[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Who says dromaeosaurs can't fly?

Jeff Hecht (jeff@jeffhecht.com) wrote:

<I think the problem may be that cladistics implicitly assumes that a 
character should have evolved only once -- not many times as you suggest.
As I understand it, this is the base of creating a "most parsimonious"
tree that is considered the most plausible one.>

  This is not really true. The tree length is arranged by the minimum
number of steps it takes to get from point A to point B, ancestral
condition (all zero state) to any end taxon, and all taxa calculated. A
character mcan evolve more than one, reverse, reappear, and reverse again,
in a lineage, and still be in the tree. Indeed, in my recent running of
the analysis of *Epidendrosaurus* in the matrix presented by Xu et al.,
different taxa were supported simultaneously in one tree by the same
character. The minimum distance by which such a conclusion can be made
results in a M[ost]P[arsimonious]T[ree], and the shorter the tree, the
better, as it were. Senseless evolution has so far not been known to
persist, as even nature's showboats, birds and fish, appear to develop the
variety of pattern and color and "spandrels" as a result of evolutionary
process, and each appear to be functional in either behavior, ecology,
feeding, etc. Parsimony is a method, as described earlier, by which human
wishful thinking is eliminated or diminished from an analysis and by which
personal preference is detracted from a conclusion. In some cases, if the
trees are few in a result, one can present all trees for the student of
the study to "select" from, but normally this is not possible if the
conclusions produce hundreds of results. My last run was so extensive
based on the variable positions of oviraptorids there were literally
thousands of trees; otherwise, the tree was stable.

<It's a problem because developmental genetics is showing that a lot of
major traits are controlled by two genetic switches. One gene turns on a
second gene -- that is, one gene controls whether or not a second gene
will be expressed, and that second gene controls (for example) limb
development. We have only a very primitive understanding of that process
at present, but it's central the emerging discipline of Evo-Devo --
evolutionary development.>

  Though this is good for developmental biology and ontogeny as studies,
cladistics will simply test the relationship of the C, G, A, and T
molecules in sequence, and does not care what genes do what. As recently
detailed in _Natural History_, the same genes in monotremes and placentals
do not control gender, showing that gene-control similarity doesn't make
placentals any special, or that some basal eutherians have the ability to
produce either gender with the sex-determining X or Y chromosones. Without
concluding on it, Zimmer in that article lit upon a subject, how the shift
on chromosome of sex-determination genes may have occured within
eutherians. But this is a process, and tells us nothing about
relationships, as the genes still place those mole voles closer to other
placentals than are monotremes or marsupials.


> I don't understand the details, and I suspect a LOT remains to be 
> learned. But just as it makes genetics a lot more complicated than 
> the simple mendelian model, it also suggests evolutionary lineages 
> can be very complex. -- Jeff Hecht

Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

Do you Yahoo!?
Yahoo! News - Today's headlines