"Multiscale selection: a framework to quantify natural selection at all spatial scales"Doekes, HiljeIn spatially structured populations, natural selection results from patterns, processes and interactions that act at different spatial scales. Consequently, the strength and sign of selection on a trait may differ depending on the spatial scale considered. For instance, in local environments selection on altruistic behaviours tends to be negative, favouring cheaters over altruists, but this local force can be overshadowed by selection at larger scales, favouring clusters of altruists over clusters of cheaters. For populations subdivided into distinct groups, this interplay between selection and spatial structure can be analysed formally through multilevel selection theory. Many populations, however, are not structured in this way, but rather (self-)organise into dynamic patterns unfolding at various spatial scales. We therefore present a mathematical framework for multiscale selection, which quantifies how patterns and processes occurring at different spatial scales contribute to natural selection in any spatially structured population. By combining Price's equation and kernel estimates we define the Local Selection Differential (LSD): a measure of the selection acting on a trait within a given local environment. Based on the LSD, natural selection in a population can be decomposed into two parts: the contribution of local selection, acting within local environments of a certain size, and the contribution of interlocal selection, acting among them. Varying the size of the local environments subsequently allows one to measure the contribution of each length scale to selection. To illustrate the use of this multiscale selection framework, we apply it to two simulation models of the evolution of traits known to be affected by spatial population structure: altruism and pathogen transmissibility. In both models, the spatial decomposition of selection reveals that local and interlocal selection can have opposite signs, thus providing a mathematically rigorous underpinning to intuitive explanations of how processes at different spatial scales may compete. It furthermore identifies in what way and to what extent processes at different spatial scales contribute to selection, and allows one to relate these scales to the scale of emergent spatial patterns. While the decomposition of natural selection in local and interlocal components is reminiscent of the multilevel decomposition of within- and between-group selection, the multiscale decomposition does not require the presence of well-defined groups or an assumed correspondence between the range of ecological interactions and the scale of the groups or local environments. Thus, it offers a new perspective on the interplay between spatial structure and evolution. |
« back