Dear conchlers,

> I've  wondered about this myself.  The grouping of a number of similar
> genera into a "tribe" often seems to be a semi-arbitrary process, with
> frequent disagreements amongst the experts in a given field.  A better
> question may be "If you have a group of genera which you deem
> sufficiently similar to warrent grouping them together in a less than
> family - oriented manner, how do you decide whether to call the
> resulting taxa a "tribe" (a term very seldom used in molluscan taxonomy,
> for some reason), or a "sub-family"??  Is there any hard and fast
> difference between the two taxonomic categories, other than that the
> "tribe " is deemed to be a slightly more cohesive or tighter grouping
> than that of "sub-family"?

This question, is extremely hard, and possibly not to be answered within the
framework of "Classical Linneaen Taxonomy", often based on morphological
similarities and differences Let's get one step earlier: what defines a
genus, or even, what defines a species? At present (and possibly never) it
seems to be impossible to define the species category inclusive of all
living creatures, that is including all animals, plants, fungi, unicellular
organisms, archaeobacteria etc..
This is also true for the defenition of "Genus", "Subgenus" and other
supraspecific categories (there is also a category subtribus).. When we
simply look at the practice
among collectors and scientists within "well known" families, such as
Cypraeidae and Strombidae, there is some debate going on: ane person uses
simply one genus "Cypraea" as a "catch-all", while others recognise several
genera, even divided again into subgenera, the same is true for Strombus
(with, rather oddly when you take a good look at the shells, recognising
Lambis as a separate genus, whilst Lambis is in fact nothing but a Strombus
with a few digits more, which indeed is rather conspicious for the human
eye, but the widely flaring, thick edged outer lip of e.g. "Strombus"
latissimus differs, with a little fifference in viewpoint, just as much as
the non flaring, thin edged lip of "Strombus" fragilis).
What helps a bit in this case (without providing clear answers) is the
fossil record, which allows one to "follow back in time" the trail of a
certain species; trying to reconstruct a (few) branche(s) of a tree, trying
to reconstruct where species (of groups of species branched off) and lable
these in higher categories, such as genera.
One might say that the more species there are in a Phylum, or subphylum, the
more refimement in categories is needed for such a reconstruction. the
prefixes "super", "sub" and "infra" are used for that purpose, and "tribus"
was invented for a category between subfamily and species.
When a scientist uses the term "tribus", he or she does so to define a group
of genera closer related to one another as would be indicated by the use of
the term "Family" or "Subfamily", as part of the reconstruction of the
phylogeny of a species. trying to include as many data as possible
(morphological, fossil, anatomical, DNA). This may lead to the for some of
us strange situation that one family is divided into two tribes, with one of
them consisting of only one genus with one species, and the other tribus
with 10 genera, each consisting of a dozen or so species.
But it remains more or less arbitrarily, where viewpoints of scientists also
matter.

Gijs

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