This paper is concerned with the evolution of the species phenotype (morphology and behaviour) when sexual organisms identify mutant traits by their unusual (or rare) appearance. Mate-seeking individuals are assumed, however, to have no means of distinguishing the occasional beneficial mutation from the others. We show that the resulting preference for mates with predominantly common traits (koinophilia) transforms the prevailing phenotype (as perceived by conspecifics) into an evolutionary stable strategy (ESS), as defined by Maynard-Smith (1974). This has far reaching evolutionary implications, permitting, in particular, the evolution of true 'group adaptations'. A subsidiary finding shows that koinophilia always has a substantial, immediate, selective advantage over panmixis, rendering koinophilia, itself, an ESS with respect to panmixis. This is particularly pronounced when the mutation rate is high, but beneficial mutations with high fitnesses are rare. The mean fitness of mutations has only a minor effect on the relative advantage of koinophilia.