miércoles, 23 de junio de 2010

A perspective from the Biological Species Concept

Susana Ortíz B.

The hight biodiversity and complexity of the biological systems, is one of the greatest challenges at the time to recognize and delimit species as real entities in nature (Shcherbakov, 2010; Purvis and Hector, 2000; Sites and Crandall, 1997). Efforts to describe and identify these entities have resulted in the formulation of multiples species concepts contrasting with differents philosophical and epistemological approaches. Perhaps, the most widely discussed concept is the Biological Species (BSC). This concept stipulates the specie as a group of interbreeding natural populations that are reproductively isolated from other such groups and share a common place (Mayr 1942-1996). Although, the concept has been widely adopted in various areas is controversial and unyokeable under certain context. Criticism have been focused against some aspects including its lack of universality (Balakrishnan, 2005). This limitation is manifest by its inapplicability to sexual taxa (Wheeler and Platnick, 2000), and fossils; the latter, given the impossibility of identify its reproductive potential.

The capacity of interbreeding is the main criterion to define species in the BSC, which in theory implies that species no interbreed with members of other species, and they are reproductively isolated (Ayala, 2010). However, this does not mean only geographical isolation, structural and behavioral barriers could be considered prevent interbreeding. Based on the above, the concept is not clear in defining the causal order between interbreeding and reproductive isolation. This is consistent with the formation of hybrids and also the practical difficulty of determining a specie in allopatric populations, ignoring the potential to interbreed with other species under contingent conditions and the ability to produce real fertile offspring (Mallet, 1995). So, because mechanisms of reproductive isolation differ among taxa, the BSC cannot be absolute in the determination of species (Claridge, 1997).

An important aspect addressed in all conceptualizations of species is related to the definition of its status as a class or as individual. According to Goldstein & DeSalle (2000)the BSC defines the species as a class, which provides an ambiguous criterion to group organisms and result in the BSC might not be monophyletic (Donoghue, 1985). However, according to Dobzhansky (1950), the biological specie is the largest and most inclusive mendelian population then recognizes the evolution and thus the species as fundamental units of evolution, while higher taxonomic categories such as gender, cataloging families and orders are artificial, made for convenience and do not necessarily reflect evolutionary relationships (Jody, 2001). Moreover, the BSC concept is conceived in a population notion, where "populations" are seen as reproductive units which share a common gene pool in a context of reproductive isolation. Although the concept frames the unique properties of biological systems such as reproduction and crossover potential, other properties are not directly unlink, for example, recognition of conspecifics and gene flow (Mallet, 1995).

Additionally, the concept dispose that the taxonomy of natural species should be the conceptual schema of genetic populations, because a community of organisms that cross, is a gene pool which gives an identity that makes relatively recognizable species (Dobzhansky, 1937), but this appears not be the case in cryptic species. In despite of the mechanisms that can give rise to real species in nature and that are not recognized by the BSC, it have a strong theoretical component, which as Mayr (1996) said not only refers to the description and recognition of the species in nature, but the causes and processes associated with them.

Finally, the species concept can be extended to virology, but maybe the BSC is not the most adecuate; In the first place, viruses are genetic and evolutionary entities but is impetuous to affirm that are biological entities. Nonetheless, some features of BSC are applicable, for example, the specie is the evolutionary face, also as the BSC the members of a species resemble one another, and differ from other species. The difference between BSC and specie in viruses lies in some processes and mechanisms responsible of this identity. Obviously, viruses do not interbreed, but the recombination could be considered loosely as a mechanism that similarly allows genetic exchange, although this event is quite rare in viruses (Lai, 1992). To conclude, clearly these are just some aspects and implications of the BSC in virology, but probably there are many issues to be adressed in this and others areas, for now, even though the BSC is an important conceptual and operational base, it cannot offer universal yardstick to delimit species in nature.

References

Ayala, F., J. & Arp, R. 2009. Contemporary Debates in Philosophy of Biology. John Wiley and Sons.426p

Balakrishnan, R. 2005. Species Concepts, Species Boundaries and Species Identification: A View from the Tropics. Syst Biol.2005; 54: 689 - 693.
Dobzhansky, T. 1937. Genetics and the Origin of Species. Columbia University Press, New York.

Dobzhansky, T. 1950. Mendelian populations and their evolution. Am. Natur. 84:401-418.

Donoghue, M. J. 1985. A critique of the biological species concept and recommendations for a phylogenetic alternative. Bryologist 88:172-181.

Goldstein P.Z, DeSalle R.2000 Phylogenetic species, nested hierarchies, and character fixation. Cladistics. 16, 364–384.

Jody, H. 2001. Genes, Categories, and Species: 2001. The Evolutionary and Cognitive Causes of the Species Problem. Oxford University Press US. 217p

LAI, M. M. C. 1992. RNA recombination in animal and plant viruses. Microbiol. Rev. 56:61–79.

Mallet, J. 1995. A species definition for the modern synthesis. Trans. Ecol. Evol. 10:294–299.

Mayr, E. 1996. What is a species and what is not? Phil. Sci. 63:262-277.

Mayr, E. 1942. Systematics and the Origin of Species from the Viewpoint of a Zoologist. Columbia University Press: New York.

Purvis, A. & Hector, A. 2000. Getting the measure of biodiversity. Nature. 405: 212-219.

Shcherbakov. V., P. 2010. Biological species is the only possible form of existence for higher organisms: the evolutionary meaning of sexual reproduction. Biology Direct. 5:1-14.

Sites, J. W., Jr., and K. A. Crandall. 1997. Testing species boundaries in biodiversity studies. Cons. Biol. 11:1289-1297.

Wheeler, Q. D., and N. I. Platnick. 2000. A critique from the Wheeler and Platnick Phylogenetic Species Concept Perspective: Problems with Alternative Concepts of Species. In: Species Concepts and Phylogenetic Theory (Ed. Q. D. Wheeler and R. Meier), pp. 133-145, Columbia University Press, New York.

Claridge, M. F., H. A. Dawah, and M. R.Wilson. 1997b. Species:The units of biodiversity. London: Chapman & Hall.