The units of evolution, paraphyly, etc.

Tom DiBenedetto tdib at UMICH.EDU
Mon Jun 16 13:08:07 CDT 1997


Harvey Ballard writes:
>My problem with accepting only monophyletic species involves three points. (1) A >number of papers have recently suggested that an enormous number of >paraphyletic species probably exist, in keeping with evolutionary hypotheses of >certain speciation models.

In systematics, a species is a taxon; it is something we identify and
name according
to our standards. To say that an enormous number of paraphyletic
species exists is merely to say that we have decided to identify and
name a number of groups of organisms which are paraphyletic, but meet
some criterea (your personal crtierea) for being considered a
species.
Yours are not the only criterea. The problem, which I have tried to
point out before, is the use of a biological species concept in a
historical framework. In systematics, "species" is a rank applied to
a taxon, and a taxon is a group of organisms identified through an
analysis of their characters. A biological species concept uses very
different criterea for recognition, and is not conceptually embedded
in a historical context (a hierarchy of branching lineages). That a
species recognized under a biological concept is artificial when
viewed in a historical context is not surprisisng. But it is not a
characteristic of nature we are discussing here, but a confusion of
different menings of the same word.

(2) We biologists worldwide, and individually, should be exerting
much MORE effort to catalog and characterizing biodiversity from an
alpha taxonomic standpoint before it slips from our grasps in the
next few decades, and to leave taxa UNNAMED means to leave them
uncharacterized--a suggestion that I find unconscionable and cowardly
in the face of the biodiversity crisis assailing us.

I dont think this is a strong argument. One can certainly name any
specimen one comes across, if one can distinguish diagnsotic
characters for it. If not, then what? Or if the "species" proves,
under histroical analysis to be undiagnosable, then its name is
rendered in quotation marks, or otherwise designated as undiagnosable
within a larger context. I dont see how this need have any adverse
effect on conservation-type isssues. Unless you mean we should
pretend to know more than we do...

(3) Considering that organic evolution proceeds in a
"non-monophyletic" fashion a goodly share of the time, systems of
classification that refuse to recognize paraphyletic species reject a
substantial proportion of the products of evolution and, in essence,
create an "unnatural" classification system.

How can you say this? A paraphyletic group is a group which YOU
choose to recognize, because of your own criterea, and which consists
of an ancestor and some, but not all of its descendants. How can you
claim that nature produces such groups?




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