  This line of reasoning highlights the next phase of the debate. From a discussion about fitness, the debate has evolved into a general debate concerning the nature of natural selection and drift. A number of alternative views have emerged in thinking about these topics. Some (e.g., Walsh, Lewens, Ariew, and Matthen) think about natural selection solely as population wide patterns caused by non selective processes: this view deprives fitness of any causal or explanatory power. Others (e.g., Millstein and Stephens) see fitness and natural selection as a cause, operating at the population level, to bring about the differential reproduction rates. Finally others argue that, if it is to play an explanatory role in our theory, fitness has to be an individual and causal concept. As these debates suggest, far from being merely a 19th century slogan, understanding the meaning of the “survival of the fittest” is of philosophical and biological urgency.