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Re: Defining Ornithischia (was Re:)
Entire text repeated (the original was in double-spaced HTML, which gets
mangled or truncated on the list), my few comments interspersed:
----- Original Message -----
From: "Lucas Marti" <firstname.lastname@example.org>
Sent: Thursday, September 20, 2007 6:46 AM
Subject: Re: Defining Ornithischia (was Re:)
Perhaps it will help. I don´t know. Extract from "Evolution" Mark Ridley
Description and Diagnosis in Formal Taxonomy
We're talking about the difference between definition and diagnosis, not
between description and diagnosis...
The point of the example of the two eagle species is intended merely to
demonstrate that species are defined in practice by observable phenetic
characters. We should also notice a terminological formality,
distinguishing between a formal description of a species and a diagnosis.
The formal definition is the description of the species a in terms of
phenetic characters a that a taxonomist originally supplied when naming
the species. Certain rules exist about the naming of new species, and the
characters specified in the formal definition are the "defining"
characters of the species in a strictly formal sense.
Except that everyone is allowed to change them, so calling them "defining"
(For the record, it's also misleading that the ICZN consistently calls
"species name" what the ICBN calls "specific epithet". According to the ICZN
itself, the name of a species is the binominal as a whole, genus name +
I also don't understand the emphasis on "phenetic characters". You are
allowed to use, say, only autapomorphies (innovations that the species in
question doesn't share with its closest relatives) -- that doesn't matter.
The formally defining characters of a species may be difficult to observe
in practice. They might, for instance, be some fine details of the
creature's genitalia, which can be recognized only by an expert using a
microscope. Taxonomists do not on purpose pick obscure characters to put
in their definitions, but if the only distinct characters that the
species' first taxonomist noticed were obscure ones then they will provide
its formal definition. If the formally defining characters are
inconvenient to observe, subsequent taxonomists will try to find other
characters that are more easily observable. These useful characters, if
they are not in the formal description, provide what is called a
"diagnosis." A diagnosis does not have the legalistic power of a
description to determine which names are attached to which specimens, but
it is more useful in the day to day practical taxonomic task of
recognizing which species specimens belong to.
I have never seen "diagnosis" used for such a meaning. The legalistic power
is not contained in the description itself -- it is contained in the
designation of the type specimen(s), and in the _presence_ of _a_
The description, including the "definition", is allowed to be wrong. I'm
allowed to make up a completely bogus "definition"; I'd do my colleagues a
disservice, but the letter of the Code would be fulfilled, and the name
would still be valid as long as it can be shown (by someone) that the type
specimen doesn't belong to an already named species.
As research progresses, better characters (i.e., more characteristic of
the species and more easily recognized) may be found than those in the
first formal description. The formal definition then loses its practical
interest, and the characters given in a work like Peterson's Birds are
more likely to be diagnostic than formally defining.
They can be completely wrong, too.
These two sentences sound like "defining characters" actually exist in
nature. Apparently the author comes from a tradition where a few of the
diagnostic characters are arbitrarily chosen as "defining characters"... the
ICZN doesn't require that.
When an evolutionary biologist discusses the definition of species, the
formal distinction between description and diagnosis is beside the point.
All that matters is that phenetic characters are used to recognize
species, as in the eagles. The distinction is worth knowing about,
however: both in order to avoid unnecessary muddles, and for other reasons
a[lso: ?] taxonomic formalities are important in the politics of
conservation, for instance.
"Definition" is not mentioned here, and the last sentence doesn't mean much.
[Nobody needs that.]
Lic. Lucas Jose Marti
----- Mensaje original ----