Joachim Pense  wrote:

> C. P. Weidling wrote:
> 
> > 
> > I think what the original poster was looking for was some way of counting
> > how many separate individual species there are between some current species
> > of life and the original primordial cell.  With horses you might count
> > through many now extinct forms of horse back to eophippus.  Then count
> > back to the common ancestor of mammals.  With humans you'd count back
> > to the common ancestor of primates, then back to the common ancestor of
> > mammals and from then on, it would be the same for horse or human.  If
> > there were more primitive horses between modern horse and that common
> > mammalian ancestor than primates, then the horse would have a longer
> > trace than the human.  (Have I got it right?)
> > 
> 
> Is there even a clear way to define "species" here? IIRC,
you define two
> species to be separate by the possibility of mating and breeding between
>   individuals of the two. But you cannot test this if one (or even both)
> of the species is/are extinct.
> 
> Joachim
> 
> (Remark: My background is similar to the OP's, so sorry if what I say is
> trivial from an evolutionary biologists' point of view.)

It is often noted that paleospecies are morphological, and that
inferences as to interbreeding are tenuous at best in such cases.
Paleospecies may underestimate the number of observed actual
reproductive species threefold. Also bear in mind that paleospecies are
often named and described on the basis of less than a handful of
fossils, making clinal variation hard to identify (this is not always
true). There is a problem of the resolution of the data.
-- 
John Wilkins
wilkins.id.au
"Men mark it when they hit, but do not mark it when they miss" 
                                               - Francis Bacon
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