Re: Digit Loss

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

George wrote:


> No, evolution is driven by >natural selection< on random mutations. 
> Critical
> point! Otherwise there would be a whole lot more transitional organisms
> around today.

Not entirely.  Natural selection may *not* act on mutations which result in 
no change in the animal's fitness.  Tyrannosaurs had ceased to use their 
hands for grappling or manipulating prey.  Therefore it doesn't matter how 
many fingers a tyrannosaur had.  Heck, a few more random mutations and 
_Tyrannosaurus_ may have had no fingers at all - just a pair of fleshy 
protuberances attached to each humerus.

As I said previously, if one follows an essentially Darwinian thesis of how 
evolution occurs, then selection acts on "bad" traits (by removing them from 
the gene pool) and on "good" traits (by ensuring they are retained and 
passed on).  BUT - mutations which are neither "good" nor "bad" may become 
fixed by chance alone.


Steve Brusatte wrote:

> I agree with much of that, but I do think that selective pressures that 
> govern digit numbers are strong, too.  Why don't digits re-evolve in 
> theropods?

Easy.  Once a gene is "switched off" it becomes removed from natural 
selection.  It can then accumulate any number of mutations without harming 
the animal - it is now a "dead" gene.  A "working" gene, by contrast, is 
very sensitive to mutation: certain mutations may be beneficial, many will 
have no effect at all, but most will be harmful.  The effect 
(good/neutral/bad) depends on where in the gene the mutation occurs.

With a "switched-off" gene, the chances of the gene becoming re-activated 
and functional are very low, because of this accumulation of mutations.


> I realize that re-evolving a digit is like evolving a new structure, and is 
> >unlikely in the short term.  However, theropods had millions of years to 
> evolve.  >Why is there no case of a theropod re-evolving a digit?

Using _Tyrannosaurus_ as an example, what would be the selective advantage 
of an extra finger (i.e. re-evolving the lost digit?).  There would be no 
potent selective forces to act on mutations which did result in re-evolving 
a finger.


> There had to be some sort of pressure.

No, because further mutations in the gene (which were allowed to persist 
after the gene was "switched off") rendered the gene resistant to further 
selective pressure.

You can tinker with the car in your garage, and maybe improve its 
performance.  But you'll be darn careful to be sure that you don't damage 
any important parts.  Meanwhile, your old car sitting in the scapyrard can 
sustain a lot of damage, and you won't care - you don't use the old bomb any 
more.  However, each piece of damage the old car sustains reduces the 
chances of you (or anyone else) driving it back on the road one day.


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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