Abstract
Variance in longevity among individuals may arise as an effect of heterogeneity (differences in mortality rates experienced at the same age or stage) or as an effect of individual stochasticity (the outcome of random demographic events during the life cycle). Decomposing the variance into components due to heterogeneity and stochasticity is crucial for evolutionary analyses.In this study, we analyze longevity from ten studies of invertebrates in the laboratory, and use the results to partition the variance in longevity into its components. To do so, we fit finite mixtures of Weibull survival functions to each data set by maximum likelihood, using the EM algorithm. We used the Bayesian Information Criterion to select the most well supported model. The results of the mixture analysis were used to construct an age × stage-classified matrix model, with heterogeneity groups as stages, from which we calculated the variance in longevity and its components. Almost all data sets revealed evidence of some degree of heterogeneity. The median contribution of unobserved heterogeneity to the total variance was 35%, with the remaining 65% due to stochasticity. The differences among groups in mean longevity were typically on the order of 30% of the overall life expectancy. There was considerable variation among data sets in both the magnitude of heterogeneity and the proportion of variance due to heterogeneity, but no clear patterns were apparent in relation to sex, taxon, or environmental conditions.
Highlights
Individual variance in fitness components is central to evolutionary demography and ecology, since variation between individuals in their traits and the resulting consequences for fitness are the basis for natural selection
In this paper we address several questions: (1) is there evidence for unobserved heterogeneity in mortality of invertebrates under controlled conditions, (2) if so, how much, and how are individuals distributed among heterogeneity groups, and (3) how much of the variance in longevity is due to heterogeneity and how much to individual stochasticity
1,203,646 20.84 85.65 171 In Electronic Supplementary Material [ESM-1], we provide the complete set of all estimates, not just those for the model selected by minimizing Bayesian Information Criterion (BIC)
Summary
Individual variance in fitness components is central to evolutionary demography and ecology, since variation between individuals in their traits and the resulting consequences for fitness are the basis for natural selection. Population Ecology (2018) 60:89–99 which variance in fitness components can be accounted for by individual stochasticity is still open (Cam et al 2016) This question is fundamental to evolutionary demography because the two sources of variance have very different implications. It can arise from many other causes, variance due to heterogeneity may have a genetic basis, and play a role in selection. Because variance due to individual stochasticity arises from individuals experiencing identical vital rates, by definition it cannot have a genetic basis It may even slow down selection by obscuring genetic variance that does exist (Steiner and Tuljapurkar 2012). Attributing observed variance in fitness components to heterogeneity overestimates the potential for selection
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