Quantitative Genetic Analysis Reveals Trade-Offs between Age at First Reproduction and Fertility in an Historical Population

James H. Jones, Stanford University

I test the hypothesis that life-cycle traits in a frontier population show significant heritabilities and are constrained by additive genetic covariance structure. Genealogical and demographic data, including age at first reproduction (AFR), average interbirth interval (IBI), reproductive span, and fraction of births surviving to their 15th birthday, were drawn from the Utah Population Database. Genetic parameters were estimated from multiple genealogical samples using MCMC. All traits show significant heritabilities, indicating substantial additive genetic contribution to observed values. Significant negative covariances were found between AFR and IBI and AFR and reproductive span. Early reproduction trades off against overall fertility because of longer birth intervals. I calculate the selection gradient on these traits from the characteristic equation fit to data. The predicted net response to selection yields lower IBI, lower recruitment success, and later AFR despite the selection gradient pushing in the opposite direction for the latter two traits.