cladism's greatest weakness

[Richard Jensen]

On Tue, 21 Sep 1999, Tom DiBenedetto wrote:

> >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).
>
> This discussion is totally bound up in the subtlties of what you
> consider a species to be. I dont know of any cladists who accept
> Hennigs notion about ancestral species going extinct. Speaking just for
> my self, it has always seemed pretty clear to me that when a
> species-level taxon diverges (whether through "budding" or an even
> division), the taxon (the lineage) becomes a higher taxon (becomes a
> diverged, i.e. a more complex lineage). If we sent a colony off to
> Mars, and they became reproductivly isolated, then the taxon Homo
> sapiens would no longer be a species level taxon, it would refer to a
> more complex lineage, and would be a higher taxon. Nothing has "gone
> extinct" or disappeared.

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.

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  |