Re: The Science of God & Human Evolution

Tim Ikeda (tikeda@sprintmail.com)
Fri, 11 Dec 1998 00:07:23 -0500

If this is talking about the same book I browsed tonight at a
bookstore (should've be doing my other shopping...but I took a
quick break), there was one brief passage about the accumulation
of mutations that didn't sound at all right.

Schroeder was describing the number of differences in the DNA sequence
that had been acquired in human evolution. Between humans and chimps,
that difference is on the order of a few million bases, and Shroeder
passes that information along correctly. Then he takes estimates on the
number of mutations that we can expect to occur between generations in
humans. The numbers he cites were on the order of one mutation per ten
cell divisions (somatic cells?) or one in 10,000-100,000 per generation
in gametes.

Given those rates, we can't be talking about "mutations" in the same
way. When we talk about mutations in DNA sequences this typically
refers to individual bases or positions in the chromosomes. That will
includes neutral, positive and negative changes. But when we use numbers
like 1/10,000 per generation, that's got to be referring to the rate of
emergence of noticeable or selectable traits (i.e. positive or negative
changes that affect the phenotype -- it completely misses neutral
mutations or ones which have little noticeable affect). Thus the mutation
rates based on DNA sequence changes vs. rates quantitated on the basis
of having a phenotypic effect are going to be very, very different.
In actuality the rate of mutation is somewhere in the neighborhood
of 1 in 5E9 per base per division (probably correct within an order of
magnitude). For humans, that means it would be hard to squeak by with
less than a few DNA base mutations per generation. If he used the wrong
starting figures, I'm not surprised that Schroeder's probability
calculations did't add up.

Regards,
Tim Ikeda
tikeda@sprintmail.hormel.com
(despam address before use)