Summary
The presence, in man, of a significant high sex ratio at birth (Mb) is a biological fact, and its evolution is not satisfactorily explained by differential mortality of males.
Such a high sex ratio could have evolved primarily as a buffering interaction between growth rate or increase of human population (Rb) and age difference at marriage (d). The interaction of Rb with d will transform a high Mb, to a lower sex ratio at mating (Mm).
The model developed to account for the interaction also shows that d is able to adjust and to achieve a balanced Mm over a relatively broad range. The limits set on d are both biological and cultural; in practically all cultures males father children when they are older than their spouses. A buffered range develops where the balanced Mm is maintained in spite of temporal fluctuations or genetic fixations in Mb, Rb, and some additional factors.
A realistic necessary condition for the operation of the buffered system is the combination of Rb > 1ยท00, d > 0, and Mb > 100. Buffering of male excesses will be achieved more efficiently than buffering of female excesses, i.e. the range is asymmetrical.
Inside the range, selective pressures will be caused by the longer generation time of the males, rather than by unmated excess of either sex.
According to prevailing fitness models, females should be born in excess, since the longer generation time of males is disadvantageous and should be balanced by the advantage of being the rare sex. That males in fact are born in excess demonstrates a larger significant discrepancy with fitness models than previously assumed.
The buffering of sex ratio by age difference at mating is advantageous mainly in populations which are organized in small groups, as it was in early human history. This pattern also facilitated group selection.
The buffered system herein described evolved in growing populations. In declining populations it causes a cumulative maximal increase of d, and severe social problems.
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Beiles, A. A buffered interaction between sex ratio, age difference at marriage, and population growth in humans, and their significance for sex ratio evolution. Heredity 33, 265โ278 (1974). https://doi.org/10.1038/hdy.1974.92
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DOI: https://doi.org/10.1038/hdy.1974.92