panzar at CLPGH.ORG
Fri Jun 13 10:13:48 CDT 1997
>>. Budding off does not affect higher
>>(vertical) taxonomic designations, only horizontal ones. If Mus newus evolved
>>from Mus oldus, but populations of M. oldus still exist, then M. oldus becomes
>>a paraphyletic species, but genus Mus is not affected.
>OK, If I go out in the field and discover two Mus species, which one should
>be called M. oldus and which is M. newus? In other words, sister taxa have
>equal age! My point is that any attempt to place temporal aspects on taxa
>(including species) will lead to unsolvable paradoxes. The monophyletic
*This* is why cladists don't like paraphyly, not the earlier, incorrect
argument that the arrival of M. newus causes higher levels to be paraphyletic.
As to which is oldus and which is newus, the theoretical answer is easy. If A
and sister group X are more closely related than B and X, you have evidence
that the common ancestor of A & B is A; species A is M. oldus and B is M.
newus. This statement is based on one of the primary underlying tenets in
cladistics, that evolutionary rate is more or less constant at the molecular
level (to be able to assume that degree of divergence measures age of
Cladistics theory cannot handle such a situation, which is the reason cladists
say it can't happen--not because it is impossible in nature, but because their
methodology can't allow for it.
>. If we actually saw a taxon split into two, then
>what we had seen would not be a taxon 'budding off' a new taxon of the same
>rank, but the birth of a new hierarchical rank. Note that this would not
Yes, but this is *not* what we're arguing. If a taxon has a subgroup that
changes (far enough to call another species or *any* other rank) while another
subgroup is still recognizably the same as it was, then we have "budding off",
not the "birth of a new hierarchical rank". Traditional (old-fashioned,
non-scientific, or whatever name you choose to use) taxonomy has no problem
accommodating this possibility; cladistic theory has not (yet?) become robust
enough to deal with it "scientifically".
Robin K Panza panzar at clpgh.org
Section of Birds, Carnegie MNH
Pittsburgh PA 15213 USA
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