Re: Phytodinosauria status

["Tim Williams" <twilliams_alpha@hotmail.com>]

George wrote:

> I think this interpretation may be incorrect. Note, e.g., that Carnotaurus
> retains four short ceratosaurian digits in an otherwise pretty useless
> (whatever that means) forelimb. I think digits are lost only when they are
> >positively detrimental<, not just useless. In other words, a forelimb 
> with
> three digits will, when reduced, generally retain the three digits.


Here you are making an assumption about the evolutionary process: that 
natural selection only acts to remove features that are detrimental, not 
just neutral ("useless").

In tyrannosaurids and _Carnotaurus_ alike, the forelimbs became very small, 
and a certain point (when the forelimbs ceased to have any grasping ability) 
the number of fingers (digits) became totally unimportant.  Why 
tyrannosaurids have two fingers and _Carnotaurus_ has four may have nothing 
to do with the selective advantage of javing two digits, and more to do with 
the number (and/or position) of mutations that occurred in the lineage 
leading to tyrannosaurids.

To put it another way, once the forelimbs were reduced to a certain size, 
and were no longer used for grasping, the exact number of digits was removed 
from selective forces.  A certain mutation pops up in the gene that governs 
the number of fingers in tyrannosaurids, with the result that the hand has 
one less finger.  No big deal - the mutation is not harmful to the animal, 
so the two-fingered mutation persists.  This mutation (or equivalent 
mutations) may not have occurred in _Carnotaurus_ nor its ancestors.

To couch this in neo-Darwinian terms, "bad" traits are removed by selection, 
and "good" traits are retained by selection - but neutral mutations (those 
which are neither good nor bad, and have no effect on the organism's 
fitness) may be retained through drift.


Tim



------------------------------------------------------------

Timothy J. Williams

USDA/ARS Researcher
Agronomy Hall
Iowa State University
Ames IA 50014

Phone: 515 294 9233
Fax:   515 294 3163

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