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Re: Digit loss




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
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