Re: Digit loss

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

Tracy Ford wrote:

  > I have not problem with this. But how would radom mutation keep 
>repeating
> body morphs if environment didn't have a role in it? From the thread I 
> gather everything is radom chance, or am I misreading what others have 
> said?



Well, you're misinterpreting what I suggested regarding detrimental vs
beneficial vs neutral mutations.

I'm not claiming that all evolution is random - Heaven forbid!  Convergence 
alone would suggest that certain morphologies are beneficial, and so are 
repeatedly selected for (contra Prof. S.J. Gould).

But, in certain cases, such as when appendages or organs no longer serve a 
function (or lose most of their original function) through selection, they 
are no longer selected for traits that improve or regain that function.

IMHO, the reduction of tyrannosaurid forelimbs was not a random change.  
But, I would aver that the loss of a manual digit (producing a didactyl 
manus) did persist by random chance, not by selection.

I'm not necessarily saying that tyrannosaurid forelimbs were utterly 
useless.  What I am saying is that whatever function tyrannosaurid hands 
were used for, it didn't matter if they had three fingers or two fingers or 
one finger.  A loss of a finger (from three to two) was not detrimental to 
the animal's fitness.  This trait was retained not because it was good 
(beneficial) but because it was neutral (had no effect on the tyrannosaur, 
one way or the other).

For other theropods (like a dromaeosaurid) the loss of a finger through 
mutation would have drastic and detrimental consequences, especially 
regarding the theropod's predatory abilities.  This would not be the case 
for tyrannosaurids.


Tim

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

Timothy J. Williams

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

Phone: 515 294 9233
Fax:   515 294 3163
_________________________________________________________________
Get your FREE download of MSN Explorer at http://explorer.msn.com