notes | speciation.01

‘Speciation is the evolutionary process by which new biological species arise. Term used to descibe the splitting of lineages or ‘cladogensis’ as opposed to ‘anagesnsis’ or ‘phyletic evolution’ occuring within lineages. Whether genetic drift is a minor or major contributor to speciation is the subject of much ongoing discussion. There are four geographic modes of speciation in nature based on the exdtent to which speciating populations are geographically isolated from one another: allopatric, peripatric, parapatric, and sympatric. Speciation may also be induced artificially, through animal husbandry or laboratory experiments. Observed examples of each kind of speciation are provided throughout.All forms of natural speciation have taken place over the course of evolution; however it still remains a subject of debate as to the relative importance of each mechanism in driving biodiversity.

Allopatric:
During allopatric speciation, a population splits into two geogrpahically isolated allopatric populations(for example, by habitat fragmentaiton due to geographical change such as mountain building or social change such as emigration). The isolated populations then undergo genotypic and/or phenotypic divergence as they (a) become subject to dissimilar slective pressures or (b) they independently undergo genetic drift. When the populations come back into contact, they have evolved such that they are reproductively isolated and are no longer capable of exchanging genes.

Observed Instances
Island genetic, the tendency of small, isolated genetic pools to produce unusual traits, has been observed in mayn circumstances, including insular dwarfism and the radical changes among certain famous island chains, for example of Komodo. see adaptive evolutionary radiation.

Peripatric
In peripatric speciation, new species are formed in isolated, small peripheral populations that are prevented from exhanging genes with the main population. It is related to the concept of a founder effect, since small populations often undergo bottlenecks. Genetic drift is often proposed to play a significant role in peripatric speciation.

Parapetric
In parapatric speciation, the zones of two diverging populations are serpate but do overlap. There is only partial seperation afforded by geography, so individuals of each species may come in contact or cross teh barrier form time to time, but recuded fitness of the heterzugote leads to selection for behaviours or machanisms that prevent breeding between two species. Ecologists refer to parapetric and peripatric speciation in terms of ecological niches. A niche must be available in orer for a new species to be successful.

Sympatric:
In sympatric speciation, species diverge while inhabiting the same place. Often cited examples of sympatric speciation are found in insects that become dependent on different host plants int he same area. However, teh existence of sympatric speciation as a mechanism of speciation is still hotley contested. People have argued that the evidences of sy mpatric speciation are in fact examples of micro-allopatric, or heteropatric speciation. The most widely accepted example of sympatric speciation is that of the chichlids of Lake Nubugabo in East Africa, which is thought to be due to sexual selection. Sympatric speciation refers to the formation of two or more descendents species from a single ancestral species all occupying the same geogrpahic location.

Artificial speciation:
New species have been created by domesticated animal husbandry, but the intial dates and methods of the initiation of such species are not clear. For example, domestic sheep were created by hybridisation, and no longer produce viable offspring with ovis orientalis, one species from which they are descended. Domestic cattle, on the other hand, can be considered the same species as several varieties of wild ox, as they readily produce fertile offspring with them.

Human speciation
Humans have genetic simliarties with chimpanzees and gorilllas, suggesting common ancestors. Analysis of genetic drift and recombination using a Markov model suggests humans and chimpanzees speciated apart 4.1 million years ago.

Heteropatric Speciation:
Heteropatric and heteropatry are terms from biogeography, referring to organisms whose geographical ranges overlap or are even identical, so that they occur together at least in some places, but which occupy ecological niches distinct enough to prevent frequent hybridization. Such organisms are sually closely related, their distribution and ecology being the result of heteropatric speciation.Heteropatric speciation is a special case of sympatric speciation that occurs when different ecotypes or races of the same species geographically coexist but exploit different niches in the same patchy or heterogreneous environment. Thus heteropatric speciation is a refinement of our notion of sympatric speciation in that it represents a behavorial rather than geographic barrier to the flow of genes among diverging groups within a population. The importance of behavioral versus geographic distiction, Wayne Getz and Veijo Kaitala introduced the term heteropatry in their extensions of Maynard Smiths analysis of conditions that facilitate symaptric speciation.


Wikipedia; Speciation

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