Evolutionary Biology/Speciation

The biological species concept, which was first introduced by Ernst Mayr, defines a species as a population whose members have the potential to interbreed to produce viable, fertile offspring, but do not produce healthy offspring with other species.

Different Modes
There are different modes of speciation that can lead to the origin of species. We will focus on allopatric and sympatric speciation, but will also introduce two other modes: parapatric and peripatric. Gene flow, the loss or gain of alleles in a population due to the migration of fertile individuals or between gamete populations, is reduced in all of these speciation modes. A recent trend towards designating speciation events as ecological speciation or nonecological speciation is motivated by a desire to focus more on the process, rather than the geography, leading to speciation events. This dichotomy may be somewhat misleading, however, as empirical work indicates that ecological factors are important in driving speciation regardless of whether or not gene flow is present, leading many to question whether speciation is ever truly nonecological.

Allopatric Speciation
A geographic barrier is the predominant means by which allopatric speciation occurs.

How it works

 * 1) There are two populations that interact by interbreeding, which allows for gene flow (genetic exchange), within a broad geographic range. The resulting offspring are fertile because these populations are of the same species.
 * 2) A geographic barrier arises, isolating the two populations. The newly isolated populations now go through a period of isolation. Some of the individuals within a population might exhibit a trait more suitable for the new environment. This variant allows for differential survivorship and reproduction. Those individuals that carry the trait are more likely to survive in the new environment. Therefore, they will leave more offspring. Gradually, the variant trait will become dominant. Theoretically, this would happen with both of the newly isolated populations resulting in reproductive isolation.
 * 3) The geographic barrier ends. The two populations now have the opportunity to interbreed again. If the populations do not recognize each other as potential mates or cannot produce fertile offspring, speciation has occurred because the populations are reproductively isolated. The two populations have become two different species. If speciation has not occurred, the populations can still interbreed successfully to produce viable, fertile offspring.

An organism's mobility plays a significant role in allopatric specition. Two populations of birds would not be hindered by an emerging mountain range because of their ability to fly. The size and remoteness of a population also play a major role in speciation. There is a greater chance of a species forming in a small, isolated population because natural selection and genetic drift are more likely to alter the gene pool there.

These barriers are not limited to the more recognizable ones, mountain ranges and rivers, though. Populations can also become reproductively isolated by members of a population colonizing a new remote area without the presence of their parent population.

Sympatric Speciation
Speciation does not only occur when there is a geographic barrier separating two populations; Sympatric speciation occurs when divergent selection causes preferred mating with genetically similar individuals but not with the parent population, hence reproductively isolating one part of the population from another.

Sympatric Speciation in Plants
Plants frequently undergo this type of speciation. The most common type of sympatric speciation is a mutant condition called polyploidy, which is caused by a doubling of chromosomes during cell division. Autopolyploid mutations occur when an individual has more than two chromosome sets, all derived from a single species. A more common type of polyploidy species is an allopolyploid, which forms a hybrid from two species. Sympatric speciation generally occurs in plants through hybrids of related species and errors in cell division, producing fertile polyploids.

Sympatric Speciation in Animals
Sympatric speciation is less common in animals than plants, but still exists. Animals can become reproductively isolated by becoming dependent on different resources than those used by their parent species. It is possible for there to be a genetic change that selects for different resources. Sympatric speciation in animals, which isolates the species reproductively, results from preferences for different habitats, food, resources, and breeding partners.

One of the best examples of sympatric speciation in animals is with the 200 different species of cichlids that inhabit Lake Victoria in East Africa. Sympatric speciation has occurred here as a result of variations in available resources, as well as selective mating based on coloration. This idea was tested by a group of scientists at Holland’s University of Leiden using P.pundamilia (blue back) and P. nyererei (red back). Although females of the P. pundamilia species only mated with males of the same species in normal lighting, they mated with the P. nyererei males in monochromatic orange lighting which made the two species appear identical. The resulting hybrids between the two species were still fertile. Mate choice based on coloration led to sympatric speciation within the cichlid species in this case.

Parapatric Speciation
In the absence of a geographic barrier, parapatric speciation can also lead to the emergence of a new species. In this type of speciation, gene flow is reduced because individuals are reproducing with the mates closest to them. Therefore, mating is not random. The phenotypes are gradually divergent from the extreme forms, but are all equally present within the population. Gene flow is reduced within the population. Parapatric speciation cannot produce separate species in the absence of divergent selection.

Peripatric Speciation
This type of speciation correlates (?) with allopatric speciation. It occurs in a few members of the population, which exhibit a different appearance than the majority. Genetic drift plays the major role in speciation. This mode is the hardest to support with evidence.

Resources
Campbell, Reece. Biology, Sixth edition. Benjamin Cummings. 2001.

http://evolution.berkeley.edu/evosite/evo101/VC1dParapatric.shtml

http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ev/m3/s2/index.htm

http://www.pbs.org/wgbh/evolution/library/05/2/l_052_05.html