15. Demographic Sources of Variation in Fitness

Heritable variation in fitness is required for natural selection, which makes identification of the sources of variation in fitness a crucial question in evolutionary biology. A neglected source of variance is the demography of the population. Demographic processes can generate a large amount of variance in fitness, but these processes are stochastic and the variance results from the random outcomes of survival, development, and reproduction, and will therefore be non-heritable. To quantify the variance in fitness due to individual stochasticity, the mean and variance of lifetime reproductive output (LRO) are calculated from age-specific fertility and mortality rates. These rates are incorporated into a stochastic model (a Markov chain with rewards) and the statistical properties of lifetime reproduction, including Crow’s Index of the opportunity for selection, are calculated. We present the basic theory for these calculations, and compare results with empirical measurements of the opportunity for selection. In the case of a historical population in Finland, 57% of the empirically observed opportunity for selection can be explained by individual stochasticity resulting from demographic processes. Analyzing the contribution of demography to variance in fitness will improve our understanding of the selective pressures operating on human populations.