Paraphyly

[Curtis Clark]

At 10:17 AM 06/13/97 -0400, Robert J. Soreng wrote:
>Having said this, I'll toss into the ring a position that I've heard
>some cladists recently take, that polyphylly is not theoretically
>clearly distinguished from paraphylly.  I have not read anything about
>this argument and can not defend it one way or the other, but am curious
>as to what may have developed from it.

The idea behind this is that a polyphyletic group and a paraphyletic group
don't differ by group membership, but rather only by the inclusion or
exclusion of hypothetical ancestors. Thus the Cyperales sensu Cronquist
could be paraphyletic, rather than polyphyletic, if it consisted of the
Cyperaceae, Poaceae, and their common ancestor, but explicitly excluded the
Joinvilleaceae and Juncaceae (just as Reptilia explicitly excludes Mammalia
and Aves). But if Joinvilleaceae and Juncaceae were also united in an Order
that included *their* common ancestor, that ancestor would simultaneously
be a member of two taxa at the same rank, a clear no-no.

It is also possible to make a paraphyletic group with the same membership
as a  monophyletic group, again by excluding the common ancestor. I'm no
fan of paraphyly, but I think the argument that paraphyly is
indistinguishable from polyphyly is only true of groups where there are
*no* hypothesese of kinship.


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Curtis Clark                       http://www.is.csupomona.edu/~jcclark/
Biological Sciences Department                     Voice: (909) 869-4062
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