cladism's greatest weakness

[Richard Jensen]

> I believe there is a logical reason that we do not find groups such as the
> one described above (sister taxa; one that has evolved an apomorphy, one
> that has not).  To do so is to recognize a privative group, or a group, the
> essence of which is absence.  Aristotle noticed the ambiguity of
> recognizing privative groups; they cannot be subdivided, and as applied to
> species or lineages, neither can be logically characterized by the absence
> of nothing.  Put bluntly, the anagenetic (static, or unchanged) taxon
> cannot produce evidence that it is evolving independently from its sister
> lineage.  Thus, there is no reason for us to suspect that the two taxa
> represent distinct species (or lineages).

I beg to disagree.  We have two lineages, X (the original) and Y (the
peripheral isolate).  X retains all the properties that allow us to
recognize it as X.  Y has some new property that makes it reproductively
isolated from X.  Given that this reproductive isolation exists, we have
every reason to suspect that X and Y are evolving separately.  The fact
that X is unchanged does not prevent our recognition of X as a distinct
entity different from Y; after all, Y has a unique character (call it
anything you like (I'll call it A') that allows
us to recognize it as different from X. This would be a case where the
"absence of something" allows us to recognize that one taxon (X) is
different from another taxon (Y). My key to the two taxa would simply have
a couplet

        A' present..........Y
        A' absent ..........X


>
>
> If we don't recognize both taxa as being unique descendents of a common
> ancestor, then we are left with a conundrum:  H. sapiens on earth must at
> the same time be an ancestor and its own descendent.  This is logically
> absurd.  It seems to me that to argue for the recognition of privations
> requires rejecting the notion of evolutionary lineages (or species) as time
> extended individuals.  Not a unique proposition, but not one I'm ready to
> embrace either.

Well, given that we now view all H. sapiens on Earth as a single taxon,
aren't we already (as a species) simultaneously both ancestor and
descendant?  Living members of H. sapiens are all derived from now extinct
members of H. sapiens, but they and we all belonged to H. sapiens.

Richard J. Jensen      |   E-MAIL: rjensen at saintmarys.edu
Dept. of Biology       |   TELEPHONE: 219-284-4674
Saint Mary's College   |   FAX: 219-284-4716
Notre Dame, IN  46556  |