Dear Art,
I guess there is not such a formula, neither for species, nor families or
other levels.
I guess it may also depend on the size of the group, its evolutionary
history and other factors. I'll give one example.
There is a group of "worm"-līke sea creatures, the Priapulida. They exist
from at least Cambian age. There are about 10-15 species known world wide.
Their "bauplan" differs so fundamentally from other living animals, that
Priapulida consist a Phylum, just like Mollusca. However, recognizing all
other taxonomic categories, which are recognized within Mollusca
(subphylum, class, subclass, order and so on), would be a bit odd for the
classification of only 15 species.
Within Arthropods, especially Insecta, it is common usage to use the term
"Tribus" between Subfamily and Genus level. To my knowledge within Mollusca
this is only done within the Pectinidae, where within the subfamily
Pectininae the tribes Hinnitini, Decatopectinini, Eburneopectinini and
Pectinini are recognized, and within Chlamydinae another 5 tribes.
(Lindner, 1999: Muscheln und Schnecken der Weltmeere: 154-155).
What then? I really don't know. I guess such a "chart" is -at least for the
moment- utopian. Dubois (a french Herpetologist) has advocated the
possibility of hybridizations as one factor for recognition of genus. To my
knowledge his views are not generally accepted, but he has an interesting
point, I reckon. The problem however is how to test it and what to do with
unicellular organisms, self fertilizing species and such.
Comparison of DNA may also provide a clue. Things aren't that simple....
Indeed, the discussion is interesting, but, at this moment, fruitless I am
afraid.
Gijs
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> Van: Art Weil <[log in to unmask]>
> Aan: [log in to unmask]
> Onderwerp: Re: Olivellidae??
> Datum: zaterdag 28 augustus 1999 12:21
>
> Dear Gijs;-
> OK! I'm easily convinced---in almost any direction.
> What the Oliva/Olivella brings up to me is this question: Is
there a
> formula for separation of families? In other words: how different and in
> what respect must the differences be to split a family?
> Now that I think of it, the same question might be asked of
Genera and
> Species. It would be nice to have some sort of chart that could quantify
> the quality of differences. It is asking for the simplification of
> something that is not at all simple. But it would help us to understand
> better why an Afgan Hound and a Pomeranian are more similar than an
> Olive and an Ollivella.
> Interesting discussion.
> Art
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