[pbs] Dangerous Topic: Taxonomy
Joe Shaw
jshaw at opuntiads.com
Sat Oct 14 12:44:11 EDT 2006
Hi Gang,
A discussion on another list (cacti) prompted me to look up the definition
of a phylum, or merely a taxon. I found the following wording and realized
it is almost precisely what I learned in college in my plant taxonomy class.
One key part of the definition is the word "arbitrary."
Definition:
"A phylum is part of the hierarchy of classification of organisms. It is an
arbitrary grouping; that is, it is developed from a combination of
scientific observation, theorizing, and guesswork in an attempt to find
order in the complexity of living and extinct life forms. The same is true
of all classification levels above and below it except for species, which
consist of organisms known to be capable, at least potentially, of
interbreeding" (FROM: "Phylum," Microsoft® Encarta® Online Encyclopedia
2000; http://encarta.msn.com © 1997-2000 Microsoft Corporation. All rights
reserved.)
It occurred to me that much miscommunication (or misunderstanding) might
derive from the arbitrariness of taxonomic decisions. Such decisions as
where to place a division (e.g., these plants belong to 2 different genera)
are inherently tricky when there is no universal definition of "how far
apart organisms must be in order to belong to different genera." The
decisions are not silly, or taken without great deliberation, but they do
involve some educated guesswork and hypothesis-testing and hence, have an
arbitrary element.
It is no wonder that people disagree about where to draw the boundaries.
The boundaries are placed according to human decisions, but there is no
higher authority that controls "where" to place a boundary. Human-created
rules (such as the ICBN) have no inherent mechanism to force anyone to agree
with anyone else about where to place a split, or where to lump.
In 2003 the Angiospermy Phylogeny Group (AGP II report) lumped several
monocot families "back" into the Alliaceae and the Asparagaceae. If this
arrangement catches on, such families as the Agapanthaceae and others will
be footnotes in history. The decision is arbitrary but well-thought-out and
the rationale for such lumping is contained within the article (see pages
403 and 404 for the discussion). I've put the article online so interested
parties can read it. It was published in the Botanical Journal of the
Linnean Society, 2003, 141:399-436.
LINK: Angiosperm Phylogeny Group II Report, 2003
http://www.opuntiads.com/pdf/APG%20II_2003.pdf
Cordially,
Joe
Conroe, TX
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