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Re: New finds



At 2:52 AM -0400 7/11/02, Dinogeorge@aol.com wrote:

<< And to add a dinosaur thread, the new find also shows a mosaic of traits seen in later hominids, evidence that the traditional linear model is breaking down for human evolution just as it seems to break down for the evolution of birds into dinosaurs. >>

The so-called "linear model" is and always has been nothing more than an
artifact of arranging taxa in a cladogram. To convert any cladogram into a
"linear model," or Hennigian comb as it's sometimes called, simply single out
one lineage to be the spine of the comb and arrange all the other lineages as
"side branches" of the selected lineage. Singling out the lineage that leads
to man, for example, produced the notorious "scala naturae" that dominated
phylogeny for circa 200 years. So the model is not breaking down; we're
simply finding lots more side branches. As for mosaicism, if you get enough
of it, it makes practically all cladistic analyses suspect. Too much
convergence turns the cladograms into mush.

Actually, I think the 'linear model' probably dates back to genealogy, where there is supposed to be a single line of descent.


I don't want to get into another 'cladistics' debate, but I would ask one question -- isn't this more of a problem when you look at on a finer scale. The analysis works fine for showing that birds are closest to theropods, but then might break down when you try to identify _which_ theropods. The problem seems to be, if I understand correctly, that some traits are suppressed but can reappear later on if selected for. If that's the case, we may be dealing with (at least) two levels of genetic mechanism -- the genes themselves, and whatever switches the individual genes off and on. -- Jeff Hecht