sábado, 28 de agosto de 2010

Phylogenetic Species Concepts: Diagnosable Version

Gualdrón-Diaz J. C.

Since of need individual for different researchers, have been development at least 22 concepts of species to characterize diversity, some of these researchers have been motivated by operational or empirical definitions, others by theoretical necessity, while some encouraged by peculiarities of organisms studied (Mayden 1997). However, all the concepts to present problems and disadvantage, some more than others, many of these are notably incompatible in the accounts the diversity and not all concepts have been equally well characterized or explicitly defined (Mayden 1997). Despite this, all attempts to define species depending on the needs and interests of each.

Of all the concepts raised, I agree with the definition of phylogenetic species. The phylogenetic species concept (PSC) has its origin in the writing of Willi Hennig (1966) and subsequent transformations of phylogenetic theory. Hennig recognized that the Biological Species Concept (Mayr, 1942-1963) was problematic with relative to the chronological history of species and proposed modifications designed to fix this species concept.

Several positive aspects of the PSC make them particurly attractive as operations in discovering biodiversity, and resolving some of the perceived problems with other concepts (Mayden and Wood, 1995). Currently at least three different concepts of species are identified as phylogenetic.The different PSCs form three general classes; one emphasizing monophyly, one emphasizing diagnosable, and one emphasizing both (Mayden 1997).

I concur with the diagnosable version of phylogenetic species concept, which have been proposed by several authors who defined the species as: “...a diagnosable cluster of individuals within which there is a parental pattern of ancestry and descent, beyond which there is not, and which exhibits a pattern of phylogenetic ancestry and descent among units of like kind” (Eldredge and Cracraft, 1980); ...”the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent”(Cracraft, 1983); “...simply the smallest detected samples of self perpetuating organisms that have sets of characters” (Nelson and Platnick, 1981); “...the smallest aggregation of populations (sexual) or lineages (asexual) diagnosable by a unique combination of character states in comparable individuals (semaphoronts)” (Nixon and Wheeler, 1990; Wheeler and Platnick, 2000).

This definition emphasizes that a priori species are diagnosable regardless of a criterion of monophyly. Although the correspondence between characters, homologs, and apomorphies is of critical importance to cladistics (Patterson 1985), such distinctions are irrelevant to species recognition. In this sense, this concept differs from other proposed phylogenetic concepts, which require identification apomorphies to diagnose species; According to this, those not possesing autapomorphic attributes do no constitute a species.The monophyly, for proponents of this concept, apply only at a level of organization above species. Species are delimited by the distributions of fixed, diagnostic characters across populations (Mayden 1997) and there is no inherently arbitrary divergence or distinction between species or subspecies in a polytypic species (Cracraft, 1983, Warren, 1992). subspecies have no ontological status.

The positive aspects to this concept over others are: First, process is not invoked before patters is observed, to initially distinguish species, this do so in preparation for and prior to a cladistic analysis (Wheeler and Platnick, 2000). Second, phylogenetic methodologies are argued to be applicable only to genealogical relationships of species and supraspecific taxa, not below the level of integration of species wherein tokogenetic relationships of infraspecific entities are the norm (Wheeler and Nixon, 1990; Nixon and Wheeler, 1990).

Finally, this concept also have the ability to recognize both biparental and uniparental species, and possess no implied modes of selection nor speciation, besides it is more compatible with phylogenetic theory because speciation events are marked by character transformations (Mayden 1997, Wheeler and Platnick, 2000).

References

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


Eldredge, N., and J. Cracraft. 1980. Phylogenetic patterns and the evolutionary process. New York: Columbia University Press.

Hennig, W. 1966. Phylogenetic systematics. Urbana, IL: University of Illinois Press.

Mayden, R. L., and R. M. Wood. 1995. Systematics, Species Concepts, and the Evolutionarily Signficant Unit in Biodiversity and Conservation Biology. Special Publication, American Fisheries Society, Bethesda, Maryland. (No. 17: 58-113).

Mayden, R. L. 1997. A hierarchy of species concepts: the denouement in the saga of the species problem. In Species:The units of biodiversity. Edited by M. F. Claridge, H. A. Dawah, and M. R. Wilson. London: Chapman & Hall. pp. 381-424.

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


Mayr, E. 1963. Animal species and evolution. Cambridge: Belknap Press of Harvard University Press.


Nixon, K.C. and Wheeler, Q.D. 1990. An amplification of the phylogenetic species concept. Cladistics 6: 211-223.

Nelson, G., and Platnick, N. I. 1981. Systematics and biogeography: Cladistics and vicariance. New York: Columbia University Press.

Paterson, H. 1985. The recognition concept of species. Pp. 21-29 in E. Vrba, ed. Species and speciation. Transvaal Museum, Pretoria.

Warren, M. L., 1992. Variation of the spotted sunfish, Lepomis punctatus complex (Centrarchidae): meristics, morphometrics, pigmentation and species limits. Bull. Ala. Mus. Nat. Hist. 12:1-47.


Wheeler, Q. D., and Platnick, N. I. 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.