An answer to paraphyly?

Graham Thackrah G.Thackrah at NHM.AC.UK
Sat Jun 14 12:31:07 CDT 1997


In reply to the recent postings on TAXACOM here are a few of my thoughts.

As far as I understand it, after a speciation event, two entirely new taxa
are described, if such an event could be witnessed. The "old" species they
evolved from ceases to exist as far as cladistic methodology is concerned.

If one thinks about it this must be the case. If a species consists of a
homogenous (with respect to their diagnostic features) population of
individuals any group of organisms "budding off" would have to disrupt this
homogeneity. If the entire population of species A is separated into two
groups then the two groups will share some characteristics and not others.
When it comes time to describe these two entities one would naturally
describe two distinct species. Their common ancestor would be the homogenous
population prior to the separation, NOT one of the extant taxa. There is no
way to my mind that one of the two groups can be described as an ancestor to
anything and certainly not to each other.

This surely makes paraphyly (in the cladistic sense) a logical impossibility?

Any conflict between this and present classificatory systems is one to be
resolved by those involved in classificatory work. If it is decided, for
reasons of expediency, that paraphyletic taxa should have a place in
classificatory schemes then it should be realised (and explicitly stated)
that expediency is the only reason for doing this.

Graham Thackrah.
M.Sc. Advanced Methods in Taxonomy and Biodiversity.




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