[Not half, but whole!]

This subject was brought up in a different thread, and as opposed to hi-jacking the thread, I thought a new thread would be appropriate.

So here's the question:

What constitutes a species?

As Linnaeus started with Systema Naturae it began as dividing into smaller and smaller groups based on observable similarities. The problems with this system is that you have species that are enormously diverse (canine, acropora, etc. that have massively variable forms of the same species) and some species that are virtually indistinguishable from each other(certain coral species, mollusks, etc)

We seem to have moved into the molecular examination. DNA is often seen as the next tool in most biological sciences. DNA is being used to trace lineages, but can DNA be used to define what a species is? Is there a definable mathematic designation? Can we develop a model that says x species has this basic DNA and deviation within z% constitutes the same species?

[C1ay]

The single most common definition of speciation is that it is a process of division that leads to reproductive isolation...

[Eclogite]

As long as we recognise that species is an artificial construct then we need not be too tight in how we define it.

[Rade]

There is no standard definition of a species--it is a major recognized problem in biology. See this review:

An introduction to species concepts and speciation of fishes
Author: Ruffing, Renea A
Kocovsky, Patrick M
Stauffer, Jay R
Citation: Fish and Fisheries
Volume: 3, Issue: 3, September 1, 2002. pp. 143-145

Also, if you want to study this topic, you should begin by reading these important publications, many are classics in biology studied in graduate school--they are from the above review paper:

References

Behnke, R.J. (1972) The systematics of salmonid fishes or recently glaciated lakes. Journalo of the Fisheries Research Board of Canada 29,639^671.
Davis, J.I. (1996) Phylogenetic, molecular variation, and species concepts. Bioscience 46 (7),502^511.
Ghiselin, M.T. (1997) Metaphysics and the Origin of Species. State University of NewYork Press, Albany.
Ghiselin, M.T. (2002) Species concepts: the basis for controversy and reconciliation. Fish and Fisheries 3,151^160.
Gould, S.J. (1979) Aquahog is a quahog. Natural History 88, 18^26.
Kottelat, M. (1997) European freshwater fishes. Biologia 52 (Suppl.5),1^27.
Mayden, R.L. (1997) A hierarchy of species concepts: the denouement of the species problem. In: The Units of Biodiversity ^ Species in Practice. (eds M.F. Claridge, H .A. DawahandM.R.Wilson). SpecialVol.54. SystematicsAssociation, ChapmanandHall Ltd, London, pp.381^424.
Mayden, R.L. (2002) On biological species, species concepts and individuation in the natural world. Fish and Fisheries 3,171^196.
Mayr, E. (1963) Animal Species and Evolution. HarvardUniversity Press, Cambridge.
Mayr, E. (1996)What a species is and what is not? Philosophy of Science 63,262^277.
Mayr, E. and Ashlock, P.D. (1991) Principles of Systematic Zoology. McGraw-H ill,NewYork.
Mayr, E. and Provine,W.B. (1980) The Evolutionary Synthesis: Perspectives on the Unification of Biology. Harvard University Press, Cambridge.
Nelson, J.S. (1999) Editorial and introduction: the species concept in fish biology. In:The Species Concept in Fish Biology (eds J.S. Nelson and P.J.B. Hart). Reviews in Fish Biology and Fisheries 9,277^280.
Simpson, G.G. (1951) The species concept. Evolution 5, 285^298.
Stau¡er, J.R. and McKaye, K.R. (2001) The naming of cichlids. Journalo f Aquariculture and Aquatic Sciences. Cichlid Research State of theArt10,1^16.
Turner, G.F. (1999) What is a fish species? in the species concept. In: Fish Biology (eds J.S. Nelson and P.J.B. Hart). Reviews in Fish Biology and Fisheries 9,291^297.
Waples, R.S. (1995) Evolutionarily significant units and the conservation of biological diversity under the Endangered Species Act. American Fisheries Society Symposium 17,8^27.
Wiley, E.O. (1978) The evolutionary species concept reconsidered. Systematic Zoology 27,17^26.
Wiley, E.O. (2002) On species and speciation with reference to the fishes. Fish and Fisheries 3,161^170.
Wilson, E.O. (1992) The Diversity of Life. W.W. Norton, NewYork.

