an answer to paraphyly

Doug Yanega dyanega at MONO.ICB.UFMG.BR
Sun Jun 15 17:20:46 CDT 1997


All my mail for the last week was deleted from my server before I read it,
so I'm coming in late here, and forgive me if I reiterate someone's
comments.

Robin Panza wrote:

>>As far as I understand it, after a speciation event, two entirely new taxa
>>are described, if such an event could be witnessed. The "old" species they
>>evolved from ceases to exist as far as cladistic methodology is concerned.
>                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>Ah, but that's the point!  It has nothing to do with nature, just a byproduct
>of a theory that is inadequate to describe nature.  If a population is
>separated from the "parent group"
[snip]
>None of these will have any effect on the parent population, no matter how much
>or how little the colonists change.  Cladists must give the parent taxon a new
>name, even though it hasn't changed, just because there is now a differentiated
>offshoot somewhere.

Somehow this seems to be confusing cladism as a theoretical ideal or
philosophy versus its actual practice. In practice, we have no way of
*knowing* when an event like this has occurred - all we can do is
reconstruct a hypothetical tree of relationships, and give species names to
the terminal taxa. I don't suppose many cladists will argue that it is
impossible or even unlikely that many terminal taxa are *actually*
paraphyletic in the sense you describe above (where the taxon has given
rise to other taxa in the past), but there's no way for us to reconstruct
that sort of history no matter what logic or methodology we apply. All we
normally EVER know is the identity of the terminal taxa, not what the
ancestors were for any of them! Shoot, if we knew that, we wouldn't need to
build trees using indirect methods, would we? Further, if one of your
hypothetical parental populations buds off multiple independent taxa, for
example, we're talking about a "natural" polytomy, where several taxa are
all essentially equally related to one another, but this is also a
situation that is very unlikely to ever be evident from *any* sort of
analysis, be it molecular or morphological, cladistic or phenetic. ALL
methodologies are inadequate to deal with such a case (unless you count
"omniscience" as a methodology ;-) ), and likewise the situation you
describe is one where ALL systems of phylogeny reconstruction and
classification will fall short. We'll never know for sure that species X is
an offshoot of species Y - that "if such an event could be witnessed"
clause in the original message is pretty darn important, I think - I don't
think this a genuine (nor unique) shortcoming of cladistics, to claim that
*if* we could witness speciation, we'd find that cladistic methodology
compelled us to do something illogical. Maybe it would, but that's one heck
of a big "if".

Sincerely,

Doug Yanega    Depto. de Biologia Geral, Instituto de Ciencias Biologicas,
Univ. Fed. de Minas Gerais, Cx.P. 486, 30.161-970 Belo Horizonte, MG   BRAZIL
phone: 031-448-1223, fax: 031-44-5481  (from U.S., prefix 011-55)
                  http://www.icb.ufmg.br/~dyanega/
  "There are some enterprises in which a careful disorderliness
        is the true method" - Herman Melville, Moby Dick, Chap. 82




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