genetic vs morphological trace of phylogeny

Curtis Clark jcclark at CSUPOMONA.EDU
Tue Apr 13 18:07:43 CDT 2004

At 09:59 2004-04-13, John Grehan wrote:
>Caccone and Powell (Evolution, 1989) [...]
>  "Virtually all molecular phylogenetic studies, including these, have a
>major underlying assumption: the genetic similarity of difference among
>taxa is an indication of phylogenetic relatedness. Lineages that diverged
>more recently will be genetically more similar to one another than will be
>lineages with more ancient splits"

This was a long time ago, as such things progress. "Genetically more
similar" implies a distance measure, and that is no longer done. If
substitution rates are much higher in one lineage than another, sister
lineages may be genetically "less similar" than one or the other is to the
outgroup (just as birds are "less similar" to lizards than crocodiles are).

Nothing can be tested with absolute rigor. But our understanding of the
parameters of DNA evolution is much better, and we can consequently model
the process better.

I'm no longer exactly responding to John, per se, but addressing those
points that he brings up that I believe need clarification for others on
the list. I should mention also (and John to his credit has never accused
me of this) that I do not discount the similarities among orangs, humans,
and fossil hominids. I suspect that they will turn out to be
symplesiomorphies, but I haven't studied the specific issue well enough to
have an informed position.

Curtis Clark        
Web Coordinator, Cal Poly Pomona                 +1 909 979 6371
Professor, Biological Sciences                   +1 909 869 4062

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