lunes, 27 de octubre de 2008

Biological Species Concept

Morales-Guerrero, A. M.
Universidad Industrial de Santander
“The BSC can illuminate
only a small fragment
of the Tree of Life” (Agapow et al. 2004).
There are two different questions involved to the Species Concept:
I. The question concerning the reality of phenomena or of objects in nature we want to name with the term “species” (this is the theoretical problem).
II. The criteria for the identification of species (and this is practical problem) (Wolfgang, 2005).

The solution of the practical problem depends on the definition of the term “species” we want to use (Wolfgang, 2005); currently the issue of species “reality” is central to decision of how to approach species delimitation and to understand the ontology of species (Goldstein & DeSalle, 2000). I share the view that species are spatio-temporally bounded entities (rather than classes defined by some common property), that species per se are not involved in processes - they are effects, not effectors (Kluge, 1990).

A short definition of the BSC is: ‘‘Species are groups of interbreeding natural populations that are reproductively isolated from other such groups’’ (Mayr, 1996). In this definition of the BSC, the species are considered as “kinds” (i.e. categories or classes) distinguishable from other species by the criterion of reproductive isolation and not overall phenotypic similarity.

The biological species concept is important because it places the taxonomy of natural species within the conceptual scheme of population genetics but has been criticized for several reasons; including the lack of a temporal dimension (Balakrishnan, 2005), for this reason is not possible to talk about age of the species or origin of the species, as a consequence the BSC definition may be only for populations that coexist in time-space and which live in sympatry. In this way the BSC also denies any idea about the fossil species due to the inability to measure its reproductive potential (Fernández et al. 1995), others problems are the practical impossibility of ascertaining reproductive isolation between populations in the wild and the inapplicability concept for asexual organisms (Balakrishnan, 2005), so this definition leaves a vast number of organisms with a nebulous status (Agapow et al. 2004). Finally there are also problems in the structure of the concept, because the BSC confuses the pattern with the process or the isolation with the speciation (Fernández et al. 1995).

The criteria for the identification of species or Operational methods (concerned with how a species may be delimited rather than what it represents) are a necessity (Sites & Crandall, 1997). Today, the empirical issue of the species delimitation is receiving increased attention and several methods have been proposed for delimiting species in a statistically rigorous framework.

According to Site & Marshall (2004), the methods for species delimitation are divided into two:
“Nontree-based methods delimit on the basis of gene flow assessments, whereas tree-based methods delimit species as historical lineages”

Templeton (2001) recently advocated application of his NCA method (Templeton et al. 1995) to the problem of species delimitation; this method is based in the reconstruction of tree and reproductive isolation and was designed for the concept of cohesive species. NCA takes into account all of the available information on the geographic and phylogenetic position of haplotypes and statistically tests for their association, and it can be applied to many different levels within a clade to determine whether a speciation event can be inferred with significant statistical support given the data available (Templeton, 2001). Though this was developed for tests cohesive species, is applicable to delimit species according to BSC, because this indirectly estimating gene flow within and between hypothesized species, so the reproductive isolation may thus be looked upon as a informative but not necessary condition for delimiting species boundaries, where it does exist, it is likely to unambiguously delimit species.
References:
-Agapow, PM. Bininda, OR. Crandall, KA. Gittleman, JL. Mace, GM. Marshall, JC. & Purvis, A. 2004. The impact of species concept on biodiversity studies. Q Rev Biol. 79(2):161-79.
-Balakrishnan, R.2005. Species concepts, species boundaries and species identification: A view from the tropics. Systematic biology. 54: 689-693.
-Fernandez, F., Hoyos, J.M. & D.R. Miranda. 1995. Especie: Es o Son? Número especial Evolución. Innovación y Ciencia. Colombia. 4(1):32-37.
-Goldstein, P. & DeSalle, R. 2000. Phylogenetic species nested hierarchies and character fixation. Cladistic. 16: 364-384.
-Kluge, AG. 1990. "Species as historical individuals." Biology and Philosophy. 5 (4): 417-431.
-Mayr, E. (1996): What is a species and what is not?. Philosphy of Science. 63: 262–277.
-Sites, JW. Jr, & K. A. Crandall. 1997. Testing species boundaries in biodiversity studies. Cons. Biol. 11:1289–1297.
-Sites JW Jr, Marshall, JC. 2004. Operational criteria for Delimiting Species. Annu. Rev. Ecol. Evol. Syst. 35:199–227.
-Templeton, AR. 2001. Using phylogeographic analyses of gene trees to test species status and boundaries. Mol. Ecol. 10:779–91.
-Templeton, AR. Routman, E. & CA. Phillips 1995. Separating Population Structure from Population History: A Cladistic Analysis of the Geographical Distribution of Mitochondrial DNA Haplotypes in the Tiger Salamander, Ambystoma tigrinurn. Genetics. 140: 767-782.
-Wolfgang Wagele, J. 2005 Foundations of Phylogenetic Systematics. verlag Dr. friedrich pfeil. Munchen.

1 comentario:

Rafael Miranda dijo...

al parecer una cosa es Mayr (el concepto) y otra la aplicacion actual (NCA), adicionalmente el mismo concepto esta lleno de ambiguedades!! sera que NCA si/no ve subsespecies/demes/razas? de hecho son necesarias las subsp en el BSC?; ademas, porque no dejar el BSC como una clase y por lo tanto vacio de contenido ontologico? y de ser asi rechazar toda aplicacion que trate de derivar el BSC a organismos reales?