The problem is we can't usually tell the time-depth of species
sufficiently accurately to determine whether one form or another is
ancestral. (The case of humans and other homonoid species is unique
because the fossil and sub-fossil evidence is subjected to a level of
scrutiny that forms in other genera aren't subjected to, plus we know
human anatomy very well).
For example, take the Corellas, it's a possible scenario to say that
once an ancestral species ranged widely over Australia, but drought
reduced it to three areas, a western refuge, a central refuge and a
south-eastern refuge, from whence emerged the Western Corella, the
Little Corella and the Long-billed Corella. With wetter climates the
central population expanded and is now in contact again with the
Western Corella, and Long-billed Corella.
Is the Little Corella the ancestral species? Or do the Long-billed or
Westerns look more like the ancestor? I remember reading that a fossil
of a Corella type parrot was found at Riversleigh from the Miocene
that looked very like the modern Long-billed. So perhaps the story is
that all Corellas were long-billed before the Pleistocene drying and
we should call the Little the Short-billed Corella and the Western the
Medium-billed Corella!
This is going to be a fascinating thread.
John Leonard
On 1 November 2010 14:55, Ralph Reid <> wrote:
> Greetings all,
>
> A short poser.
>
> Given that all existing species (particularly those of the animal kingdom)
> are descended from a somewhat similar ancestor species, but divergent enough
> to be considered a separate species; and given that the general definition of
> a species is that a species of animal does not/cannot interbreed with another
> separate species (although, admittedly, there are many exceptions - tigers
> and lions interbreed, horses and donkeys interbreed as just two examples, but
> with sterile offspring). Then, by this definition a species must generally
> have evolved/changed its characte ristics to the extent that members of this
> species now cannot inter-breed with members of the ancestor species.
>
> Does this imply that the descendant species (generally being more fitted to
> exploit a particular environmental niche that the ancestor species)
> ultimately replace the ancestor species, driving that ancestor species into
> extinction?
>
> Or do the two species continue to co-exist side-by-side, possibly each
> exploiting somewhat different environmental niches, due to sympatric
> speciation?
>
> If one considers the genus homo, for example, why are all the previous
> variants of hominins not continuing to live today alongside homo sapiens?
>
> So the question arises - are there any known, proven instances of an ancestor
> species continuing existing with a descendant species? Are there any known
> ornithological examples of this?
>
> Regards
>
> Ralph Reid
>
>
>
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--
John Leonard
Canberra
Australia
www.jleonard.net
I want to be with the 99,999 other things.
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