cladism's greatest weakness

[Richard Zander]

----- Original Message -----
Byron J Adams wrote:

> Regarding speciation, somebody (I don't recall who) said:
>
> > >No, ancestral species do not have to
> > >disappear (it is not a convention), but given the data sets, we seldom
> find
> > >a sister branch (as a computational tool, mind you) with no syn- or
> > >autapomorphies (so practically speaking ancestors disappear even if the
> > >speciation event that destroys them is just gradual change).
>
> 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).

Interpretation of lineages as surviving ancestors is possible. If a terminal
taxon has no autapomorphies (no branch length) it still exists, and is
privative only in the absence of traits interpretable as advanced and not
also in its sister group/taxon.  Examples are given in:

Zander, R. H. 1998. A phylogrammatic evolutionary analysis of the moss genus
Didymodon in North America North of Mexico. Bull. Buffalo Soc. Nat. Sci.:
36: 81-115.


Richard H. Zander, Curator of Botany
Buffalo Museum of Science
1020 Humboldt Pkwy
Buffalo, NY 14211 USA
email: rzander at sciencebuff.org
voice: 716-895-5200 x 351