Striking a balance, weighting and Cladistics

[Curtis Clark]

At 10:34 AM 2/15/01, Ken Kinman wrote:
>    I don't how many nucleotide changes were involved in the evolution of
>feathers, but it was a large number.  The above statement boggles my mind
>and had my jaw dropping.
>    Even individual nucleotides are of differential importance and will
>someday be weighted routinely in some fashion.

Ken, I'm more important than you are. Just ask any of my students who are
hoping to get good grades at the end of the quarter. But I'm sure that in a
different context, you are more important than I.

The issue is context. Every homology is equally important in elucidating
phylogeny. That fact is independent of their importance in any other context.

Just yesterday, I lectured about how the deposition of lignin in secondary
cell walls was the key step in the evolution of vascular plants--that all
the other apomorphies of vascular plants were foregone conclusions once
that step had occurred. But that's an adaptive (some might say
adaptationist) context. *Any one* of those synapomorphies (assuming they
are truly homologies) is sufficient for us to recognize the clade of
vascular plants.

Ironically, adaptive importance works against us if we are unclear whether
a feature is a homology, since it can more likely be the result of convergence.


--
Curtis Clark                  http://www.csupomona.edu/~jcclark/
Biological Sciences Department             Voice: (909) 869-4062
California State Polytechnic University      FAX: (909) 869-4078
Pomona CA 91768-4032  USA                  jcclark at csupomona.edu