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Selection for increased brood size in historical human populations.
Samuli Helle1, Virpi Lummaa, Jukka Jokela
1Section of Ecology, Department of Biology, University of Turku, FIN-20014 Turku, Finland. samuli.helle@utu.fi
Evolution; International Journal of Organic Evolution
|April 8, 2004
Summary
Human twinning rates in Sami populations indicate natural selection favored twins. Women with twin births had higher lifetime reproductive success and fitness, suggesting twinning was adaptive, not a byproduct.
Area of Science:
- Evolutionary biology
- Human reproductive strategies
Background:
- Twinning rates are debated: adaptive trait vs. byproduct of other reproductive selections.
- Historical Sami populations offer a unique dataset to test these hypotheses.
Observation:
- Analysis of Sami historical data (1710-1890) compared fitness of mothers with singleton vs. twin births.
- Key reproductive parameters like age at start/cessation of reproduction, lifetime fecundity, and offspring survival were examined.
Findings:
- Women with twin births exhibited earlier reproduction, later cessation, higher lifetime fecundity, and more offspring reaching adulthood.
- Twin deliveries resulted in an average of 1.2 offspring surviving to adulthood, compared to 0.8 for singleton deliveries.
- Singleton births were only more beneficial for mothers starting reproduction very late (>37 years) or with exceptionally long reproductive lifespans (>20 years).
Implications:
- Twinning in Sami populations was likely driven by natural selection, not a maladaptive byproduct.
- The fitness benefits of twinning were context-dependent, influenced by maternal age and reproductive lifespan.
- These findings challenge previous assumptions about the evolutionary drivers of human twinning.