The combined effect of selection and drift on phenotypic change during bottlenecks

The relative contributions of selection and genetic drift to phenotypic evolution remain difficult to disentangle. Although often framed as alternative causes of evolution, both operate simultaneously, particularly during population bottlenecks, range expansions, and extreme climatic events. Motivated by previous findings where a population of threespine stickleback (Gasterosteus aculeatus Linnaeus, 1758) experienced a severe drought, we explore how selection and drift interact to produce phenotypic change during a bottleneck. We implement forward-time Wright-Fisher simulations across a range of selection coefficients and bottleneck population sizes to assess the plausibility of different causal scenarios. We use this case study to highlight that single-generation phenotypic responses to selection are constrained, but that bottlenecks expand the variance, making extreme phenotypic shifts possible. Critically, the simulations also illustrate a fundamental non-identifiability problem, where a wide range of parameter combinations produce indistinguishable patterns, making definitive causal inference from a single evolutionary episode inherently challenging. We discuss how genomic data, long-term monitoring programs, and experimental approaches can help resolve the causal contributions of selection and drift. As contexts involving strong selection and severe bottlenecks become increasingly frequent, such as range expansions and extreme climatic events, developing such tools is essential for understanding evolutionary responses.