Speciation

Speciation
To understand speciation, it is important to have a definition of a species. A species is a group of populations through which genes can flow and whose offspring have a fitness equal to the parents. As evolution progresses, gene flow is eventually stopped between one population and the species as a whole. As a population undergoes changes in gene frequencies it could diverge entirely until it is reproductively distinct from other populations of the species from which it diverged. This is called phyletic evolution or anagenesis. Alternatively the population could evolve into two distinct species that exist simultaneously. This is called true speciation or cladogenesis. The development of the cereal crops is an example.


Cladogenesis occurs because reproductive-isolating mechanisms prevents two sub-populations from interbreeding. This reproductive barrier could be the result of 1) the isolation of one portion of the population by some physical barrier; 2) a sub-population becoming established in a new ecological niche not previously occupied by the species; or 3) a polymorphism becoming established in a population before it moves to a new ecological niche.

Speciation involves the genetic change in a subgroup of a population that renders the new population incapable of reproducing offspring with the original population. If a reproductive barrier occurs because of isolation by a physical barrier, the isolated population can evolve and develop into a new species. This process is termed allopatric speciation. This has been considered for a long time to be the primary mode of speciation.

Speciation can also occur when a subpopulation migrates into a new niche. This is termed parapatric speciation, and this process seems to have been used by some annual plants.

The final form of speciation is called sympatric speciation. This type of speciation occurs when a subpopulation that occupies the same niche as the remainder of the species develops a unique mutation that prevents it from mating with the original population. That new species may also have an ecological advantage which permits its establishment as a species in the same niche. A good example of this method of speciation is the development of the new saltmarsh species Spartina townsendii, that was derived from S. alterniflora (American saltmarsh grass) and S. maritima (European saltmarsh grass), but is reproductively incompatible with either parent. This new species is better adapted to the coastal regions of Holland than either of the parental species and was able to better establish itself in that niche.

Copyright © 1997. Phillip McClean

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