Soft sweeps and beyond: understanding the patterns and probabilities of selection footprints under rapid adaptation

1. The tempo and mode of adaptive evolution determine how natural selection shapes patterns of genetic diversity in DNA polymorphism data. While slow mutation-limited adaptation leads to classical footprints of 'hard' selective sweeps, these patterns are different when adaptation responds quickly to a novel selection pressure, acting either on standing genetic variation or on recurrent new mutation. In the past decade, corresponding footprints of 'soft' selective sweeps have been described both in theoretical models and in empirical data. 2. Here, we summarize the key theoretical concepts and contrast model predictions with observed patterns in Drosophila, humans, and microbes. 3. Evidence in all cases shows that 'soft' patterns of rapid adaptation are frequent. However, theory and data also point to a role of complex adaptive histories in rapid evolution. 4. While existing theory allows for important implications on the tempo and mode of the adaptive process, complex footprints observed in data are, as yet, insufficiently covered by models. They call for in-depth empirical study and further model development.