Quantifying the impact of gene flow on phenotype-environment mismatch: a demonstration with the scarlet monkeyflower Mimulus cardinalis.

Geographic range margins offer testing grounds for limits to adaptation. If range limits are concordant with niche limits, range expansions require the evolution of new phenotypes that can maintain populations beyond current range margins. However, many species' range margins appear static over time, suggesting limits on the ability of marginal populations to evolve appropriate phenotypes. A potential explanation is the swamping gene flow hypothesis, which posits that asymmetrical gene flow from large, well-adapted central populations prevents marginal populations from locally adapting. We present an empirical framework for combining gene flow, environment, and fitness-related phenotypes to infer the potential for maladaptation, and we demonstrate its application using the scarlet monkeyflower Mimulus cardinalis. We grew individuals sampled from populations on a latitudinal transect under varied temperatures and determined the phenotypic deviation (PD), the mismatch between phenotype and local environment. We inferred gene flow among populations and predicted that populations receiving the most temperature- or latitude-weighted immigration would show the greatest PD and that these populations were likely marginal. We found asymmetrical gene flow from central to marginal populations. Populations with more latitude-weighted immigration had significantly greater PD but were not necessarily marginal. Gene flow may limit local adaptation in this species, but swamping gene flow is unlikely to explain its northern range limit.