Speciation is a hypothetical process by which new species arise. In fact, speciation has never been observed directly or indirectly.[Citation Needed] An atheist believes in speciation regardless of the evidence, because it serves as his substitute for the existence of God.[Citation Needed]
Evolutionists, typically atheists, insist that speciation should occur 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 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.
A species is:
"a group of organisms, with similar morphological, physiological, biochemical and behavioral 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.
Modes of Speciation
The process of speciation can be conveniently broken into four separate '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.
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.
- Continental Drift
- 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.
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
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 differences 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 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.
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 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.
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 happens when the association between two species is very close, that they may speciate in parallel. This is especially likely to happen between parasites and their hosts.
Often this type of speciation is found in symbiotic organisms.
Rarely do scientists find hosts and parasites with exactly matching phylogenies. However, sometimes the phylogenies indicate that cospeciation did happen along with some host-switching.
Examples of Cospeciation
Speciation and Evolution
Sometimes speciation is described as "evolution in action." While this description is essentially correct, speciation does not prove the general theory of evolution, specifically, universal common descent.
Creationists assert that many species may exist in a created kind due to diversification, and that speciation is expected in a creation model as well as the evolutionary model.
- Biology 2, Jones M. & Gregory J., 2001, Cambridge University Press, ISBN 0-521-79714-4