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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" <lucasjosemarti@yahoo.com.ar>
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" is misleading...


(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 + "species 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_ description.


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.

Cheers

Lic. Lucas Jose Marti
FCNyM. UNLP.
Tel: (54)0221-4871856

----- Mensaje original ----
[Nobody needs that.]