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Hum Biol. 1984 Feb;56(1):35-46.

Anthropometric traits, balanced selection and fertility.



Evidence from Mexican-American families regarding the question of balanced selection in anthropometric traits in relation to fertility or "Darwinian fitness" is presented and assessed. The data are organized on the basis of 3 kinds of selection on continuously distributed characters in natural selection: 1) directional, favoring 1 extreme; 2) stabilizing, (balanced), in which the average phenotype has greater reproductive fitness over the extremes; and 3) destabilizing, in which the extremes have the highest Darwinian fitness. There have been marked differences in the results of previous studies on the subjest. The present study is based on data collected in 1941 on Mexican immigrant parents and their American born children in Texas and of parents and children in central nad northern Mexico. At least 1 child in each family had to be an adult. Information was obtained as fully as possible for all children, living and dead. 176 families were studied in Texas and 129 in Mexico. 230 families in which the mother was at least 40 years old were included. Modal, minimal, and maximal classes were established in parental populations (both sexes combined) for morphological traits and for the sexes separately. The variability of fertility was determined according to mprphological class of mother and father separately and their interaction by analysis of variance with 2-factor interchange. The traits: (weight, stature, min. frontal, bizygomatic, bigonial, hand length, hand width, ear index, body surface area, weight/surface ratio) and directional in 10 other parental traits (head length, head width) menton-crinion, menton-nasion, nose height, nose width, ear height, ear width, cephalic index, and nose index), except nose width in the latter group which was in the destabilizing category. The directional traits in regard to mean munber of children were generally in the order of minimal, modal, and maximal class. None of the differences between the means, however, were statistically significant. It has been calculated that a difference of 1% in fertility between 2 groups would require 38,000 families in each group for statistical significance at the 5% level. Whether fertility in the sense of a large number of offspring is a requisite of natural selection in the case of human beings appears to be moot in view of the pervasive influence of "cultural selection" which so significantly mediates mating patterns and family size.

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