Today I visited the poll on Efrain de Luna's IAB blog site and
discovered I am the avatar for those in favor of paraphyly. I am
flattered, but it is ironic because I am not not voting. I won't vote
because I don't see this as such a black and white issue. I do have a
position that is not shared by many contemporary phylogenists. I hope
it is worth expanding on.
This discussion took off when Johannes Enroth responded to a query
about local endemics by mentioning a distinctive island endemic
recently found to be nested within a widespread, common species. I
stuck my foot in it by suggesting it would be silly to consider
breaking up the common species in order to recognize the rare, local
endemic species. Things got hot when Brent Mishler claimed as "truth"
that the rank of species is not real. I chided him for exaggerating
his authority and tried to explain how I use the concept of species in
my work. He responded by urging me to read his article making a case
for the unreality of species. I have read it and it was good to do so.
It helps me understand better why Mishler believes what he does and
helps me articulate better why I see things differently.
Although Mishler writes with an authoritative voice, there are several
weak points in his arguments, including logical fallacies and factual
distortions, which undermine his case for rejecting species as a
useful ("real") concept. I will address two important theoretical
points.
Mishler cites Darwin as supporting the notion that there is no point
in the evolution of lineages that species become distinct. This is
revealing, as it exposes Mishler as a gradualist (just like Darwin, to
be sure); a position that I think of as having been "debunked" with
the exposition of punctuated equilibrium. It is not surprising,
however, that Mishler adheres to gradualism because phylogenetic
analysis of DNA sequences does not provide a means of detecting the
nature of tempo in evolution. Being constrained by the blinders of
apomorphic cladistics, Mishler and his colleagues are stuck in
gradualist thinking, their data unsuited for discerning the
discontinuities derived from surges in evolutionary processes.
Affection for gradualism might help explain the fallacy of the
molecular clock. Only paleontologists, it appears, have unequivocal
data to demonstrate that there are discontinuities in evolutionary
rates along lineages through time. It is existence of these
discontinuities that are critical to the coherence of discontinuities
by which we recognize species.
The other major weakness in Mishler's case against a useful species
concept is his insistence that if a species were to be real, it would
have to be characterized by monophyly based on apomorphies. Now, I
like monophyletic groups as much as anybody. It seems clear to me that
most species are, indeed, monophyletic, as a survey of recent papers
will show. The problem with demanding that monophyly (sensu Mishler)
is a required criterion for species recognition, and positing it is a
sufficient criterion, is that it ignores an important aspect of
evolution, one critical to understanding species as unique biological
entities.
As with the issue with gradualism, doing phylogenies means being
blind to data that reveal the diversity of patterns of species and
speciation. Although devotion to detecting and grouping apomorphies is
critical to the generation of good phylogenies, there is more to
understanding evolution than clarifying phylogeny. Mishler seems to
forget that Darwin's most significant contribution to evolutionary
theory was not just that all organisms have a lineage that is unbroken
through time, but that the direction of the changes is influenced by
natural selection. And that natural selection is just as likely to
select for stability based on adaptive value of plesiomorphies as it
is to select for change due to advantages of apomorphies. Natural
selection may produce species that have unusual breeding systems
inimical to monophyly. Natural selection coupled with a dynamic
environment serves to emphasize the distinctness of the entities we
can call species. Recognizing species as the basic unit upon which
natural selection acts distinguishes the species rank as different
from all others. Knowledge isn't perfect on this, and things get murky
when random effects overshadow natural selection, but I believe this
concept is more useful than inconsistent monophyletic ranks.
I do mean ranks. Phylogenies are not rank free. Reading the phylocode
literature, one comes to realize that a perfectly resolved phylogeny
has n-1 ranks, all of which could be named. Add one more object and
the number of ranks changes. Now, that's chaos!
There are a number of practical matters covered in Mishler's paper
which I could dispute, such as the utility of the Phylocode or
applications to conservation, but I will restrict this contribution to
analysis of theoretical issues regarding species. I hope I have
expressed myself clearly. I am better at writing keys than writing
about the theoretical basis for the taxonomic system that is behind
them.
References:
Brent's paper:
My latest key, written for web browsers:
http://oregonstate.edu/dept/botany/herbarium/racoweb/ARACKEY.htm
David H. Wagner, Ph.D.
Northwest Botanical Institute
P.O. Box 30064
Eugene, OR 97403-1064
davidwagner@mac.com
541-344-3327
http://web.mac.com/davidwagner/Site/FernZenMosses.html
On Oct 7, 2009, at 8:04 PM, Brent Mishler wrote:
> All I am saying is that the
> rank of species is unreal.
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