An initially interbreeding population that splits into two or more distinct species sharing a common range exemplifies sympatric speciation. Such speciation may be a product of reproductive isolation – which prevents hybrid offspring from being viable or able to reproduce, thereby reducing gene flow – that results in genetic divergence. Sympatric speciation does not imply secondary contact, which is speciation or divergence in allopatry followed by range expansions leading to an area of sympatry. Sympatric species or taxa in secondary contact may or may not interbreed. In biology, two related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter one another. An initially interbreeding population that splits into two or more distinct species sharing a common range exemplifies sympatric speciation. Such speciation may be a product of reproductive isolation – which prevents hybrid offspring from being viable or able to reproduce, thereby reducing gene flow – that results in genetic divergence. Sympatric speciation does not imply secondary contact, which is speciation or divergence in allopatry followed by range expansions leading to an area of sympatry. Sympatric species or taxa in secondary contact may or may not interbreed. Four main types of population pairs exist in nature. Sympatric populations (or species) contrast with parapatric populations, which contact one another in adjacent but not shared ranges and do not interbreed; peripatric species, which are separated only by areas in which neither organism occurs; and allopatric species, which occur in entirely distinct ranges that are neither adjacent nor overlapping. Allopatric populations isolated from one another by geographical factors (e.g., mountain ranges or bodies of water) may experience genetic—and, ultimately, phenotypic—changes in response to their varying environments. These may drive allopatric speciation, which is arguably the dominant mode of speciation. The lack of geographic isolation as a definitive barrier between sympatric species has yielded controversy among ecologists, biologists, and zoologists regarding the validity of the term. As such, researchers have long debated the conditions under which sympatry truly applies, especially with respect to parasitism. Because parasitic organisms often inhabit multiple hosts during a life cycle, evolutionary biologist Ernst Mayr stated that internal parasites existing within different hosts demonstrate allopatry, not sympatry. Today, however, many biologists consider parasites and their hosts to be sympatric (see examples below). Conversely, zoologist Michael J. D. White considered two populations sympatric if genetic interbreeding was viable within the habitat overlap. This may be further specified as sympatry occurring within one deme; that is, reproductive individuals must be able to locate one another in the same population in order to be sympatric. Others question the ability of sympatry to result in complete speciation: until recently, many researchers considered it nonexistent, doubting that selection alone could create disparate, but not geographically separated, species. In 2003, biologist Karen McCoy suggested that sympatry can act as a mode of speciation only when 'the probability of mating between two individuals depend on their genotypes, dispersed throughout the range of the population during the period of reproduction'. In essence, sympatric speciation does require very strong forces of natural selection to be acting on heritable traits, as there is no geographic isolation to aid in the splitting process. Yet, recent research has begun to indicate that sympatric speciation is not as uncommon as was once assumed. Syntopy is a special case of sympatry. It means the joint occurrence of two species in the same habitat at the same time. Just as the broader term sympatry, 'syntopy' is used especially for close species that might hybridise or even be sister species. Sympatric species occur together in the same region, but do not necessarily share the same localities as syntopic species do. Areas of syntopy are of interest because they allow to study how similar species may coexist without outcompeting each other. As an example, the two bat species Myotis auriculus and M. evotis were found to be syntopic in North America. In contrast, the marbled newt and the northern crested newt have a large sympatric range in western France, but differ in their habitat preferences and only rarely occur syntopically in the same breeding ponds. The lack of geographic constraint in isolating sympatric populations implies that the emerging species avoid interbreeding via other mechanisms. Before speciation is complete, two diverging populations may still produce viable offspring. As speciation progresses, isolating mechanisms – such as gametic incompatibility that renders fertilization of the egg impossible – are selected for in order to increase the reproductive divide between the two populations. Sympatric groups frequently show a greater ability to discriminate between their own species and other closely related species than do allopatric groups. This is shown in the study of hybrid zones. It is also apparent in the differences in levels of prezygotic isolation (by factors that prevent formation of a viable zygote) in both sympatric and allopatric populations. There are two main theories regarding this process: 1) differential fusion, which suggests that only populations with a keen ability to discriminate between species will persist in sympatry; and 2) character displacement, which implies that distinguishing characteristics will be heightened in areas where the species co-occur in order to facilitate discrimination. Reinforcement is the process by which natural selection reinforces reproductive isolation. In sympatry, reinforcement increases species discrimination and sexual adaptation in order to avoid maladaptive hybridization and encourage speciation. If hybrid offspring are either sterile or less-fit than non-hybrid offspring, mating between members of two different species will be selected against. Natural selection decreases the probability of such hybridization by selecting for the ability to identify mates of one's own species from those of another species.