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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.

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.



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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