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Re: a rose by any other name(was fish & dogs)



In a message dated 12/3/00 3:54:54 PM EST, k.clements@auckland.ac.nz writes:

<< Just what are the "benefits" of using a paraphyletic group such as 
 this in a systematic sense? Ichthyology textbooks usually cover all 
 aquatic vertebrates with fins that breathe water, but none pretend that 
 this grouping means anything in evolutionary terms. >>

Number One, all paraphyletic groups preserve evolutionary information, since 
a paraphyletic group by definition has a unique common ancestor. It is 
completely wrong to state that the group "fish" doesn't mean "anything in 
evolutionary terms," since we presume they're all descended from a common 
ancestor and have acquired many of their shared features (fins, gills, 
sinuous endoskeleton, etc.) from this common ancestor. It is >polyphyletic< 
groups that have no evolutionary meaning, not paraphyletic groups. 
Polyphyletic groups for this reason must be avoided in taxonomies, but there 
is no such valid stricture against paraphyletic groups.

Number Two, taxonomies can be devised to present not only evolutionary 
connections but also morphological consistency within their groups. It makes 
perfect sense to have a taxonomic group called "fish" in opposition to, as 
well as ancestral to, a taxonomic group called "tetrapods," within the larger 
group called "vertebrates." Almost all fish are water-breathers, almost all 
tetrapods are air-breathers, and having formal groups that preserve this 
distinction (for example) facilitates scientific discourse. Taxonomies that 
permit occasional paraphyletic groups that preserve a particular grade of 
organismic organization are certainly easier to use than strictly cladistic 
taxonomies with their intensely fragmented basal-group structures, wherein we 
name a group, then a subgroup of the group with the basalmost genus omitted, 
then a subgroup of the subgroup with the next most basal genus omitted, and 
forth.