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Re: Archaeopteryx (rant)
The problem is that cladists require taxonomic groups to be clades, which
is as arbitrary a convention as any other in taxonomy.
No. Dead wrong. This convention is not arbitrary. It has as its
foundation the concept of common descent.
What makes a group natural, that is, taxonomically valid, is descent from a
common ancestor--and the common ancestor itself need not even be included.
That's all that's necessary for biological taxonomy.
Using that rationale, one could justify polyphyletic groups as well. After
all, life on planet Earth has (as far as we know) a single origin. One
could arbitrarily exclude any species you want to from a group, and use the
remaining species to form a new "taxonomically valid" group.
Here's a more specific example of how flawed your definition is. Rhinos,
hippos and elephants can all claim descent from the same hoofed ancestor;
thus one could propose Pachydermata as a "taxonomically valid" group
containing only rhinos, hippos and elephants. The fact that horses, tapirs,
hyraxes, pigs, antelopes, giraffes, deer, sheep, goats, wildebeest and
whales are all excluded from Pachydermata does not mean that it's an invalid
group. As you state: "What makes a group natural, that is, taxonomically
valid, is descent from a common ancestor--and the common ancestor itself
need not even be included." Pachydermata fits your definition precisely.
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