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Re: Ancestors... (was Re: Stratigraphy, biogeography & cladograms )

Pharris Nicholas J wrote:

> On Wed, 20 Jan 1999, Stewart, Dwight wrote:
> >
> >
> >          Well, I think it's one caveat of species that the animals have to
> > be able to produce viable offspring, naturally.  But, I took viable as
> > meaning that
> >       the offspring are capable of reproducing.


Hrm, I'm sure this has been covered on the list before. Pretty sure because I
think I was one that covered it:)

Species definitions are flexible depending on your frame of reference and
organism that you are working on. To throw this into fodder for the dino list,
paleontologists use what is known as a morphological definition, meaning
basically that they call it a different species when the organism looks
sufficiently different to them that they believe it would be a different species
in real life. This is very subjective and tends to be likely on the conservative
side. After all, all they have to work with are bones, how are they supposed to
know if the animals could have bred or not?
There are several other definitions out there but the one that most people
relate to is what is known as the biological species concept: a population or
series of populations of organisms that can freely interbreed with one another
UNDER NATURAL CONDITIONS but not with members of another species.
Why may you ask did I capitalize under natural conditions? Glad you asked. This
is the part that most people forget. Many species, like the domestic cat and
leopard cat as mentioned, can breed if put together and may in rare
circumstances even give rise to fertile offspring. However, in normal natural
conditions they will never see each other and thus their populations may be
considered separate species. And it is not that they won't see each other
because of separated populations. They won't because they typically inhabit (as
much as any domesticated animal can be said to inhabit a particular habitat)
different types of habitats or different niches within a habitat..
Personally, I feel that the domestic dog has gone far beyond a single species
despite that is all that is recognized officially because many of the breeds can
only interbreed by sometimes extreme help. Chihuahuas and Irish Wolfhounds just
don't go together without herculean efforts. But I digress.
Anyway, the biological species concept only holds (and not always even then) for
sexually reproducing animals. It does not hold for plants and is laughably
absurd for single-celled organisms. The genetics of speciation and viable
definitions of species for these critters is at best chaotic and complex (making
the complexities of vertebrate genetics bloody tame) and at worst a nightmare
that should not and must not be named (gibber gibber).
Well, that is the very short of it, with a minor bit of editorializing. Hope
this helps to clear up any confusion. If I caused any further confusion that can
be taken care of easily enough, at least until the point is reached when I can
say congratulations! You've reached maximum confusion and the present state of
understanding in the field of the genetics of speciation:)

Joe Daniel