Monday, 29 December 2008

On the definition of "species"

Apparently there are some 18(? must check) definitions for 'species'. The creationist's arguments imply that 'evolutionists' only use one of the definitions - or use the one that suits them at the time...

In the former case, it would be interesting to note any examples of speciation for each particular definition.

In the latter case, well, much the same - but if a speciation event by each of the different definitions can be shown, is that not sufficient to prove that speciation does occur via natural selection?

Things to look up:
1) How many definitions of 'species' or 'speciation' are there (How many does evolution claim? How many do creationists claim?)
2) Examples of speciation under each definition.

1 comment:

  1. From:

    Various categories of definition of species: folk, biological, morphological, genetic, paleontological, evolutionary, phylogenetic and biosystematic. Four listed are: folk, biological, morphological and phylogenetic. Refers to Stuessy 1990 for further definitions (Will try to find a link for that reference)

    "One aspect is the idea of reproductive compatability and continuity within a species. Dogs beget dogs, they never beget cats! This has a firm grounding in folk knowledge. The second notion is that there is a discontinuity of variation between species. In other words, you can tell species apart by looking at them (Cronquist 1988)."

    "The biological species concept (BSC) defines a species as a reproductive community." (paraphrased)

    Tracing the origin of the biological definition...

    "Du Rietz defined a species as '... the smallest natural populations permanently separated from each other by a distinct discontinuity in the series of biotypes.'"

    "... that stage of evolutionary progress at which the once actually or potentially interbreeding array of forms becomes segregated into two or more separate arrays which are physiologically incapable of interbreeding." (Dobzhansky 1937)

    "... groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups."(Mayr, 1942)
    "... a reproductive community of populations (reproductively isolated from others) that occupies a specific niche in nature."(Mayr, later than above)

    The commentary goes on to state:
    "There is an abundance of asexual populations that this definition just doesn't apply to (Budd and Mishler 1990)." ... "A similar situation is found in the prokaryotes. Though genes can be exchanged among bacteria by a number of mechanisms, sexuality, as defined in eukaryotes, is unknown in the prokaryotes."

    "Another reason why using the BSC to delimit species is impractical is that breeding experiments can often be inconclusive. Interbreeding in nature can be heavily influenced by variable and unstable environmental factors. ... If we can't duplicate natural conditions of breeding, a failure to breed doesn't mean that the critters can't (or don't) interbreed in the wild. ... In addition, experimentally showing that A doesn't interbreed with B doesn't preclude both interbreeding with C." ...
    "The cladists argue that sexual compatibility is a primitive trait. Organisms that are no longer closely related may have retained the ability for genetic recombination with each other through sex. This is not a derived characteristic. Because of this it is invalid for defining monophyletic taxa."

    I am quoting swathes of material without making any comment - it stands by itself, but perhaps I need to finish reading the page before summarising it any further!