Speciation

Also see:
Punctuated Equilibrium

Allopatric Speciation Peripatric Speciation Parapatric Speciation Sympatric Speciation

Speciation Introduction Peripatric Speciation

Speciation is the evolutionary process by which new biological species arise. Four speciation processes are recognized that are differentiated by the extent to which a population from within a population becomes geographically isolated from the parent population.

Allopatric Speciation

In allopatric speciation, a population splits into two geographically isolated populations due to formation of a barrier between portions of a population, for example, because of mountain building as depicted in the animal to the left. The isolated populations then experience differentiating genotypic and phenotypic divergence as a result of different selective pressures in their differing environments. Additionally, genetic drift will occur differently, eventually differentiating the population’s genotypes and phenotypes. The separated populations will also experience different mutations that may persist differently in their respective environments. Should the populations be eventually recombine in the same environment, they have each evolved differently to an extent that they remain isolated because changes are too great to enable genetic mixing through sexual reproduction. Allopatric speciation is not necessarily precluded if some individuals from one group cross the barrier and mate with members of the other group. In other words, allopatric speciation can occur if gene flow between groups is greatly reduced, but not entirely eliminated.

Allopatric Speciation Sequence
A) Original population of brown butterflies	B) Mountain formation separates the population into two sexually isolated subpopulations	C) Survival is favored by evolution of color green in one sexually isolated environment	D) The old brown and new green species eventually migrate and mix but do not mate

Peripatric Speciation

Peripatric speciation closely resembles allopatric speciation. New species arise in isolated, smaller peripheral populations that are prevented from exchanging genes with the main population. It is related to the concept of a founder effect, since small populations often undergo bottlenecks. The essential mechanism of paripatric speciation is genetic drift. In peripatric speciation, small population size would makespeciation a more likely consequence of geographic isolation since genetic drift acts more quickly in small populations. Genetic drift, and perhaps strong selective pressures, would cause rapid genetic change in a small population. This genetic change could be sufficiently substantial to result in a new species. Because the extent of genetic drift can’t be measures after the fact, testing the hypothesis of this mode of speciation is very difficult.

Parapatric Speciation

Parapatric speciation occurs during incomplete, only partial separation of the geographical environments of two diverging populations. Individuals within each species may come in contact or cross habitats at times, but reduced fitness of the heterozygote leads to selection for behaviors or mechanisms that prevent their inter-breeding. Thus, in parapatric speciation, selection acts on existing variation within a single environment, as opposed to selection proceeded differently in separated environmental niches, as in the case of peripatric and allopatric speciation. Ecologists refer to parapatric and peripatric speciation in terms of ecological niches. In parapatric speciation there is no physical barrier to gene flow. The population members are share the same environment, but fail to mate randomly. Instead, individuals are more likely to mate with their geographic neighbors than with individuals in a more distant part of the environment. In this mode, divergence may happen because of reduced gene flow within the population and varying selection pressures across the population’s physical range.

Sympatric Speciation

In sympatric speciation, two or more progeny species arise from a single parent species all within the same geographic environment, that is, species diverge while inhabiting the same place. The existence of the sympatric speciation mechanism remains a scientific debate since gene flow due to interbreeding would be expected to overwhelm any emerging genetic differentiation within a combined population.