===

Here is a working definition of a "species" that I like because it combines the concepts of genes (alleles) with populations as selective units, isolation, natural selection (that is, the non-random reproduction of genetopes) and time. Time is very important for the process of speciation, and most likely unique for each species evolved. I copy a section of the paper where it was published:

A Genetical Theory of Species Selection
Author: Rice, Sean H.
Citation: Journal of Theoretical Biology
Volume: 177, Issue: 3, December 7, 1995. pp. 237 - 245

Defining Species (by S.H. Rice):

For purposes of this discussion, a species is defined as a population of individuals that is reproductively isolated from other such populations over a period that is long relative to the time that it would take for a newly arisen allele to go to fixation by drift ["This last condition simply ensures that the population is isolated for a long enough time that we can observe some evolutionary dynamic within it"]

[Ganoderma]

no single definition can work, life is too diverse.

taxonomists say that only different genera may not inter breed. Cactus break that rule like non other. Various colubrid snakes are commonly hybridized as well, breaking the rules....and the list grows.

what defines a specie? a lot of things i don't think anyone will agree on lol

[JulianKeller]

spe·cies (spē'shēz, -sēz)
n., pl. species.
Biology.
A fundamental category of taxonomic classification, ranking below a genus or subgenus and consisting of related organisms capable of interbreeding.
An organism belonging to such a category, represented in binomial nomenclature by an uncapitalized Latin adjective or noun following a capitalized genus name, as in Ananas comosus, the pineapple, and Equus caballus, the horse.
Logic. A class of individuals or objects grouped by virtue of their common attributes and assigned a common name; a division subordinate to a genus.

A kind, variety, or type: “No species of performing artist is as self-critical as a dancer” (Susan Sontag).
The human race; humankind.
Roman Catholic Church.
The outward appearance or form of the Eucharistic elements that is retained after their consecration.
Either of the consecrated elements of the Eucharist.
Obsolete.
An outward form or appearance.
Specie.

[C1ay]

See ring species for examples that break the concise definition you have posted.

[coldcreation]

Look at this article:

Exclusive: Half man, half chimp - should we beware the apeman's coming?

A LEADING scientist has warned a new species of "humanzee," created from breeding apes with humans, could become a reality unless the government acts to stop scientists experimenting.

...if a female chimpanzee was inseminated with human sperm the two species would be closely enough related that a hybrid could be born.

...Leading scientists say there is no reason why the two species could not breed, although they question why anyone would want to try such a technique.

..."If you put human sperm into a frog it would probably create an embryo, but it probably wouldn't go very far," he said.

"But if you do it with a non-human primate it's not beyond the realms of possibility that it could be born alive."

But in the same article it is written:

"Owing to the significant differences between human and animal genomes, they are incompatible and the development of a foetus or progeny is impossible.

And:

HYBRIDS ARE AT CROSS PURPOSES

EVEN though hybrids of humans and animals have never been created, many other creatures have been crossed successfully.

Lions and tigers have been bred to create ligers, the world's largest cats.

And there are also zorses (zebra and horse), wholphins (whale and dolphin), tigons (tiger and lion), lepjags (leopard and jaguar) and zonkeys (zebra and donkey).

As well as these hybrid mammals, there are also hybrid birds, fish, insects and plants.

Many hybrids, such as mules, are sterile, which prevents the movement of genes from one species to another, keeping both species distinct. However, some can reproduce and there are scientists who believe that grey wolves and coyotes mated thousands of years ago to create a new species, the red wolf.

More commonly, hybrids mate with one of their parent species, which can influence the genetic mix of what gets passed along to subsequent generations.

Hybrids can have desirable traits, often being fitter or larger than either parent.

Most hybrid animals have been bred in captivity, but there are examples of the process occurring in the wild.

This is far more common in plants than animals but in April 2006 a hunter in Canada's North-west Territories shot a polar bear whose fur had an orange tint.

Research showed that it had a grizzly bear father, and it became known as a pizzly.

In 2003, DNA analysis confirmed that five odd-looking felines found in Maine and Minnesota were bobcat-lynx hybrids, dubbed blynxes.

Wow...



CC

[mynah]

Interesting question, but a quote I read comes to mind:

Man classifies, nature does not.

[freeztar]

Originally Posted by mynah

Interesting question, but a quote I read comes to mind:

Man classifies, nature does not.

I love it, short and sweet. Where is it from?