>Just a reminder, natural selection and evolution work on populations, not
>individuals.

Um, no.  Evolution operates on the level of the population, changing gene
frequencies from one generation to the next; natural selection works on
individuals.  It is the individual who has more or fewer offspring than the
next individual.

 Therefore, the
>more individuals that have a "genotype" that expresses itself with early
>menarche, the more children they are able to bear and the increase in that
>"genotype" in the next and future generations.

Yes, but I wasn't talking about individual genetic variation in age at
menarche (which certainly exists, just as does individual genetic variation
in potential for height), I was talking about environmental influences on
age at menarche.  As near as we can tell, all the secular trend in height
and age at menarche has been due to environmental changes, not to genetic
changes in age at menarche.



Katherine A. Dettwyler, Ph.D.
Associate Professor of Anthropology and Nutrition
Texas A&M University