cladism's greatest weakness

Tom DiBenedetto tdib at UMICH.EDU
Wed Sep 22 19:04:12 CDT 1999

Richard Jensen wrote:

>>Tom DiBenedetto wrote:
>>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 think that this response points us to the heart of the problem with
species concepts. The notion of "species" (and "genus") were and still
are, at heart, systematic concepts, deriving from the ancient Greek
terms roughly corresponding to the specific and the general in any sort
of grouping arrangement. But the term species has come to be identified
as well with functional groups, in the "individual, kin group,
population, species, community" continuum. And this is the source of
much confusion. When I refer to reproductive isolation, I was not
placing species in this functional spectrum, but rather meant to keep
it within the systematic framework, in which we recognize species as
those lineage branches which have severed their genealogical ties with
other groups, and thus assume a discrete identity.
The humans who would remain on earth still function as a species, in an
unbroken chain with the ancestral humans, but so do the martians. It is
simply that the martians and the earthlings do not interact in a
genalogical manner with each other. So both are species unto themselves
(post-split), but both also retain the same relationship with the
ancestral species. The ancestral taxon, named and identified as a
discrete unit, without one or more of its descendants would be
paraphyletic. It would be an artifical naming decision. The ancestral
taxon now encompasses both of its descendants, by the very criterea by
which we recognize any taxon (ancestor and its descendants). The
ancestral taxon IS a higher taxon, encompassing two daughter species.
I think this is what Hennig was trying to approximate with his "species
go extinct at branching" notion. It would have been better put as
"species-level taxa are promoted to higher level taxa at branching". I
dont understand why there is resistance to the notion that our
systematic ranks (which should be tied to the hierarchy of taxic
divergence) should evolve as the taxa themselves evolve.

>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.

But it seems obvious to me that if we are to take evolution seriously,
we must recognize that this species that you have has two sets of
descendants, which do not share genes with each other. Clearly both
descendants are functioning as species, and clearly both have the same
relationship with the ancestral species as the other, so both should
have the same taxonomic identity relative to the ancestor. And if both
are species, then the ancestor and its descendants form a higher taxon.

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