Age at menarche as a fitness trait: further considerations.

The extensive twin study of Treloar and Manin (1990) shows that most of the genetic variance of the age at menarche (AM) is nonadditive, typical of traits affecting genetic fitness. This is an important addition to our understanding of this major, but still elusive, physiological landmark. I wish to comment on their key assumption that earlier AM, on average, does in fact produce increased fertility. First, it Seems remarkable to me that there appear to be very few tests of this assumption, such as correlating AM with number ofIive~born children ever born (granting the problem of recall of AM in older women) or even with the age of the first-born child. The authors do not mention such tests. The only explicit test known to me is my own, and it supports the assumption , showing a very significant (P < 10-5) negative regression, in Caucasians, of number of pregnancies (at about 26 years of age) on AM (see study mentioned below). I agree with Treloar and Martin that it is a strong inference that. earlier AM, as an indicator of earlier sexual maturity, should result in increased fertility. This earlier maturity could have been especially important in our evolutionary past, before the Agricultural Revolution, when women (and men) often failed to survive to the end of their potential reproductive period. Second, it may not be widely realized that earlier fertility can have a selective advantage in two ways. The obvious way is an increase in total fertility (number of live-born children ever born to a woman). But even if total fertility is unchanged, having children earlier still has a selective advantage. Consider two women who have completed their reproduction, each with a total of two live-born children. A had one child at age ·20 years and one at age 25 years. B had each child two years earlier, at ages 18 and 23 years. A's annual rate of production of children (and, proportionally , transmitted genes) is 1120 + 1125 = .090; B's annual rate is 1118 + 1123 = .099,10% larger. If this earlier production of children by B is due to genes causing earlier menarche, B would have a selective advantage; the "early menarche" genes would be selected for. This point, the selective advantage ofhav-ing children earlier, has been made before in a different context: study of a gene (for Huntington disease) which changes the maternal age distribution …