martes, 20 de julio de 2010

An approach from phylogenetic species concept

Susan@ Ortiz B.

Even though the species concept is an important topic in the recognition and conservation of biological diversity, presently there are no a universal concept and choosing one in particular is controversial in all cases. Perhaps, just like Wheeler (1999) mentioned, there are more species concepts in use today than any other time; however, the Phylogenetics Species Concept (PSC) could be considered a good approach. According to cladistic principles, the PSC suggested by Rosen (1978), is focused to population or group of populations geographically constrained defined by one on more unique apomorphous features. After a decade, a modified version was proposed by Queiroz and Donoghue (1988), as the smallest unit determined cladistically at least one specific character. Evidently, this concept is based on autapomorphic background and therefore the species are defined under monophyly criterion. However, some practical difficulties arise about recognition of ancestral species based on autapomorphies, since, the character becomes a synapomorphy and the ancestral taxon actually differs from its descendant due to lack of the evolutionary novelty. Additionally the PSC point out against BSC the absence of reproductive isolation as a plesiomorphic character inappropriate in the delimitation of species (Wheeler and Meier, 2000) and the absence of reproductive boundaries between ancestral and descendant populations (Hennig, 1966).

On the other hand, Eldredge and Cracraft (1980) and Cracraft (1983) addressed the PSC towards diagnosable characters, establishing the specie as a diagnosable cluster or the smallest diagnosable cluster of individuals within which there is a parental pattern of ancestry and descent. Afterwards Nelson and Wheeler (1990) redefine it as the smallest aggregation of populations or lineages diagnosable by a unique combination of character states. As well this concept also is based on the relationships between species and its characters, although not necessarily imply monophyly, being that the autapomorphic characters are diagnosable, but no all diagnosable characters can be autapomorphies. This approach seems to evoke the Typological Species Concept with regard to constant diagnostic differences to identify species (Mayr, 1991); nonetheless the measure is less arbitrary because it follows a parental pattern of ancestry and descent. A critique against this concept indicates that the species exist in nature independently of the systematic’s ability to diagnose them (Haro, 1999). Obviously, diagnosable apomorphies must be sought, but this is a systematic’s problem that does not affect the organism as real entity. Another difficulty is to define the boundaries of diagnostic in polytypic species with a wide variety, which could lead to false increases in diversity and underestimation of intraspecific variation (Fernandez et al., 1995).

Despite the above, the PSC is an operational concept compatible with phylogenetic theory designed to recognize and define the groups for any characteristic, from morphological to molecular. Some fundamental components of the PSC are the weighting of the recognition against reproductive isolation and lack of a temporal dimension under BSC (Willmann and Meier, 2000), its applicability to all evolving systems, whether sexual or asexual (Balakrishnan, 2005) and the recognition that species based on observable, testable characters simply avoid the confusion imparted by considerations of modes of speciation (Wheeler, 1999). Likewise, according to Balakrishnan (2005) the major practical troubles include determining whether shared traits have attained fixation in the population (Wiens and Servedio, 2000), determining how many diagnostic traits to consider and the inapplicability of this approach to most continuously varying quantitative traits (Willmann and Meier, 2000). Finally, although PSC has not completely solved the conflict between the real species existing in nature and the criteria and tool to recognize, it constitutes an attractive approach and a step in this direction. Also emphasize the diversity of mechanisms that can give rise to real species in nature and that are not all recognized by the BSC.


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