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Biological Species Concept

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Autor: reviewessays 06 November 2010

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Biological Species Concept (BSC)

What are biological species? At first glance, this seems like an easy question to answer. Homo sapiens is a species, and so is Canis familaris (dog). Many species can be easily distinguished. When we turn to the technical literature on species, the nature of species becomes much less clear. Biologists offer a dozen definitions of the term "species". These definitions are not fringe accounts of species but prominent definitions in the current biological literature. Philosophers also disagree on the nature of species. Here the concern is the ontological status of species. Some philosophers believe that species are natural kinds. Others maintain that species are particulars or individuals. The concept of species plays an important role both in and outside of biology. Because of the important role of this concept, many biologists proposed definitions for this concept.

Over the last few decades, the Biological Species Concept (BSC) has become predominately the dominant species definition used in biology. This concept defines a species as a reproductive community. This though has had much refinement through the years. The earliest precursor to the concept is in Du Rietz (1930) then later Dobzhansky added to this definition in 1937. But even after this the definition was highly restrictive, the definition of a species that is accepted as the Biological Species Concept was founded by Ernst Mayr; “...groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups”. However, this is a definition on what happens in nature. Mayr later amended this definition to include an ecological component; “... a reproductive community of populations (reproductively isolated from others) that occupies a specific niche in nature. The BSC is greatly accepted among vertebrate zoologists and entomologists. Two reasons account for this addition to the definition of Biological Species Concept. Firstly, these are the groups that the authors of the BSC worked with (Mayr is an Ornithologist & Dobzhansky has worked mainly with Drosophila). More importantly, Sexual reproduction is the predominate form of reproduction in these groups. It is not coincidental that the BSC is less widely used amongst botanists. Terrestrial plants exhibit much greater diversity in their mode of reproduction than vertebrates and insects.

There have been many criticisms of the BSC in its theoretical validity and practical utility. For example, the application of the BSC to a number of groups is problematic because of interspecific hybridization between clearly delimited species. It can’t be applied to species that reproduce asexually (e.g. Bdelloid rotifers, eugelenoid flagellates). Asexual forms of normally sexual organisms are also known. Prokaryotes are also left out by the concept because sexuality as defined in the eukaryotes is unknown (Society for Developmental Biology, 1955). The Biological Species Concept is also questionable in those land plants that primarily self-pollinate. Practically the BSC has its limitations in the most obvious form of fossils. It can’t be applied to this evolutionary distinct group because they no longer mate. It also has limitations when practically applied to delimit species. The BSC suggests breeding experiments as the test of whether an organism is a distinct species. But this is a test rarely made, as the number of crosses needed to delimit a species can be massive. Thus, time, effort, and money needed to carry out such tests that are prohibitive. Not only this but also the experiment carried out are often inconclusive. In practice even strong believers of the BSC use phenetic similarities and discontinuties for delimiting species.

Although more widely known, several alternatives to the Biological Species Concept exist. The Phenetic (or Morphological / Recognition) Species Concept proposes an alternative to the BSC that has been called a "renewed practical species definition". This defines species as; "... the smallest groups that are consistently and persistently distinct and distinguishable by ordinary means." Problems with this definition can be seen, once again depending on the background of the user. For example "ordinary means" includes any techniques that are widely available, cheap and relatively easy to apply (Kerry L. Shaw, 2002). These means will differ among different groups of organisms. For example, to a botanist working with angiosperms ordinary means might mean a hand lens; to an entomologist working with beetles might mean a dissecting microscope; to a phycologist working with diatoms, it might mean a scanning electron microscope. What means are ordinary are determined by what is needed to examine the organisms in question. Thus, once again we see that it is a subjective view depending on how the biologist wants to read the definition.

It also has similar difficulties to the BSC in defining between asexual species and existence of hybrids. There are several phylogenetic species definitions. All of them suggest hat classifications should reflect the best supported hypotheses of the phylogeny of the organisms. The biologist Baum described two types of phylogenetic species concepts, one of thes is that a species must be monophyletic and share one or more derived character. There are two meanings to monophyletic according to the scientist Mayr. The first defines a monophyletic group as all the descendants of a common ancestor and the ancestor. The second defines a monophyletic group as a group of organisms that are more closely related to each other than to any other organisms. The species concepts are only theoretical and by no means no standard as to which species should be grouped (Ernst Mayr, 1996). However, it can be argued that without a more stuructured approached proper discussion can not occur due to conflicting species names. And so, if there are quite large problems with all of the species concepts, the question about what is used in practicehas to be asked.

Most taxonomists use one or more of four main criteria. The first one is the individuals should bear a close resemblance to one another such that they are always readily recognisable as members of that group. The second criterion, there are gaps between the spectra of variation exhibite by related species; if there are no such gaps then there is a case for amalgamating the taxa a single species. Also, each species occupies a definable geographical area (wide or narrow) and is demonstrably suited to the environmental conditions which it encounters. Lastly, in sexual taxa, the individuals should be capable of interbreeding with little or no loss of fertility, and there are should be some reduction in the level or success (measured in terms of hybrid fetility or competitiveness) of crossing with other species. Of course, as has been seen, no one of these criteria is absolute and it is more often left to the taxonomists own judgement.

Quite frequently a classification system is brought about from the wrong reasons. Between two taxa similarities and differences can be found which have to be considered, and it is simply up to the taxonomists’ discretion as to which differences or simila rities should be empahasised. Thus, differences are naturally going to arise between taxonomists. The system used can be brought about for convienience, from historical aspects and to save argument. It may be a lot easier to stick with a current concept, although requiring radical changes because of the upheaval and confusion that may be caused. As seen much has been written on the different concepts and improvements to these concepts but these amounts to little more than personal judgements aimed at producing a workable classification (Mann D.G. 1989).

In general most Biologists adopt the definition of species that is most suited to the type of animal or plant that they are working with at the time and use their own judgement as to what that means. It is common practice amongst most taxonomists to look for discontinuities in variation which can be used to delimit the kingdoms,divisions etc.. Between a group of closley related taxa it can be useful, although highly subjective, to use the crtieria of equivalence or comparibility. Usually however, the criteria of discontinuity are more accurate than comparability, even if the taxa are widely different.

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