Cladistics and "Eclecticism"

[Richard Jensen]

Thomas DiBenedetto wrote:

> -----Original Message-----
> From: Richard Jensen
> It's considered extinction because the ancestral taxon which just gave rise
> to a
> single new terminal is no longer a terminal.  As cladists have explained it
> to
> me, if my favorite species undergoes an event in which a peripheral
> population
> is isolated and speciates, then my favorite species no longer exists "as a
> species!"
> ************
> No longer exists as a _species level taxon_, but rather as a higher level
> taxon. This can be called exinction?????
> I would never call that extinction, so I am a bit frustrated that critics of
> cladistics call it extinction, then turn around and blame cladistics for
> having an absurd assumption about things going extinct at bifurcation.
> Once again, I sense a confusion between an ecological sense of "species" and
> a systematic sense of species, as I tried to explain a few weeks ago.
> ***********

I didn't say anything went extinct.  What I said was that cladists have
explained it to me as the taxon (species A) "going extinct" at the species
level.  As you said, it no longer exists as a "species level" taxon.

> Jensen:
> I believe that species A, sister to species B, can give rise (as described
> above) to species C.  What was a branch with two terminals, is now a branch
> with
> three terminals, two species that continue to exist as before and a third
> species that now undergoes its own evolutionary fate, etc.
> ***********
> Once again you are talking about species as ecological units. The
> claasification is about historical relationships.
>   In your example, would you represent the relationships as a trichotomy? Is
> this an accurate representation, given that you have stipulated that C arose
> from A, and not from the common ancestor of A and B? Is it not more accurate
> to say that you now have that original branch with two sub-branches (the A
> branch and the B branch) and that the new species C is a sub-sub branch
> diverging off of the A sub branch?

I didn't say it was a polytomy.  I said C arose from A. The branch with two
terminals
                        ------B
                  ---- |
                        ------A

now has three terminals

                        ------B
                  ---- |
                        ------A
                            |
                             ---C
In my view, we have a clade with three species, two of which are as the original
species (A,B), and a third derived from A.  There is no reason, as I see it, for
any revision of systematic/taxonomic relationships.  B is sister to A and its
derivatives and C is sister to A.  You are right, nothing has gone extinct.
What has happened is that a new taxon has evolved from and already existing
taxon, without altering the previously existing taxon.

If A and B are nested within a large genus, then we still have the genus and we
now have a new species in addition to the already existing species.  The
difference of opinion, as I see it, is that, under your view,  a newly derived
species cannot coexist with its "ancestral" species; I believe it can.

Dick
--
Richard J. Jensen              TEL: 219-284-4674
Department of Biology      FAX: 219-284-4716
Saint Mary's College         E-mail: rjensen at saintmarys.edu
Notre Dame, IN  46556     http://www.saintmarys.edu/~rjensen