"Conceptual simplicity, recursiveness, and formal separation of levels of selection are attractive features of these equations. But, of course, being able to point to a relevant and generally non-zero part of selective change is far from showing that group selection can override individual selection when the two are in conflict. Moreover, even the possibility of devising model circumstances in which a positive group-selection term (first term) outweighs a negative individual selection one (second term, assuming no further levels), gives no guarantee that ‘altruism’ can evolve by group selection: we have to consider whether the population can get into the specified state, and, if it can, whether its present trend will continue."