FREE ESSAY ON SPECIES

SPECIES

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. References 
Mayr, Ernst, 1904-/Systematics and the origin of species : from 
the viewpoint of a zoologist/1942/QH 366 
Cronquist, Arthur / The evolution and classification of flowering 
plants/1968/QK 980 Stace, Clive A., Clive Anthony, 1938-/ Plant taxonomy and 
biosystematics/1991/QK 990 
Stuessy, Tod F / Plant taxonomy : the systematic evaluation of 
comparative data/1990/QK 95 
Evolution : a biological and palaeontological approach / editor 
[for the Course Team] Peter Skelton/1993/QH 366 
http://wfscnet.tamu.edu/courses/wfsc403/ch_7.htm - Interspecific Competition 
http://sevilleta.unm.edu/~lruedas/systmat.html - Phylogenetic Species Concept