Essay/Term paper: Species concept

Essay, term paper, research paper:  College Papers

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 sp

ecies 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 ca

n 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 gr

oups 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. T

here 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 s

pecies 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 t

o 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 compar ibility ,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