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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
Word Count: 1256
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