miércoles, 18 de agosto de 2010

Who is Right?

Susana Ortiz B.

Although it is true the species concept has been controversial and approached from multiple perspectives throughout history, its applicability and acceptance depends on how well to recognize, identify and understand the species in nature hence its relevance in areas like conservation, ecology, evolution, taxonomy and of course systematic. The Biological Species Concept (BSC) is perhaps the most widely accepted species concept in biology, it defines species in terms of interbreeding and reproductive isolation (Mayr, 1963). This lats feature based on barriers to gene flow between populations includes not only geographic isolation but also prezygotic factors such mate choice, and fertilization incompatibilities and postzygotic factors i.e., hybrid inviability and sterility caused by genomic incompatibilities (Dobzhansky, 1937). However, in practice this concept involves some difficulties mainly in relation to hybridization, asexual populations, adimensionality and not distinction between reproductive isolation and the speciation´s process (Mallet, 2007; Fernandez et al., 1994; Templeton, 1989).

In view of this scenario, emerges the Phylogenetic Species Concept (PSC), which stablishes the recognition as criterion for species delimitation through characters; despite the multiple versions, the PSC proposed by Eldredge and Cracaft (1980) and later rephrased by Cracraft (1983) conceives the specie as “…the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent”, the above entails some operational advantages, such as its applicability to fossils and all evolving systems of living things whether sexual and asexual populations and you do not need to know or understand the process driving speciation in order to recognize species (Claridge et al. 1997; Nelson and Placnick, 1981) . Under this concept the pattern of characters distribution in nature provides testable evidence of the existence of species in nature (Wheeler, 1999), additionally it seems to return to typological species concept given its confinement to diagnostic characters, nonetheless, the PSC emphasizes the relationship between unique combination of inherited characters either molecular or morphological and an ancestral–descendant sequence.

According to Cracraft (1989), species dened under this diagnostic criterion are real taxa suitable for phylogenetic analysis and evolutionary studies, being so is necessary to take special care of strict application rather in small groups and polymorphic species if unknown genetics aspects, geographic distribution and demography of the group, this in order to avoid underestimating the intraspecific variation and therefore the wrong multiplication of species (Mallet, 2007; Fernadez et al., 1995); However, to counteract such diagnosable groups have no authentic parental pattern of ancestry and descent (Cracraft, 1989). Another relevant aspect of the diagnostic species concept is the fundament of this diagnosticability that is the characters as synapomorphies or autapomorphies or simply characters as descriptors indistinctly of their historical background. This latest maybe is the most permissive given that it has not conflict in to recognize cladogenesis or anagenesis indistinctly and can be monophyletic or non-monophyletic. On this basis, also the concept is ambiguous because it could behave like synapomorphy, autapomorphy or simplesiomorphy in the same branch, also the major practical difficulties include determining whether shared traits have attained fixation in the population (Wiens and Servedio, 2000) and the inapplicability of this approach to most continuously varying quantitative traits (Willmann and Meier, 2000).

The monophyly criterion of species is an important component in synapomorphic and autapomorphic versions of the PSC, thus, they do not recognize the phyletic change or anagenesis, just the cladogenesis as evolutionary process. Following the above, in cladogenesis from a stem species, subspecies may first appear and then a new species with acquisition of apomorphies, nevertheless, the autapomorphic concept could recognize subspecies as species before the consolidation of the cladogenesis (De Haro, 2005); other problem is that it excludes the existence of stem-species by definition, since these can neither be monophyletic, nor can they possess autapomorphies, relative to its own descendant species since they inherit their "autapomorphies", so that the former autapomorphies of the stem-species become the autapomorphies of the resulting monophylum and thus become symplesiomorphies of the stem-species (Wheeler, 1999; Wheeler and Meier, 2000). On the other hand, the PSC based on synapomorphy, addresses the species problem from a perspective different, perhaps this is synapomorphy and monophyly in terms of relations among species and their characters (Hennig, 1966), but at this level should reect the branching, or cladistic, relationships among species (Goldstein and De Salle, 2000), ie, the synapomoraphies may show the phylogenetic relationships of two or more species belong to a monophyletic group, then take this group as a single species is inadequate and even more when the historical status may be unresolved. Maybe, monophyly is an ambiguous concept whose problem lies a fundamental distinction between species and monophyletic taxa, where species form mutually exclusive reticulated systems, while higher taxa form inclusive hierarchical systems (Rieppel, 2009).

Finally, although in many contexts the PSC emphasizes a diversity of mechanisms that can give rise to real species in nature and that are not all recognized by the BSC (Claridge et al. 1997), operational difficulties exist for the identification of distinct historical entities in nature regardless of the phylogenetic species concept applied (Frost and Kluge, 1995), however, seems that the most adequate species concept is perhaps the autapomorphic concept in despite of its difficulties, without this meaning that it is the right or the universal criteria by which species may be delimited, it is just a good approximation to define and recognize the diversity of life.

References

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Cracraft, J. 1983. Species concepts and speciation analysis. Current Ornithology, 1, 159–187.

Cracraft, J. (1989). Speciation and its ontology: The empirical consequences of alternative species concepts for understanding patterns and processes of differentiation. In Speciation and its Consequences (D. Otte and J. A. Endler, Eds.), pp. 28–59. Sinauer Associates, Sunderland, MA

Dobzhansky, T. 1937. “Genetics and the Origin of Species.” Columbia Carpenter, J. M. (1992). Random cladistics. Cladistics 8, 147–153.

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Templeton, A. R. 1989. The meaning of species and speciation: A genetic perspective. In “Speciation and Its Consequences” (D. Otte and J. A. Endler, Eds.), pp. 3–27. Sinauer Associates, Sunderland, MA.

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1 comentario:

Rafael Miranda dijo...

Y en realidad ¿quién ganó? y por cierto: ¿cuál es SU(S) concepto preferido?