Concept of Species

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.