Biological Species Concept


     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.

Bibliography

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