An answer to paraphyly?

Tom DiBenedetto tdib at UMICH.EDU
Mon Jun 16 11:51:15 CDT 1997


On Mon, 16 Jun 1997 07:00:17 -0500, Thomas G. Lammers wrote:

someone else opined:
>>When it comes time to describe these two entities one would naturally
>>describe two distinct species. Their common ancestor would be the homogenous
>>population prior to the separation, NOT one of the extant taxa. There is no
>>way to my mind that one of the two groups can be described as an ancestor to
>>anything and certainly not to each other.
>
>This is EXACTLY the sort of thinking I have been speaking about!   This
>seems so divorced from REALITY.

I beg to differ.  Consider the meaning of the word "ancestor". It is
a type of historical relationship, irresepctive of amount of
similarity. In historical systematics, we try to represent
*relationships*; similarity is merely a source of *evidence*.  The
ancestral population is (yes, really reallly is) ancestral to the two
descendant populations.
There is *nothing* different about the relationship between the
ancestor with one descendant and the ancestor with the other. That
there is no evidence to illuminate the full set of REAL historical
relationships is unfortunate, but it doesnt change the nature of the
real relationships.

>Okay, let's get down to brass tacks, a concrete example.

[snip, standard example of peripheral isolate which evolves apomorphy
while rest of species doesnt]

>BUT... what if we could travel back in time and collect specimens of tthose
>mainland populations BEFORE dispersal and establishment of P. fernandeziana?
>Back in the herbarium, we compare our time-machine-retrieved specimens to
>current ones.  Do we say, "Jeepers!  These specimens from 500,000 B.C (or
>whatever)  are a whole different species!  Look at how different they were
>before Planta fernandeziana diverged"?
>
>I think not.

No probably not,,,because the character data is not informative to
the actual branching pattern (the relationships amongst populations
through space and time).
We will never be able to discover everything that happened.
While you are in your time machine, why not just watch what happened.
You wouldnt need character analysis then, and you could simply, and
accurately represent the FACT that at time X there was one
population, and it gave rise over time to two populations. Both of
the two are equally descendants of the one. To claim that one
contemporary species is the ancestor of another strikes me as *very*
divorced from reality, or at least from the meaning of the word
"ancestor".




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