Speciation by lineage splitting

The majority of research effort on speciation has been on how lineages split into two. Speciation of this form has an important geographic context,

Table 12.1 The geographic context of some modes of speciation (from Levin 2000)

Size of speciating entity

Sympatric

Spatial relationship

Contiguous

Proximal

Distal

Local

Sympatric speciation

Peripatric speciation

Disjunct Speciation

Big

Parapatric speciation

Vicariance speciation

which has been the subject of much controversy (Table 12.1). The last two columns of Table 12.1 represent so-called allopatric speciation, speciation through geographic isolation. Allopatric speciation can be further subdivided according to the size of isolated populations and how far they are separated from each other. Under the peripatric model of peripheral isolates favoured by Mayr, the speciating entity is small, and close to the main parent population. Disjunct speciation differs in that the speciating entity is dispersed far from the original. In both these modes of speciation stochastic processes, such as founder effects and drift, may be important as the speciating population is small, though selection may also be operating to change gene frequencies. In vicariance speciation, isolating barriers emerge in an existing part of the geographic range, leading to isolation between the fragmented populations. Selection is likely to predominate here as the force causing genetic differentiation between species, and ultimately isolation, because the isolated populations are usually larger.

Some authors regard these modes of speciation as theoretically trivial, because we expect geographically isolated populations to differentiate over time by one process or another. In addition, there is good evidence that all the allopatric modes do exist in nature. For example, clear-cut cases of vicariance come from marine organisms separated by the Isthmus of Panama (approximately 3 Ma). A large number of closely related species are separated by this barrier from fishes and crabs to shrimps and sea urchins (Lessios 1998). Disjunct speciation has occurred in the 14 species of Darwin's finches on the Galapogos islands, all descended from vagrant individuals arriving from the American mainland about 3 Ma (Grant and Grant 1996). Several likely cases of peripatric speciation, occurring in birds on offshore archipelagos which differ from nearby mainland relatives are documented by Mayr (1940,1963) in the Carribean and East Indies. Thus, allopatric speciation in general is highly plausible.

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