[tdwg-content] DwC Occurrence [SEC=UNCLASSIFIED]

Paul Murray pmurray at anbg.gov.au
Wed Jun 1 05:09:59 CEST 2011

On 31/05/2011, at 9:10 PM, Nico Cellinese wrote:

> What I don't seem to be able to digest is the notion that same individual will later be equaled  by some to a species. That assertion is hard to swallow.

Is this do do with type specimens? Or are we talking about the OWL class vs. OWL individual problem?

With respect to taxa, it seems to me that a taxon is a rule that can decide whether or not an individual is part of the taxon or not. This can be done with a description, by specifying a list of included and excluded other taxa, or by various other ways. If two taxa defined differently select the same subset of real-world individuals, then the taxa are synonymous. This is to say: the truth of assertions about taxonomic relationships can be contingent on what is out there in the real world.

(The taxa "organisms that have the front half of a horse" and "organisms that have the back half of a horse" would be synonymous if not for the occasional zebra/horse hybrid. Actually ... this is a stickier point than it seems: is a taxon a subset of all potential individuals, or a subset of all actual individuals? Or a subset of all actual individuals ... including the ones that aren't currently alive, and that we don't happen to know about? Is a taxon the *rule*, or is it the *subset* of individuals? Is the rule - the definition or circumscription of the taxon - the "taxon concept", and the actual subset of individuals the "taxon"?)

Anyway - to return to "species = individual":

Since a taxon is simply some rule for selecting individuals out of the set of all individuals, you could quite happily define a taxon "the set of all individuals whose tokens are the type specimens of Vombatus ursinus". This is not the same as the species, or the nominal concept - it's a "concept" containing one individual. I suppose it might be called the "type taxon" of a name.

Interestingly, this taxon of one individual is not an entirely useless idea. All taxa that are named "Vombatus ursinus" by definition contain the type taxon as a subset (except in cases of misattribution). From this, and the rule "if two taxa have a common nonempty subtaxon then they overlap", we can infer "if two taxa have the same (not misattributed) name, then they overlap". This is not something that could otherwise be inferred, because in general "a overlaps b" and "b overlaps c" does *not* allow us in infer "a overlaps c". It's the existence of the common subtaxon - the type taxon - that does. It's the existence of this overlap - even of just one individual - that gives meaning to different authors using the same name to label their concepts. 

(I am not sure that I am not repeating myself - I may have brought this up on this list previously)

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