Re: 50% mutation

From: r norman (rsn__at__comcast.net)
Date: 02/21/05 

Date: Mon, 21 Feb 2005 00:48:39 -0500 (EST)

On Sun, 20 Feb 2005 16:41:30 -0500 (EST), br.hessels@planet.nl
(A.C.H.) wrote:

>A lot of evolution is the result of quantitive changes. In human
>evolution, for example, more brain, less hair, less snout etc. etc.
>
>These changes are the result of changes in the regulation of genes;
>how much, when and where a certain gene-product is produced. (And not
>of changes in the gene-products themselves.)
>
>The thing i want to point out is that the variables more-less,
>earlier-later are one-dimensional. A mutation in a regulatory sequence
>results in a phenotypic change along one dimension.
>
>Now consider a situation of directional selection, a situation where
>some adaptive change is required.
>
>The chance of a favorable mutation, a mutation with with positive
>fitness, is exactly 50%. (since either more or less, earlier or later
>necessarily results in positive fitness.)
>
>Am i missing something?

You are missing a lot of things.

A mutation in a regulatory sequence can make the regulation totally
non-functional. Speeding it or slowing it are not the only
possibilities.

The timing and magnitude of a regulatory process has little linear or
even direct relationship to the nucleotide sequence of the regulatory
site.

Even if what you say is true, it is quite possible that the timing
involves a critical zone or a critical magnitude so that both more and
less, or earlier and later are detrimental.