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