cladism's greatest weakness

[Byron J Adams]

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).

Regarding speciation via budding, Richard Jensen said:

>Given that you have used a specific criterion in your example
>(reproductive isolation), then I would argue that the taxon Homo sapiens
>is still a species (those on Earth), and those on Mars represent a new
>species of Homo (H. ares?).  Those on Earth still behave as a single
taxon
>consisting of a large number of reproductively compatible individuals.
>
>I don't see the necessity of raising H. sapiens to a higher taxon.  As
you
>say, nothing has gone extinct or disappeared.  If a have a species, and
>nothing goes extinct or disappears, then I still have a species.

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.

Byron Adams
Department of Nematology
One Shields Avenue
University of California
Davis, CA 95616
bjadams at ucdavis.edu