"When should adaptation arise from a highly polygenic response versus few large-effect changes?"Sella, GuyWilliam R Milligan, Laura K Hayward and Guy Sella This foundational question traces back to early debates around Darwin’s “Origin of species” and remains unanswered today. Strong but indirect evidence suggests that polygenic adaptation should be ubiquitous, but demonstration has proven elusive. In turn, there are hundreds of examples of large-effect adaptations in many species, but it remains unclear whether they are common in any given species. Translating such disparate evidence into general answers is complicated by the fact that different studies of adaptation are designed to answer different questions, and vary in their limitations and biases. We propose that a productive way forward would begin with reframing the question in terms of traits and asking how the genetic basis of adaptation depends on the properties of genetic variation in a trait (the trait’s genetics) and on changes in selection pressures acting on it (the trait’s ecology). We study this question in a simple but highly relevant setting. We consider a quantitative trait subject to stabilizing selection around an optimal value and model the response to selection when a population at mutation-selection-drift balance experiences a sudden shift in this optimal value. We characterize the adaptive response both quantitatively and qualitatively. Specifically, we delimit how the adaptive contributions of large-effect and polygenic changes depend on a trait’s genetics and ecology. We (briefly) discuss other salient factors that affect adaptation, how the theory we present and its future development could help us understand when different modes of adaptation are expected, providing a framework within which to interpret the diverse and growing body of evidence about the genetic basis of adaptation. |
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