queries on rank;

DanB 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,
          non-expression, etc.



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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