[tdwg-content] Taxon Concept dilemma

Richard Pyle deepreef at bishopmuseum.org
Tue Jul 6 15:10:49 CEST 2010

Hi Gregor,

> I agree in theory. However, in practice, this is hard to distinguish.
> A clear case ist that a replacement epithet does NOT change 
> circumscription. 

I don't think there is any need to distinguish anything; I think it's purely
a function of logic.  *If* we define the "essense" of what a taxonID
represents as the circumscribed set of individuals (living, dead, and
yet-to-be-born)implied to be contained within the taxon circumscription,
then it seems to me that a new taxonID is needed only when that
circumscribed set changes (exapnds or shrinks).  Because taxa are nested
sets, the placement of a circumscribed set from one parent set into another
parent set doesn't affect the contents of the child set, and hence (in this
definition of what is represented by a taxonID), doesn't trigger a need for
a new taxonID.

I think the crux of the issue hinges on the premise of the above; that is:
*are* we defining the "thing" represented by a taxonID as the circumscribed
set of individuals?  If so, then I think we can be confident that a new
taxonID is only needed when the contents of the cirumscribed set changes.
But I'm not sure we all agree that's what a taxonID is intended to

> But we already disagree about merging two 
> species: I don't think is necessarily changes the 
> circumscription, it may only correct a previous oversight 
> (i.e. that one circumscription is a subset of the other). But 
> who in practice establishes whether the larger 
> circumscription is the older or the younger name.

I think we are both in agreement on this (I mis-spoke/mis-wrote in my reply
to Dave).  I think it depends on whether the merge changes the implied
members of the circumscribed set.  For example:

Suppose Smith describes Aus bus to represent all the individuals of a
population in the Marshall Islands.

Then Jones comes along and describes Aus cus to represent all the
individuals of a population in the Hawaiian Islands, and he lists all the
morphometric differences that distinguishes his new Aus cus from Smith's Aus

Then Brown comes along and decides that Aus cus is a junior synonym of Aus
bus, and that Aus bus occurs in both the Hawaiian Islands and the Marchall

Clearly, in this case, we have three taxon conepts (= three different
circumscribed sets of individuals, and need three taxonIDs):  One for Aus
bus sensu stricto (of Smith), one for Aus cus sensu stricto (of Jones), and
one for Aus bus sensu lato (of Brown).

Now, suppose instead that Jones was unaware of Smith's description of Aus
bus from the Marshall Islands, and he describes Aus cus based on material
from the same Marshall Islands population that Smith based his description.

In this case, I agree with you -- we only have one circumscribed set of
organisms, and hence need only one taxonID to represent it, despite the fact
that two protonymIDs (=two type specimens) exist within that set.

Unfortunately, a common situation is more along the lines of the following: 

Smith describes Aus bus to represent all the individuals of a population in
the Marshall Islands.

Then Jones comes along and describes Aus cus to represent all the
individuals of a population in the Hawaiian Islands, and he is *completely
unaware* of the existence of Smith's Smith's Aus bus.

Then Brown comes along and decides that the populations in Hawaii and
Marshall Islands are *clearly* and *unabiguously* the same species, so
synonymizes the two names.

In this case, how many circumscritpions are involved?  It all hinges on my
use of the word "implied" in the notion of a the "implied members of the
circumscribed set".

This is why the only way we're going to be able to establish
RelationshipAssertions (sensu TCS) is via third-party assertions.  In other
words, someone is going to have to assert an opinion over whether the
implied members of Smith's Aus bus would have included the population in
Hawaii, and whether the implied set of Jones' Aus cus would have included
the population in the Marshall Islands.

In any case, the point is I think you are right -- the synonomization of two
names does not necessarily imply that a new taxonID is needed.

As to your comment:

> My main concern is however changes of the higher taxon. If a 
> species was originally placed in a genus circumscribed as 
> leaf spots producing elliptical spores on a specific host 
> plant, and is moved into a genus with defined conidiogenesis, 
> we have changed the circumscription, and previous 
> "identifications" may now be misapplied names.

Yes -- but the new taxonID is not needed because the genus name changed; the
new taxonID is needed because the implied set of members of the child set
changed (due to the new character-based definition).  In other words, the
act of changing the genus didn't change the circumscription scope; the
change in the character definition of the child taxon changed in.  Even if
it seems these are the same thing (i.e., that the change in character-based
definition was triggered by the genus change), they are still different
logical actions.

> The central point is that in practice, significant experience 
> and knowledge about past practices is required, to determine 
> whether a new taxon ID should be given or not. Computer 
> typists will get it wrong and experts may disagree about this.

I agree that we require significant experience and knowledge about past
practices/etc. to make assertions about the logical relationships between
two circumscribed sets represented by two taxonID values.  But not for the
reason you suggest -- rather, for the reson I described above.

> My conclusion is that if every change in genus name is given 
> a new ID, the system is manageable after assignment of ID. 
> Computers can synonymize IDs (sameAs), but they cannot 
> retrospectively split them.
> :-)

Right -- and this comes back to my fundamental problem with the notion of a
taxonID.  In order for it to be useful, we need a very clear and shared
understanding of the entity that is represented by the taxonID.  What we
need to do is atomize the ID's -- and this is best done through the
TaxonNameUsage object (which is relatively simple to define).  So in answer
toy your last quote above, in my mind *every* TNU is a "potential taxon"
(sensu Berendsohn), so *these* are the Ids we should be using and re-using
when we define taxon concepts.  My vague understanding of a taxonID is that
it is established to represent a set of tnuID's.  If it were that simple,
then taxonID really just becomes a subclass of tnuID.  However, I don't
think it's that simple, because the definition of a taxonID is not limited
to simply sets of tnuIDs -- it can also be established through
member-specimens, morphological characters, genetic characters, etc.
Although I think it's logically possible to inherit all these things
*through* tnuIDs (after all, anytime anyone ever asserts that a taxon is
defined by a set of specimens or characters, a new tnuID is born); we're a
long way from having all that sort of information parsed and digitized (as
per Kevin's earlier comment).

So, for purely *practical* reasons, I think the main justification for even
defining "taxonID" (as more than just a subclass of tnuID), is that our
content base for tnuID's is still small, and the inheritance of things like
specimens and characters through TNU instances is not yet well-developed.
For this reason -- alluded to by Kevin -- I see a practical value to having
a taxonID.

But we *still* need to define what it is!


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