Department of Anthropology, Stanford University
Saturday 4:30-4:45, Parlors
The quantitative genetic theory of life histories predicts negative additive genetic covariances between life-history characters, indicating trade-offs and intermediate optima. Despite these expectations, negative covariances are difficult to measure in practice because of data with incomplete pedigrees or large environmental variance. I test the hypothesis that life-history traits in a frontier population show significant heritabilities and that these traits are constrained by the additive genetic covariance structure. Demographic data with full genealogical information were drawn from the Utah Population Database. I use four traits: (1) age at first reproduction (AFR), (2) average interbirth interval (IBI), (3) reproductive span, and (4) fraction of births surviving to their 15th birthday (recruitment). Quantitative genetic parameters were estimated from an animal model fit to multiple genealogical samples using MCMC simulation of the joint posterior distribution.
All four traits show significant heritabilities, ranging from 0.21 to 0.56, indicating a substantial contribution to the phenotypic values by additive genetic variation. Significant negative covariances were found between AFR and average IBI and AFR and reproductive span. Early reproduction, while favored in a growing population, trades off against lower fertility overall because of longer birth intervals. I construct a demographic model to calculate the selection gradient on these life history traits. The predicted net response to selection yields a reduction in IBI, lower recruitment success, and later AFR despite the selection gradient pushing in the opposite direction for the latter two traits.
This work is supported by grant 5P30AG024957-08 from NIA.