Mating system change reduces the strength of sexual selection in an American frontier population of the 19th century
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Fig. 1
Polygamous men in the UPDB as a frequency of all married men by birth year.
Fig. 2
Variation in lifetime reproductive success (solid line and left axis) and mating success (dotted line and right axis) for males (black [blue]) and females (grey [red]) by birth year.
Fig. 3
The opportunity for selection for males (IM, blue) and females (IF, red) by birth year. The solid black line is the opportunity for sexual selection Imates, and the black dotted line represents the proportion of all selection explained by sexual selection. The strength of sexual selection decreases markedly over time, both in absolute terms and as a proportion of all selection.
Fig. 4
Bateman gradients by birth year (βM is blue, βF is red and βF is black). Dotted lines and error bars correspond to estimated gradients when serial monogamists are excluded from the analysis. These are grouped by decade of birth. Of these, the upper (blue) line corresponds to βM and the lower black line to βF
. Error bars represent the 95% confidence intervals estimated by bootstrapping over parents (using the adjusted bootstrap percentile method in R version 2.10.1 ‘boot’ package; Canty & Ripley, 2009; Davison & Hinkley, 1997; R Development Core Team, 2009 ) . In terms of fitness, polygyny strongly benefits males but weakly harms females.
Fig. 5
Relative effect of MS on RS that is caused by increasing the reproductive rate of males. The lower black line uses all data grouped into birth-year cohorts. The upper black line with dots and error bars (95% confidence intervals — see Fig. 4 legend) excludes monogamous men with MS>1. These are grouped into birth decades. Roughly half of the male benefit of polygyny derives from increased male reproductive rates; the remainder of the fitness advantage follows from the increased time over which children are fathered.
Fig. A1
Sample sizes by birth year for males (blue) and females (red), including the projected nulliparous individuals. The black line represents the sample size of polygamous men with MS>1. These are grouped into birth decades.
Fig. A2
Mean reproductive and mating success. Males and female RS (blue and red, respectively) correspond to the left axis. Mean female mating success corresponds to the right axis. As we explain in the Methods section, we have assumed that mean male mating success is 4.8% higher.
Abstract
Sexual selection, or competition among members of one sex for reproductive access to the other, is one of the strongest and fastest evolutionary processes. Comparative studies support the prediction that sexual selection is stronger in polygamous than in monogamous species. We report the first study of the effect on sexual selection of a change in mating system, from polygyny to monogamy, within a historical human population. Here we show that over the reproductive lifetimes of Utahns born between 1830 and 1894, socially induced reductions in the rate and degree of polygamy correspond to a 58% reduction in the strength of sexual selection. Polygyny conferred a strong advantage to male fitness as well as a weak disadvantage to female fitness. In contrast, mating with multiple males provided little benefit to females in this population. Polygamy benefitted males by increasing reproductive rates and by lengthening reproductive tenure. Each advantage contributed to roughly half of the increased total lifetime reproductive success. This study illustrates both the potency of sexual selection in polygynous human populations and the dramatic influence that short-term societal changes can have on evolutionary processes.
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☆Grants: This work was funded by National Science Foundation grant DEB-0717234 to J.A.M. and D.EL.P., and NIH grant RO1GM065414-06 and NSF grant DEB-0614086 to M.J.W and partially supported by National Institute on Aging grant P30-AG013283 .
☆☆Disclaimer: The study is solely our responsibility and does not necessarily represent the official views of the National Institute on Aging or the National Institutes of Health. Grant sponsor: National Institute of Aging (The Utah Study of Fertility, Longevity, and Aging); grant number: AG022095 .
