[Tcs-lc] Concepts of Higher Taxa

[Richard Pyle]

> > My understanding of the circumscription of a genus is biased towards the
> > species that are included within it and less so on its description and I
> > think this is pretty common. As we go up the hierarchy taxa are defined
> > more by their members than by anything else. An author's definition of a
> > family is usually more or less just a list of genera arranged into
> > intermediate taxa of some kind.
> 
> I don't believe definition of taxa by membership is commonly held -- at 
> least not in a strict sense.  The problem of cascading new taxa makes that 
> system unworkable (IMHO).  If adding a new member to a taxon effectively 
> means defining a new concept, then any newly identified specimen requires 
> a new set of all parent taxa all the way to root (life).  That makes every 
> taxonomy unique -- non-reusable -- and therefore worthless.

I think we need to clarify an important distinction here:

1) A taxon concept circumscription is defined by the members that it contains.

2) A taxon concept circumscription is defined by the set of members available to the original definer, and explicitly indicated in the original concept definition.

I believe in number 1.  I don't believe in number 2.

Taxon concepts are inherently messy.  Until a two-point definition system is established (and technology advances to the point where phylogeny can be known to a level of confidence approaching 100%), there will never be a mechanism to reliably know the full extent of members that are included within any concept definition.  This an aspect of taxonomy that we must not only acknowledge, but embrace.

Thus, when explictly assigning members to a concept, it will always be a matter of interpretation whether the act of doing so re-defines the concept circumscription beyond the intent of the original definer.  The evidence we have to make this judgement is primarily available in the form of taxonomic descriptions, and states of charcters.

Thus, I do not believe that statement #1 above is incompatible with the idea of reusable concept definitions.

> Intuitively, a lot of people casually equate content with a definition, 
> but I it's more like the application of rules against a set of objects 
> (filtering by circumscription); if you end up with a different set of 
> objects, they must have been different rules. Some may want to argue 
> this point, 

I'll withhold further argument, because I think we may be stuck on a semantics issue.

> but what I really want to say is that the strength of TCS 
> is that it's (supposed to be) agnostic about how one defines taxa 
> (taxonomic concepts).  Therefore, a consistent model applied to all 
> ranks (or all taxa in a rankless system), from species through kingdom
> should work.

Agreed!! I didn't realize this was the point of discussion.  Yes, by all means, TCS should be agnostic in this regard, and I think the current version achieves this very effectively.

Aloha,
Rich