I think the key point is that “taxa” represent a set of individuals implicitly (i.e., a taxon definition exists without the need to define or enumerate the complete set of individuals contained within it); whereas the class that I think we need to add to DwC is one that deals with *explicitly* defined or enumerated members. These sets of explicitly defined or enumerated members can be assigned as implicit members of a single taxon (via an instance of dwc:identification), but the difference, I think, is in is the explicit inclusion vs. implicit inclusion of members. This may seem trivial, but I believe it to be fundamental.
The problem with “populations” is that they can be defined either way (either implicitly or explicitly). If you think of them from the implicit perspective, they seem more like taxa. If you think of them from the explicit perspective, they seem more like an extension of an “Organism” instance. In a way, populations are the exception that proves the rule. Or….the exemplar that illustrates the distinction.
Aloha, Rich