Speciation

Speciation one suggested process by which new species are thought to arise, possibly by Darwin-Wallace evolution. It occurs due to natural selection and separation of populations of the same species, allowing allele frequencies in the two groups to change independently. See evolution for a detailed dissection of the many problems of this theory.

Species

A species is:

"a group of organisms, with similar morphological, physiological, biochemical and behavioural features, which can interbreed to produce fertile offspring, and are reproductively isolated from other species." - Jones M. & Gregory J., Biology 2, 2001, Cambridge University Press, ISBN 0-521-79714-4

morphological features

structural features, such as anatomy

physiological features

how the body functions

biochemical features

sequence of bases in DNA

sequence of amino acids in proteins

For example, all donkeys look alike, have similar physiology and biochemistry and the same genome. Donkeys can successfully interbreed with other donkeys to produce more fertile donkeys. Donkeys can interbreed with a similar species; horses. However, the offspring (called mules) are infertile and cannot reproduce with each other. Therefore donkeys and horses are separate species.

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

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:

• Large bodies of water.
• Deforestation.
• Mountainous areas.
• Smaller-scale barriers for small or immobile organisms.

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

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.

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.

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

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.