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Re: Digit Loss
No, evolution is driven by >natural selection< on random mutations.
point! Otherwise there would be a whole lot more transitional organisms
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
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
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
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.
Timothy J. Williams
Iowa State University
Ames IA 50014
Phone: 515 294 9233
Fax: 515 294 3163
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