Chapter 12 - Human Adaptations to Temporally and Spatially Variable Environments

Abstract

Human populations live in a diverse array of environments across the globe. Environments also fluctuate over time. Coarse-grained heterogeneity is primarily experienced between different populations or different generations, whereas fine-grained heterogeneity is experienced by individuals. Coarse-grained spatial heterogeneity broadens the conditions for protected polymorphisms even under high levels of gene flow, and the conditions for protection broaden as gene flow diminishes. Adaptive responses to coarse-grained spatial variation are modulated by the interaction of natural selection with gene flow. One common method for detecting local adaptation is to screen the genome for outliers of the fst statistic, particularly when correlated with environmental variables. A genetic cline of gradually changing gamete frequencies over space can emerge either when there is gene flow across an abrupt environmental change (ecotone) or with restricted gene flow across a gradual environmental change (gradient). Admixture has frequently occurred in human history, and genomic screening of admixed populations, both ancient and modern, has revealed genomic regions subject to local selection in the admixed population. Coarse-grained temporal heterogeneity also broadens the conditions for protected polymorphism relative to constant fitness models. Lag effects in adapting to temporal changes can cause maladaptive responses to current conditions. Another source of temporal change is niche construction, the modification of the environment by humans that often induces strong selection. Our environment includes interactions with other organisms, such as pathogens, and these organisms can evolve as well, leading to a situation of coevolution of both species. Selection and genetic drift interact to favor those genotypes most buffered against fine-grained heterogeneity, even if there is a reduction in average individual fitness. In other cases, selection favors buffering mechanisms that increase an individual's average fitness in response to fine-grained heterogeneity, including the heterogeneity associated with human social interactions.