Biological Species Concept Essay, Research Paper

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.

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