Fear response and domestic behavior of sheep, cattle, horses, and pigs were studied using four different variants of experimental stress conditions. As standard aversive stimulus, the factor of human presence during the feeding time was used under the following conditions: animals were starved for 12-14 or 2 h before the exposure and fed individually or in a group. In the animal species studied, these conditions were found to reveal phenotypic and genotypic differences in fear response to humans, which is correlated with feeding behavior by regulatory mechanisms. A comparative qualitative and quantitative study of the discrete variation of the associated passive-defensive responses and feeding behavior (as related to domestic behavior) was conducted with regard to the effect of the breed and individual genotypes and the feeding and zoosocial deprivation. A classification scheme of the individual diversity of this behavior in farm animal species is proposed, in which ontogenetic and evolutionary relationships earlier found in sheep were taken into consideration. It is suggested that the progressive ecological and zootechnical specialization alters adaptive reaction norms of stress response in breeds and leads to the formation of the homologous variation in a set of coadapted traits, i.e., constitutional types of animals genetically marked by their behavior. A conclusion is made that heritable polymorphism of domestic behavior represents a new source of adaptive variation for improving the total fitness, welfare, and productivity of farm animals by breeding.