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MODES OF SPECIATION All forms of speciation have actually taken place over the course of evolution, though it still remains a subject of debate as to the relative importance of each mechanism in driving biodiversity. 1 available online There is some debate as to the rate at which speciation events occur over geologic time. While some evolutionary biologists claim that speciation events have remained relatively constant over time, some Palaeontologist s such as Niles Eldredge and Stephen Jay Gould have argued that species usually remain unchanged over long stretches of time, and that speciation occurs only over relatively brief intervals, a view known as '' Punctuated Equilibrium ''. Allopatric (geographic) During Allopatric Speciation , a population splits into two geographically isolated Allopatric populations (for example, by Habitat Fragmentation due to geographical change such as mountain building or Emigration ). The isolated populations then undergo genotypic and/or phenotypic divergence as they (a) become subjected to dissimilar Selective 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 Parapatric (somewhat geographic) In Parapatric Speciation , the zones of two diverging populations are separate but do overlap. There is only partial separation afforded by geography, so individuals of each species may come in contact or cross the barrier from time to time, but reduced fitness of the Heterozygote leads to selection for behaviours or mechanisms which prevent breeding between the two species. Observed instances
Sympatric (non-geographic) In Sympatric Speciation , species diverge while inhabiting the same place ( Sympatric ). Examples of sympatric speciation are found in insects which become dependent on different Host plants in the same area. Increased Ploidy Level s, i.e. Polyploidy , is a mechanism often attributed to causing some speciation events in Sympatry . It should be noted that not all polyploids are completely reproductively isolated from their parental plants, so an increase in chromosome number may not result in the complete cessation of gene flow between the incipient polyploids and their parental diploids. Observed instances
Polyploidy Polyploidy OTHER RELATED TERMS Speciation by reinforcement Reinforcement is the process by which natural selection increases reproductive isolation. Reinforcement can occur as follows: When two populations previously kept apart come back into contact, the Reproductive Isolation between them might be complete or incomplete. If it is complete, speciation has occurred. If it is incomplete, hybrids would be produced. If the hybrids had lower fitness than either parental form, selection would act to increase the reproductive isolation because each form would do better not to mate with the other and form the disadvantageous hybrids. Speciation might then be speeded up by favoring genes which caused individuals to avoid mating with hybrids. Reinforcement is a necessary requirement for both the parapatric and sympatric theories of speciation. It is the process by which a hybrid zone (an area of contact between different forms of a species) develops into a full species barrier. Reinforcement is known as secondary reinforcement if the reproductive isolation has partly evolved allopatrically, and is then reinforced when the two populations come into secondary contact. Reinforcement could occur whenever two forms coexist, and the hybrids between them have lower fitness than crosses within each form. Reinforcement can be simulated by artificial selection experiments. By continually selecting for assortative mating (the tendency of like to mate with like), it has been possible to obtain significant changes in prezygotic isolation mechanisms. However, the theoretical conditions for speciation to take place by reinforcement are difficult and it is controversial whether the process takes place in nature.Ecologists speak about speciation in terms of ecological niches, where it is thought that a niche must be available in order for a new species to be successful. Citation from: http://www.blackwellpublishing.com/ridley/tutorials/Speciation15.asp (Homepage of the book ''Evolution'' by Mark Ridley ) ARTIFICIAL SPECIATION New species have been created by domesticated Animal Husbandry , but the initial dates and exact methods of the initiation of such species are not clear. The best-documented creation of new species in the laboratory was performed by W.R. Rice and G.W. Salt in the 1980s and described in their paper.2. Rice and Salt bred fruit flies, '' Drosophila Melanogaster ,'' using a maze with three different choices such as light/dark and wet/dry. Each generation was placed into the maze, and the groups of flies which came out of two of the eight exits were set apart to breed with each other in their respective groups. After 35 generations, the two groups and their offspring would not breed with each other even when doing so was their only opportunity to reproduce. The history of such attempts is described in "Laboratory experiments on speciation: What have we learned in forty years?"3 For more information see also ''Observed Instances of Speciation'' from the Talk.Origins Frequently Asked Questions . REFERENCES |
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