 Walsh, Lewens, and Ariew then state that this is the common feature that the four types of drift they outline all share; thus drift, on their view, is when a series of births, survivals, deaths, and reproductions diverges from the outcome predicted by differences in fitness. Note that, using the terminology introduced in the previous section, this is a purely outcome-oriented (“outcome-only”) definition of drift. Whatever type of process (if any) might have produced the outcome plays no role in the definition. Matthen (2010) defends a similar characterization of drift: “Departures from expected values are what population geneticists call ‘drift’” (Matthen 2010: 3).