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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 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.

'''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 occurred.

==Species==

==Species==

==Modes of 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.

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 somewhere in a continuum from no geographical isolation to complete geographical isolation.

===Peripatric 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]]

'''Peripatric speciation''' occurs when a population is divided into a 'main population' smaller, peripheral isolate which is to some degree separated 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===

===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.  

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 occruring 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.

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 occurrence in plants but particularly rare in animals as most do not naturally reproduce asexually.

Occasionally a tetraploid plant can produce gametes, but they will be diploid.  Zygotes produced by these gametes fusing with a gamete from a normal, diploid, plant will be '''triploid''' (contain three pairs of each chromosome).  Triploids can grow successfully but will be sterile as three sets of chromosomes cannot be shared equally between two daughter cells.  As this triploid plant will not be able to reproduce successfully with its parent, tetraploid, plant it is a ''new species''.

Occasionally a tetraploid plant can produce gametes, but they will be diploid.  Zygotes produced by these gametes fusing with a gamete from a normal, diploid, plant will be '''triploid''' (contain three pairs of each chromosome).  Triploids can grow successfully but will be sterile as three sets of chromosomes cannot be shared equally between two daughter cells.  As this triploid plant will not be able to reproduce successfully with its parent, tetraploid, plant it is a ''new species''.