Selection for increased brood size in historical human populations |
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Authors: | Helle Samuli Lummaa Virpi Jokela Jukka |
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Institution: | Section of Ecology, Department of Biology, University of Turku, FIN-20014 Turku, Finland E-mail:;Large Animal Research Group, Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, United Kingdom E-mail:;Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, United Kingdom;Department of Biology, University ofOulu, POB 3000, FIN-90014 Oulu, Finland E-mail: |
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Abstract: | Human twinning rates are considered to either reflect the direct fitness effects of twinning in variable environments, or to be a maladaptive by-product of selection for other maternal reproductive traits (e.g., polyovulation). We used historical data (1710-1890) of Sami populations from Northern Scandinavia to contrast these alternative hypotheses. We found that women who produced twins started their reproduction younger, ceased it later, had higher lifetime fecundity, raised more offspring to adulthood, and had higher fitness (individual lambda) than mothers of singletons in all populations studied. For example, an average of 1.2 offspring survived to adulthood from a twin delivery, irrespective of its sex ratio, whereas only 0.8 offspring survived to adulthood from a singleton delivery. Only if mothers started reproduction at very late age (> 37 yr), or had a very long reproductive life span (> 20 yr), was it more beneficial to produce only singletons. These findings suggest that twin deliveries among Sami could not be explained as a maladaptive by-product of selection for other maternal reproductive traits. In contrast, our results suggest that twinning was under natural selection, although the strength of selection was likely to have been context dependent. |
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Keywords: | Correlational selection fitness insurance ova lambda life history twins Homo sapiens |
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