At 06:36 PM 10/16/00 +0100, you wrote:
>In cases like that above, the lack attribution of phylum rank for these
>'sponges' hides the fact that the
>Porifera may very well have given rise to an independent phyla.
Glenn,
For your (and other secondary-career evolutionary biologists' )
information, the singular form of "phyla" is "phylum". There are certain
contexts (e.g., the above sentence) where you mistakenly use the plural
form. The same rule applies for genus (singular) and genera (plural).
As for the issue of defining phyla, even body plan has its problems as a
criterion. Sometimes what appears to be a major difference in body plan
turns out (upon more detailed analysis) to have a fairly subtle basis in
development. In actual fact, our current hierarchical taxonomic system
based on nested groups within groups and binomial nomenclature is incapable
of representing the complex reality of evolutionary relationships. There
are all kinds of reasons for this. For example, certain plants species may
be related by one "family tree" or phylogeny with reference to their
nuclear genome and a rather different phylogeny with reference to their
chloroplast genome. In the end, every taxonomy is a classification based
on a compromise of several different criteria, including monophyly (that
the species involved are grouped according to real historical "branches" on
the evolutionary tree of life, at least as is represented by the bulk of
the genome), morphological homology (including complex body plans, etc.),
and practical use in scientific communication.
The important point (i.e., the very point you are making, too) is that
phyla (or taxa at any rank) are not static, unchanging "kinds" or
Aristotelian "essences" in the mind of God. They represent living branches
and networks of historical continuity in evolutionary history.
Doug
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