Brent,
Your insistence on strictly monophyletic
classification leads to at least as many problems
and paradoxes as anything raised by the "species
concept" - How come you are so concerned with the
"problems" (as you perceive them) in the species
concept but yet you ignore the paradoxes raised
by using strict phylogeny (i.e., excluding
paraphyly) as the basis for classification?
For example, about species, you mention that we
do not know that phylogenetic distance between
what are currently regarded as species in say,
mosses, may not be the same as that in a genus of
(say) beetles. This point may be valid, but to
me it does not bring down the species concept.
Species in fact can often be defined by the
breeding behaviour of populations, especially for
example in birds and mammals, and to this extent
species are objectively definable. More
importantly, the species, genus and family
concept works well for most plants, in that
people from all over the word and from many
cultures are able to recognize these kinds of
natural groupings. Far from being a "problem",
in practice, the Linnaean-derived system we use
when communicating about organisms is one of the
most remarkably accepted and successful
achievements of science, or indeed of any other
multi-national human activity. It ain't broke.
But even if there are "problems" with the species
concept, why do you ignore the greater (to me, at
least) problems and paradoxes with strict
monophyletic classifications? Others have
already mentioned here the issue with
allotetraploids that originate multiple times.
If independently-derived allotetraploid
individuals breed with each other (as indeed they
do) but not with their parents, what happens to
your monophyletic system?
To me, a more general concern with strict
monophyletic classsification is that it seems to
ignore different degrees of selection. A good
example ( amongst many) concerns the relationship
between the Hawaiian Silverswords and the
California genus Madia. Bruce Baldwin's work, as
I understand it, has shown that Hawaiian
Silverswords arose from within Madia, by
migration of an ancestor from California to
Hawaii. If you adhere to strict monophyly, this
seems to mean that one cannot place Silverswords
in one or more separate genera (or "clades" in
your terms) without splitting Madia here in
California. This creates multiple problems: the
Hawaiian plants are much more different from
Californian Madias than these current Madias are
from each other. This is easy to understand,
because presumably there was much stronger
selection for morphological change in the new
environments of Hawaii than there was in the
already-adapted Madias that remained at home.
Yet, monophyly requires them to remain in Madia
forever, no matter how much they change, or
alternatively, Madias in California must be split
no matter how much they remain unchanged. If
this seems bad now, imagine a thought experiment
a hundred million years in the future. Even if
Silverswords have by then developed wings and
learned to fly, under monphyly they will still be
just good old Madias, unless the California
plants happen to change and become splittable too.
As Alan Smith likes to mention, the other thing
that could happen is that one branch of Madia in
California could go extinct, as the species it
contains are rare. This could save monophyly in
this case. But classifying Hawaiian plants
depending on whether and when we perceive a
Californian plant as extinct seems reminiscent of
Schroedinger's Cat. Should we call it call it
the paradox of Schroedinger's Madia?
Bet wishes,
John Game
Research Associate,
University and Jepson Herbaria,
University of California,
Berekeley, CA.
jcgame@stanford.edu
510 527 7855
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