queries on rank;
danb at AMSG.AUSTMUS.OZ.AU
Wed Jan 11 14:27:01 CST 1995
Peter Stevens asked:
"1. Does anybody know of a reference where it is stated that the smaller the
genera, the more features they will have in common? It doesn't matter
whether the ref. is botanical or zoological. I seem to remember hearing
about such a statement - perhaps by C. Bremekamp, but I didn't take the
reference, and it was twenty years ago..."
This is an interesting question. Of course, smaller genera
will have more features in common because they comprise
fewer species and hence fewer chances that eceptional
characters will occur. As well, small genera may be the
result of excessive splitting and taxonomic attention, or
they may somehow be relictual, with messy
intermediates having become extinct (never doubt the value
of extinction in helping to define taxa!).
However, the converse does certainly hold. The larger
the genus, the more chance there will be characters which
are not held in common. This was discussed among other
things in an excellent paper:
Gauld, I. & L.A. Mound. 1982. Homoplasy and the delineation
of holophyletic genera in some insect groups. Systematic
Entomology 7: 73-86.
This paper should have been published in Systematic Zoology
not Systematic Entomology, because it was overlooked
by most systematists. However, it has important
things to say about cladistic problems in large taxa. Here
it should be said that many insect genera have more
than 500 species, and quite a few more than 1000 species.
One point worth noting is that large taxa often require a
"polythetic definition" , i.e., they are based on a mosaic
of characters, no character in isolation being diagnostic
for all members. Thus strong defining characters (hopefully
synapomorphies) may be non-existent, because with large
species numbers, there is a greater chance for reversal,
Dan Bickel, Entomology Section, Australian Museum, Box
A-285, Sydney South, NSW 2000 AUSTRALIA.
e-mail: danb at amsg.Austmus.oz.au
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