Parapatric speciation

In biogeography, the terms parapatric and parapatry are often used to describe the relationship between organisms whose ranges do not significantly overlap but are immediately adjacent to each other; they do not occur together except in a narrow contact zone. Parapatry is a geographical distribution opposed to sympatry (same area) and allopatry or peripatry (two similar cases of distinct areas).

Various "forms" of parapatry have been proposed and are discussed below. Coyne and Orr in Speciation categorise these forms into three groups: clinal (environmental gradients), "stepping-stone" (discrete populations), and stasipatric speciation in concordance with most of the parapatric speciation literature. Henceforth, the models are subdivided following a similar format.

Charles Darwin was the first to propose this mode of speciation. It was not until 1930, when Ronald Fisher published The Genetical Theory of Natural Selection where he outlined a verbal theoretical model of clinal speciation. In 1981, Joseph Felsenstein proposed an alternative, "discrete population" model (the "stepping-stone" model). Since Darwin, a great deal of research has been conducted on parapatric speciation—concluding that its mechanisms are theoretically plausible, "and has most certainly occurred in nature".

Models

Mathematical models, laboratory studies, and observational evidence supports the existence of parapatric speciation's occurrence in nature. The qualities of parapatry imply a partial extrinsic barrier during divergence; thus leading to a difficulty in determining whether this mode of speciation actually occurred, or if an alternative mode (notably, allopatric speciation) can explain the data. This problem poses the unanswered question as to its overall frequency in nature.

Parapatric speciation can be understood as a level of gene flow between populations where ${\displaystyle m=0}$ in allopatry (and peripatry), ${\displaystyle m=0.5}$ in sympatry, and midway between the two in parapatry. Intrinsic to this, parapatry covers the entire continuum; represented as ${\displaystyle 0<m<0.5}$. Some biologists reject this delineation, advocating the disuse of the term "parapatric" outright, "because many different spatial distributions can result in intermediate levels of gene flow". Others champion this position and suggest the abandonment of geographic classification schemes (geographic modes of speciation) altogether.

Natural selection has been shown to be the primary driver in parapatric speciation (among other modes), and the strength of selection during divergence is often an important factor. Parapatric speciation may also result from reproductive isolation caused by social selection: individuals interacting altruistically.

Environmental gradients

Due to the continuous nature of a parapatric population distribution, population niches will often overlap, producing a continuum in the species' ecological role across an environmental gradient. Whereas in allopatric or peripatric speciation—in which geographically isolated populations may evolve reproductive isolation without gene flow—the reduced gene flow of parapatric speciation will often produce a cline in which a variation in evolutionary pressures causes a change to occur in allele frequencies within the gene pool between populations. This environmental gradient ultimately results in genetically distinct sister species.

Fisher's original conception of clinal speciation relied on (unlike most modern speciation research) the morphological species concept. With this interpretation, his verbal, theoretical model can effectively produce a new species; of which was subsequently confirmed mathematically. Further mathematical models have been developed to demonstrate the possibility of clinal speciation with most relying on, what Coyne and Orr assert are, "assumptions that are either restrictive or biologically unrealistic".

A mathematical model for clinal speciation was developed by Caisse and Antonovics that found evidence that, "both genetic divergence and reproductive isolation may therefore occur between populations connected by gene flow". This research supports clinal isolation comparable to a ring species (discussed below), except that the terminal geographic ends do not meet to form a ring.

Doebeli and Dieckmann developed a mathematical model that suggested that ecological contact is an important factor in parapatric speciation and that, despite gene flow acting as a barrier to divergence in the local population, disruptive selection drives assortative mating; eventually leading to a complete reduction in gene flow. This model resembles reinforcement with the exception that there is never a secondary contact event. The authors conclude that, "spatially localized interactions along environmental gradients can facilitate speciation through frequency-dependent selection and result in patterns of geographical segregation between the emerging species." However, one study by Polechová and Barton disputes these conclusions.

Ring species

The concept of a ring species is associated with allopatric speciation as a special case; however, Coyne and Orr argue that Mayr's original conception of a ring species does not describe allopatric speciation, "but speciation occurring through the attenuation of gene flow with distance". They contend that ring species provide evidence of parapatric speciation in a non-conventional sense. They go on to conclude that:

Nevertheless, ring species are more convincing than cases of clinal isolation for showing that gene flow hampers the evolution of reproductive isolation. In clinal isolation, one can argue that reproductive isolation was caused by environmental differences that increase with distance between populations. One cannot make a similar argument for ring species because the most reproductively isolated populations occur in the same habitat.

Discrete populations

Referred to as a "stepping-stone" model by Coyne and Orr, it differs by virtue of the species population distribution pattern. Populations in discrete groups undoubtedly speciate more easily than those in a cline due to more limited gene flow. This allows for a population to evolve reproductive isolation as either selection or drift overpower gene flow between the populations. The smaller the discrete population, the species will likely undergo a higher rate of parapatric speciation.

Several mathematical models have been developed to test whether this form of parapatric speciation can occur, providing theoretical possibility and supporting biological plausibility (dependent on the models parameters and their concordance with nature). Joseph Felsenstein was the first to develop a working model. Later, Sergey Gavrilets and colleagues developed numerous analytical and dynamical models of parapatric speciation that have contributed significantly to the quantitative study of speciation. (See the "Further reading" section)

Para-allopatric speciation

Further concepts developed by Barton and Hewitt in studying 170 hybrid zones, suggested that parapatric speciation can result from the same components that cause allopatric speciation. Called para-allopatric speciation, populations begin diverging parapatrically, fully speciating only after allopatry.

Stasipatric models

One variation of parapatric speciation involves species chromosomal differences. Michael J. D. White developed the stasipatric speciation model when studying Australian morabine grasshoppers (Vandiemenella). The chromosomal structure of sub-populations of a widespread species become underdominate; leading to fixation. Subsequently, the sub-populations expand within the species larger range, hybridizing (with sterility of the offspring) in narrow hybrid zones. Futuyama and Mayer contend that this form of parapatric speciation is untenable and that chromosomal rearrangements are unlikely to cause speciation. Nevertheless, data does support that chromosomal rearrangements can possibly lead to reproductive isolation, but it does not mean speciation results as a consequence.