An answer to paraphyly?

Tom DiBenedetto tdib at UMICH.EDU
Sat Jun 14 12:22:57 CDT 1997


On Sat, 14 Jun 1997 12:31:07 +0100, Graham Thackrah wrote:

>In reply to the recent postings on TAXACOM here are a few of my thoughts.
>
>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.

I agree with much of your posting, but I would just clarify that
"species", in a classifactory scheme, is a ranking, applied to a
taxon. The "old" species is in fact a taxon, and does not cease to
exist. The taxon has become more complex, it is no longer a terminal
taxon in the historical hierarchy, and thus no longer qualifies to be
ranked as a species, under the convention of applying such a rank
only to terminal taxa. If I may mention the standard problematical
situation which others have raised; the "budding off" of a sub-taxon
from an ancestral species,,,the new "bud" presumably has a diagnostic
apomorphy and can be described as a taxon, and being terminal, can be
ranked as a species. The "non-bud" organisms can be expected to
evolve a diagnostic apomorphy (if they constitute an inter-breeding
population) and at such point they too would be recognized as a
taxon, named, and ranked as a species. In the interim (when the
"non-buds" have no diagnositic apomorphy of their own and seem
identical to the ancestral population), they are simply recognized
for what they are, an undiagnosable assemblage within the higher
taxon, or a "meta-species".  The old name for the all-inclusive taxon
(formerly ranked as species) now refers to a higher taxon and could
be ranked accordingly (although ranking is not necessary at every
node).
>
>This surely makes paraphyly (in the cladistic sense) a logical impossibility?
>
Paraphyly does not exist in nature, for paraphyly is a *human
decision* to apply a name to a certain group of organisms. If one
chooses to recognize as a taxon, a group of organisms which which do
not contain all descendants of a common ancestor, using plesiomorphic
diagnostic characters, then one has *invented* a paraphyletic group.
It could of course, be argued that monophyletic groups do not exist
in nature either, since nature consists of simple material facts
(organisms) and any grouping of these organisms are human constructs.
But within a human conceptual framework which postulates an
evolutionary history for organisms within a system of branching
lineages, monophyltic groups can be recognized as real coherent parts
of this system; paraphyletic groups are artificial distortions of it.
        I think that the root cause of so much of this confusion is
the persistant attempt to use  non-evolutionary species concepts in
avowedly evolutionary classifications. The prime offender is the
biological species concept (BSC) which is explicity "contemporary";
recognizing species on their present day distinctivness and ability
to interbreed, and is not part of a larger conceptual system which
links this definition of "species" to the concept of a systematic
taxon, nor to the other ranks which are used in classification. Under
the BSC, the "non-bud" remnants of a taxon which has budded off a
sub-taxon can be recognized as a species. But when one attempts to
move back in time (begins to consider evolution) there is no
criterion within the BSC to distinguish the "non-buds" from the
ancestral species, and so one is led to recognize a paraphyletic
group. But the problem is not nature, it is the imposition of a
non-evolutionary species concept on a classificatory system which is
now being asked to account for history, I think this is why the BSC
has fallen out of favor to a considerable extent amongst those who
advocate a scientific classification system which is avowedly
historical, and who need a concept of species which is grounded in,
or makes reference to the notions of taxa, hierarchy, and history.




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