 Another argument in the philosophical literature for thinking that behaviors are not the proper targets of natural selection explanations is that behaviors are not quasi-independent (Lewontin, 1978) in the way they need to be to evolve by selection in their own right (Sterelny, 1992; Sterelny and Fitness, 2003; Sterelny and Griffiths, 1999). This is because many behavioral strategies will depend on multi-purpose mechanisms that cannot change during evolution without those changes ramifying to the other strategies those mechanisms produce. However, Driscoll (2004) argues that this is not necessarily the case; multi-purpose mechanisms, in order to be able to produce more than one strategy, would have to have branching algorithms; variation in any of these mechanisms necessary to generate variation in any one strategy need only occur on the branch relevant to the strategy in question and need have little or no effect on the other branches. Instead, whether an evolutionary explanation is properly directed at the behavioral or psychological level depends on the case.