Biological species concept
The biological species concept gives an explanation of how species form (speciation). A biological species is a group of individuals which can breed together (panmixia). However, they cannot breed with other groups. In other words, the group is reproductively isolated from other groups.
- "The words 'reproductively isolated' are the key words of the biological species definition". Ernst Mayr.p273
According to Ernst Mayr, a new species forms when an existing species splits. Dobzhansky described the role of reproductive isolation in the formation of new species. Once a species lives in two different areas, the geographical isolation makes breeding between the groups reduce or stop. Each group develops features which make breeding between them work less well. Eventually, each group becomes a 'good' biological species, because the two species do not reproduce with each other even when they are together.
This is still considered the most common reason for species splitting, and has the technical name of allopatric speciation. It is contrasted with sympatric speciation where speciation takes place even though all the members live in the same area.
Historical developments [change]
John Ray [change]
In 1686 John Ray introduced a non-evolutionary biological concept. To him, species were distinguished by always producing the same species, and this was fixed and permanent, though considerable variation was possible within a species.
The idea of a species as a physical type of organism had a long history. This survives as the concept of a type specimen in taxonomy. It was the way Linnaeus worked in his binomial classification, and is still useful for amateur naturalists today.
Charles Darwin [change]
- "... I look at the term species as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other...".
But twenty years before, he had a much better idea. He thought of species as maintained by reproductive isolation. He even says "Hence species may be good ones and differ scarcely at all in any external character". Here he quotes the two sibling species of leaf warbler discovered in England by Gilbert White in 1768. At this early stage in his career, Darwin came very close to the modern biological species concept.p266
Modern era [change]
In the last 70 years two ideas have dominated the way professional biologists think about species.
The first is the population genetics concept. This is where a species is seen as a group which can mate together even though they are all to some extent different. It amounts to saying that a species is a gene pool.
The second is the use of DNA sequence analysis to show whether similar-looking species are genetically different from each other. This is especially useful when it is not practical to do breeding experiments.
Sibling species [change]
Many cryptic species exist in all habitats. In the marine bryozoan Celleporella hyalina, DNA sequence analysis was used to show that more than ten ecologically distinct species that had been diverging for many millions of years.
Evidence from the identification of cryptic species means that older estimates of global species richness are too low. For example, mitochondrial DNA research suggests there are at least 11 genetically distinct populations of giraffes. Similarly, the Amazonian frog Eleutherodactylus ockendeni is at least three different species which diverged over 5 million years ago.
- Mayr, Ernst 2001. What evolution is. Weidenfeld and Nicolson, London. p284
- Mayr, Enrst 1982. The growth of biological thought: diversity, evoluton and inheritance. Harvard. The rise of the biological species concept, p270–285
- Dobzhansky T. 1937. Genetics and the origin of species. New York: Columbia University Press. ISBN 0-231-05475-0
- Mayr E. 1970. Populations, species, and evolution. Harvard University Press, Cambridge, Mass.
- Wilkins, John S. (2006). "Species, kinds, and evolution". Reports of the National Center for Science Education. http://ncse.com/rncse/26/4/species-kinds-evolution. Retrieved 2009-09-24.
- Wilkins, John S. (2009). "The first biological species concept : evolving thoughts". http://scienceblogs.com/evolvingthoughts/2009/05/the_first_biological_species_c.php. Retrieved 2009-09-24.
- Darwin, Charles 1859. The origin of species. Murray, London p469
- Medawar P.B. 1977. In The Fontana dictionary of modern thought, eds Alan Bullock et al., Fontana, London. p802
- Knowlton, N (1993). "Sibling species in the sea". Annual Review of Ecology and Systematics 24 (1): 189–216. doi:10.1146/annurev.es.24.110193.001201. ISSN 0066-4162. http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.es.24.110193.001201.
- Knowlton, N (2000). "Molecular genetic analyses of species boundaries in the sea". Hydrobiologia 420 (1): 73–90. doi:10.1023/A:1003933603879. ISSN 0018-8158. http://apps.isiknowledge.com/full_record.do?product=WOS&search_mode=GeneralSearch&qid=1&SID=U2j2HLH4FAKPAj6DDmd&page=8&doc=72.
- Gómez, Africa et al. (2007). "Mating trials validate the use of DNA barcoding to reveal cryptic speciation of a marine bryozoan taxon". Proceedings. Biological Sciences / the Royal Society 274 (1607): 199–207. doi:10.1098/rspb.2006.3718. ISSN 0962-8452. PMC 1685843. PMID 17035167.
- "Giraffes and frogs provide more evidence of new species hidden in plain sight". Science Daily. 2008. http://www.sciencedaily.com/releases/2007/12/071221094911.htm.
- Brown, David et al. (2007). "Extensive population genetic structure in the giraffe". BMC Biology 5: 57. doi:10.1186/1741-7007-5-57. ISSN 1741-7007. PMC 2254591. PMID 18154651. http://www.biomedcentral.com/1741-7007/5/57.
- Elmer, Kathryn et al. (2007). "Cryptic diversity and deep divergence in an upper Amazonian leaflitter frog, Eleutherodactylus ockendeni". BMC Evolutionary Biology 7: 247. doi:10.1186/1471-2148-7-247. ISSN 1471-2148. PMC 2254618. PMID 18154647. http://www.biomedcentral.com/1471-2148/7/247.