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Line 37: − + − The commonest method of '''sympatric speciation''' is probably '''[[polyploidy]]'''. Polyploids contain more than two complete sets of chromosomes in each cell. This can occur if there is an error in [[meiosis]] (production of gametes) so that a gamete ends up with two copies of each set of chromosomes instead of one copy. Two of these gametes may fuse to produce an organism that has four sets of each chromosome in their cells, called a '''tetraploid''' organism.+ + + + + + + Line 39:
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'''Speciation''' is the process by which new species arise. Speciation occurs when gene-flow stops between two sub-populations due to geographic or behavioral isolation. During such isolation the gene pool of each sub-population reacts differently to the processes of [[natural selection]] and [[genetic drift]] and and such evolve divergently. If this evolutionary divergence becomes great enought that crosses between individuals from different sub-populations can no longer produce viable offspring each sub-population is considered to be a new species and speciation is said to have occured.
'''Speciation''' is the process by which new species arise. Speciation occurs when gene-flow stops between two sub-populations due to geographic or behavioral isolation. During such isolation the gene pool of each sub-population reacts differently to the processes of [[natural selection]] and [[genetic drift]] and and such evolve divergently. If this evolutionary divergence becomes great enough that crosses between individuals from different sub-populations can no longer produce viable offspring each sub-population is considered to be a new species and speciation is said to have occured.
==Species==
==Species==
A [[species]] is:
A [[species]] is:
Unfortunately the interbreeding test is often difficult to carry out, especially if the organisms being investigated are rare, take a long time to reproduce or do not breed in captivity.
Unfortunately the interbreeding test is often difficult to carry out, especially if the organisms being investigated are rare, take a long time to reproduce or do not breed in captivity.
==Allopatric speciation==
==Modes of Speciation==
The process of speciation can be conveniently broken into four seperate 'modes' of speciation ( allopatric, peripatric, and sympatric) that differ in the degree of geographical isolation between diverging species. Some authors also propose other names for modes of speciation or argue that all speciation events should considered to occur somehere in a continuum from no geograpical islolation to complete geographical isolation.
=== Allopatric speciation===
'''Allopatric speciation''' occurs due to geographical isolation. This is supported by the fact that many islands have their own unique group of species, for example the separation of the [[Galapagos]] islands from the mainland Americas.
'''Allopatric speciation''' occurs due to geographical isolation. This is supported by the fact that many islands have their own unique group of species, for example the separation of the [[Galapagos]] islands from the mainland Americas.
Geographical isolation can occur due to many factors, such as:
Geographical isolation can occur due to many factors, such as:
*Large bodies of water.
*Large bodies of water.
*[[Plate Tectonics|Continental Drift]]
*Deforestation.
*Deforestation.
*Mountainous areas.
*Mountainous areas.
The individuals in the separated area are exposed to different selection pressures than the rest of their species and, over time, their morphology, physiology and biochemistry change significantly so that they are now unable to produce fertile offspring when interbred with the original population.
The individuals in the separated area are exposed to different selection pressures than the rest of their species and, over time, their morphology, physiology and biochemistry change significantly so that they are now unable to produce fertile offspring when interbred with the original population.
==Sympatric speciation==
===Peripatric Speciation===
'''Peripatric speciation''' occurs when a population is divided into a 'main population' smaller, peripheral isolate which is to some degree seperated from the main population. [[Ernst Mayr]] was the first to propose that peripheral populations my play an important role in speciation and this idea gels well with his ideas about [[genetic revolutions]] and the [[founder effect]]
===Sympatric speciation===
In '''sympatric speciation''' reproductive isolation is achieved by newly diverging species without any geographic barriers to gene flow. Sympatric speciation is certainly the most controversial of the proposed methods with most biologists thinking it plays a minor role and many even questioning if it occurs at all. In recent years examples have of sympatric speciation occuring by host shifting by insects and differnces in flowering time in palms have been presented as strong evidence that multiple species of cichlid fish may have arisen without geographical barriers to gene flow in crater lakes.
