Concept of Species

Over the last few decades the Biological Species Concept (BSC) has become predominately the dominant species definition used. 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 (1942); “..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 amongst vertebrate zoologists & entomologists. Two reasons account for this .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 more greater diversity in their mode of reproduction than vertebrates and insects. There has 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 hybridisation between clearly delimited species.(Skelton). It cant 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. The Biological species concept is also questionable in those land plants that primarily self-pollinate.(Cronquist 1988). Practically the BSC has its limitations in the most obvious form of fossils.-It cant be applied to this evolutionary distinct group because they no longer mate.( Do homo Erectus and homo sapiens represent the same or different species?) It also has limitations when practically applied to delimit species. The BSC suggests breeding experiments as the test of whether a n 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. So the time, effort and money needed to carry out such tests is prohibitive. Not only this but 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 (Cronquist) 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. 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 it 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. So 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. Baum (1992) describes 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 (Nelson 1989). 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. So really, the species concepts are only theoretical and by no means no standard as to which species should be grouped. 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 on or more of four main criteria; (Stace 1990) 1.The individuals should bear a close resemblance to one another such that they are always readily recognisable as members of that group 2.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 taxtas a single species. 3.Each species occupies a definable geographical area (wide or narrow) and is demonstrably suited to the environmental conditions which it encounters. 4.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 levelll 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 consisdered ,and it is simply up to the taxonomists discretion as to which differences or simila rities should be empahasised. So 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 amount to little more than personal judgements aimed at producing a workable classification (Stace).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 is more accurate than comparibility ,even if the taxa are widely different.