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selective advantage versus random non-genetic noise

Paul Barton-Davis pauld at cs.washington.edu
Fri Sep 13 17:12:48 EST 1991

Here's a question thats been on my mind for a few years. How many
mutations confer effective selective advantage ? 

Looking at things from a macroscopic, organism level, it seems
reasonable to note that selective advantage is only rendered in the
real world with one of two effects. Firstly, you can reproduce more
often and/or more successfully. Secondly, you can kill off competing
phenotypes more successfully.

Now, in both cases, it seems generally accepted that any given
mutation will have a rather small effect. My problem comes from the
fact that external, non-genetic factors are also at work influencing
both cases. If the influence of random "noise" on reproductive success
greatly overwhelms the small increase conferred by a particular
mututation, then how does a given genotype ever get established ?

For some real numbers, consider a mammal that normally has 3 to 4
children, of which only 1-2 survive to reproduce themselves. A
mutuation occurs that makes it 0.000001% more likely that it will have
4 rather than 3. However, at the same time, the chance of a cold
winter that kills of 2 of the offspring is around 73%. Averaged over
many, many generation, this mutation confers insufficient selective
advantage to enable the new genotype to become more numerous than its
fellows ?

Or does it ? I keep meaning to write a little simulation program to
make this calculation easy for lots of different real values.

My overall view, given the above, is that many biological features are
arbitrary, and have not been selected for by evolution. They occured,
and so they exist.

What am I missing ? 

-- paul
Paul Barton-Davis	 | I am bored prefixing everything I say with "I think" 
UW CS Lab		 | or "in my opinion". Everything I say is my opinion; 
pauld at cs.washington.edu	 | the only thing of which I am certain is that there 
+1 206 543 0377		 | is very little of which one can be certain.

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