 Drift, then, on Wright’s view, plays an essential role in the evolutionary process. Fisher, by contrast, thought that mass selection on large populations was the predominant and most effective mode of evolution, leaving very little role for drift given the large population sizes. (This disagreement about the relative role of drift in evolution is one of several things that Wright and Fisher disagreed about; e.g., as is clear from the above, they also disagreed about population structure and effective population size. See Skipper 2002, 2009 and Plutynski 2005 for analyses of contemporary biologists who continue to argue the Wright-Fisher debate).