 Although there is widespread agreement that there are multiple modes of evolution, much contemporary work in biology and philosophy of biology has been focused on natural selection. Whether this focus is a good thing or not is in part what the debate over  adaptationism  is about. That is, do we have reason to think that natural selection is the most prevalent or most important mode of evolution? Should scientific methodologies be geared toward testing natural selection hypotheses or toward a variety of possible evolutionary modes? The focus on natural selection has also led to a large literature on the concept of  fitness,  given that population genetics’ definitions and other definitions of natural selection typically invoke fitness; a natural selection explanation of why X was more successful than Y might invoke X’s higher fitness. What fitness means, what entities it applies to (genes, organisms, groups, individuals, types), what sort of probabilities it invokes, if any, and how it should be calculated, are all under philosophical dispute. There is also a large literature on conceptually and empirically distinguishing natural selection from genetic drift. Migration, mutation (as a mode of evolution), and sexual selection have received less attention from philosophers of biology.