Species Essay, Research Paper

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