paraphyletic evolution

[Ken Kinman]

Curtis Clark wrote:
>Paraphyletic groups are not natural because it requires human judgment to
>decide which clades to remove and which to leave, and any two investigators
>may have different ideas. A stem species with peripheral isolates is
>natural, because *all* of the species it spawned are no longer a part of
>it--no opinions required.
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     I disagree.  It does take human judgment to decide which peripheral
isolates have achieved species status and which have not.  The literature is
full of examples where one worker's believes a population is a good species
and another worker believes it is not (the latter believing the peripheral
could either evolve into a separate species under some conditions, or be
genetically re-absorbed by the parent species under other conditions.
     Such differing opinions at species level can be far more difficult to
resolve than where to make the cut between reptiles and birds (as I related
in the previous post on Archaeopteryx, where historical contingency has made
this the almost universal choice for the most primitive clade of birds).
      You can never escape some level of arbitrariness in classification,
although strict cladists often sacrifice usefulness and stability (and
divergence information) in a vain attempt to do it.
                  -------Ken Kinman

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