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Re: Of Tarbosaurs and tabbies
On 18/3/04 2:47 am, "email@example.com" <firstname.lastname@example.org> wrote:
> On Thursday, March 18, 2004, at 12:35 AM, Thomas R. Holtz, Jr. wrote:
> OK, but doesn't that really amount to them being in the incomplete
> process of diverging from each other? Shouldn't they only become
> distinct species when they can no longer interbreed at all? Currently
> they can still do so, but the results are inconsistent, and will
> presumably eventually amount to nothing when they've evolved away from
> one another sufficiently.
A species is defined by the absence or near-absence of gene flow - it
has been defined as the point (or hazy grade) where reticulate descent
(between interbreeding individuals) becomes secondary to linear or branching
descent (from one species to another). Obviously, this is one of those
wonderful definitions which are both totally accurate and completely useless
at the same time. The various species concepts are essentially differences
of opinion on how to actually identify this point.
As has been pointed out, ability to interbreed is only one factor in
identifying species. Many 'species' _can_ interbreed, but usually _don't_
interbreed. Two species of grebe, the Clark's and the Western, can produce
offspring, but prefer partners of the same species. Males actually become
more willing to accept a mate of the other species the longer they go
without a mate of the same. Females of many cichlid fishes, also, prefer to
mate with males of the 'right' colour pattern, but can still produce
offspring with others. This has been quite dramatically shown after some
species have merged in Lake Victoria since rising eutrophication has reduced
visibility and hence the females' ability to distinguish males.
>> As Darwin
>> showed, regional varieties grade into subspecies which grade into
>> there aren't distinct breaks between these "ranks".
I've always imagined species as being like bobbles or swellings on a string
- something like this:
The individual swellings (the species) are real, but to say where one
swelling ends and another begins is fairly arbitrary.