The commonest method of sympatric speciation is probably '''[[polyploidy]]'''. Polyploids contain more than two complete sets of chromosomes in each cell. This can occur if there is an error in [[meiosis]] (production of gametes) so that a gamete ends up with two copies of each set of chromosomes instead of one copy. Two of these gametes may fuse to produce an organism that has four sets of each chromosome in their cells, called a '''tetraploid''' organism.
Tetraploids are usually sterile as four pairs of chromosomes attempt to pair up in meiosis (instead of the usual two pairs) and get muddled. Nevertheless, such organisms my be able to reproduce [[asexually]] as [[mitosis]] is not impaired. This is a fairly common occurrance in plants but particularly rare in animals as most do not naturally reproduce asexually.
Tetraploids are usually sterile as four pairs of chromosomes attempt to pair up in meiosis (instead of the usual two pairs) and get muddled. Nevertheless, such organisms my be able to reproduce [[asexually]] as [[mitosis]] is not impaired. This is a fairly common occurrance in plants but particularly rare in animals as most do not naturally reproduce asexually.
Polyploids where their chromosomes originate only from one species are called '''autopolyploids''' ('auto' meaning 'self'). If a polyploid contains chromosomes from two, closely related, species then it is an '''allopolyploid''' ('allo' meaning 'different'). Meiosis occurs easier in allopolyploids than autoployploids as the chromosome pairs from each species seek to pair up together. Allopolyploids may well be fertile. Even so, allopolypoloids will ''not'' be able to interbreed with their parent species and is a new species.
Polyploids where their chromosomes originate only from one species are called '''autopolyploids''' ('auto' meaning 'self'). If a polyploid contains chromosomes from two, closely related, species then it is an '''allopolyploid''' ('allo' meaning 'different'). Meiosis occurs easier in allopolyploids than autoployploids as the chromosome pairs from each species seek to pair up together. Allopolyploids may well be fertile. Even so, allopolypoloids will ''not'' be able to interbreed with their parent species and is a new species.
===Example of speciation through allopolyploidy===
====Example of sympatric speciation through allopolyploidy====
''Spartina anglica'' is a cord grass that grows vigorously in British salt marshes. Before 1830 the species of ''Spartina'' that grew in such marshes was ''S. maritima'' but in 1829 ''S. alterniflora'' was imported to the regions from America. ''S. maritima'' and ''S. alterniflora'' hybridised to produce a new species, ''S. townsendii'', a diploid plant with a set of chromosomes form each of ''S maritima'' and ''S. alterniflora''. As the two sets of chromosomes from ''S. townsendii'''s parents cannot pair up, meiosis cannot occur and ''S. townsendii'' is sterile. However, ''S. townsendii'' can rapidly reproduce asexually via [[rhizomes]].
''Spartina anglica'' is a cord grass that grows vigorously in British salt marshes. Before 1830 the species of ''Spartina'' that grew in such marshes was ''S. maritima'' but in 1829 ''S. alterniflora'' was imported to the regions from America. ''S. maritima'' and ''S. alterniflora'' hybridised to produce a new species, ''S. townsendii'', a diploid plant with a set of chromosomes form each of ''S maritima'' and ''S. alterniflora''. As the two sets of chromosomes from ''S. townsendii'''s parents cannot pair up, meiosis cannot occur and ''S. townsendii'' is sterile. However, ''S. townsendii'' can rapidly reproduce asexually via [[rhizomes]].
At a later time a faulty cycle of mitosis, or the fusion of two diploid gametes from ''S. townsendii'', produced a tetraploid plant. This new species, ''S. angelica'', is an allopolyploid as two pairs of chromosomes came from ''S. maritima'' and two from ''S. alterniflora''. Meiosis can now occur and ''S. angelica'' is fertile and grows so vigorously that it has practically replaced the other three species of ''Spartina'' in England.
At a later time a faulty cycle of mitosis, or the fusion of two diploid gametes from ''S. townsendii'', produced a tetraploid plant. This new species, ''S. angelica'', is an allopolyploid as two pairs of chromosomes came from ''S. maritima'' and two from ''S. alterniflora''. Meiosis can now occur and ''S. angelica'' is fertile and grows so vigorously that it has practically replaced the other three species of ''Spartina'' in England.
==Cospeciation==
==Cospeciation==