Identifying targets and agents of selection: innovative methods to evaluate the processes that contribute to local adaptation

Summary Extensive empirical work has demonstrated local adaptation to discrete environments, yet few studies have elucidated the genetic and environment mechanisms that generate it. Here, we advocate for research that broadens our understanding of local adaptation beyond pattern and towards process. We discuss how studies of local adaptation can be designed to address two unresolved questions in evolutionary ecology: Does local adaptation result from fitness trade‐offs at individual loci across habitats? How do agents of selection interact to generate local adaptation to discrete contrasting habitats types and continuous environmental gradients? To inform future investigations of the genetic basis of local adaptation, we conducted a literature review of studies that mapped quantitative trait loci ( QTL ) for fitness in native field environments using reciprocal transplant experiments with hybrid mapping populations or Genome‐wide Association Study ( GWAS ) panels. We then reviewed the literature for field experiments that disentangle the contributions of various agents of selection to local adaptation. For each question, we suggest future lines of inquiry and discuss implications for climate change and agriculture research. (i) Studies in the native habitats of five biological systems revealed that local adaptation is more often caused by conditional neutrality than genetic trade‐offs at the level of the QTL . We consider the ramifications of this result and discuss knowledge gaps in our current understanding of the genetic basis of local adaptation. (ii) We uncovered only five studies that identified the agents of selection that contribute to local adaptation, and nearly all were conducted in discrete habitats rather than across the continuous environmental gradients that many species inhabit. We introduce a novel experimental framework for illuminating the processes underlying local adaptation. A holistic view of local adaptation is critical for predicting the responses of organisms to climate change, enhancing conservation efforts, and developing strategies to improve crop resilience to environmental stress. Experiments that manipulate agents of selection in native field environments using pedigreed populations or GWAS panels offer unique opportunities for detecting the genetic and environmental mechanisms that generate local adaptation.