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RE: Digit Loss



-----Original Message-----
From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of Tim
Williams
Sent: Friday, June 08, 2001 9:48 AM
To: dinoland@lycos.com; dinogeorge@aol.com; dinosaur@usc.edu
Subject: Re: Digit Loss


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.<<

But they don't. The fingers have good claws, strong fore-limbs.
Tyrannosaurus rex has larger arms in relation to it's body than many earlier
tyrannosaurids.

>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.<<

I think you should use something other than "switched off" which can also
mean it could again be "switched on". Perhaps complete degeneration would
work better.

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.<<

You'd have to ask the tyrannosaurids. Maybe will never know, but it worked.


>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.<<

If life is just a act of mutations (which is the drift I get from this
thread) then why is there so many body morphs that keep cropping up during
evolution? I'm noticing more and more of this. Estemmenosuchids and wart
hogs, Long skulled labyrinthodonts, phytosaurs and gavials, and numerous
others. How can all this be just an act of mutation? Question open to group
discussion.


>>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.  <<

Not if you knew how to work on cars and could improve. Even I did that with
my Firebird and I'm not a mechanic, now working on my truck...
: >




Tracy L. Ford
P. O. Box 1171
Poway Ca  92074