cladism's greatest weakness

Richard Jensen rjensen at SAINTMARYS.EDU
Thu Sep 23 08:40:16 CDT 1999

On Thu, 23 Sep 1999, John Trueman wrote:

> Richard Jensen wrote:
> 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
> Going back to my earlier comment re paraphyly, X is identifiable only as
> that part of X+Y which does not have A'. In other words it is identifiable
> by plesiomorphies alone. Is there any reason it should not be considered
> paraphyletic?

Well, let's suppose X has three uniquely derived autapomorphies that allow
us to recognize it as distinct from all other taxa related to X.  Y
evolves a unique autapomorphy, but maintains the three autapomorphies
found in X.  Now, X+Y is a monophyletic group defined by three
synapomorphies, and Y is a monophyletic group defined by a single
autapomorphy.  Does X lose its status as a monophyletic group simply
because it does not have a feature that is unique to X with respect to Y?
Both are evolutionarily diagnosable entities.  My view is that Y is
diagnosed as that part of X+Y that has a unique feature not found in X,
and both share a common evolutionary history with respect to all other
closely related taxa.

So, my diagnosis for these would be

        Possesses B', C', and D'.......... X+Y
                Possesses A' ............. Y
                Lacks A' ................. X

And, if we hypothesize that A' is a feature (not simply geographic
isolation; I don't believe I ever said this was sufficient for
recognizing species) that invokes reproductive incompatibility, we have
two distinct lineages that qualify as biological species as well as
evolutionary species with respect to each other.

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

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