Changes of vectors of selection for piglet’s newborn weight during population formation in new environment conditions

Studying changes of gene pools of populations of the domestic animals caused by selection and reflecting micro evolutionary processes, represents both practical and theoretical interest. As a rule, in such works the qualitative traits controlled by the principle «one genotype - one phenotype» are used that significantly simplifies the analysis. We investigated dynamic processes in Landrace population of domestic pigs ( Sus scrofa domesticus ), having estimated a variation of one of continuous quantitative traits, the newborn piglet weight, for which similar relationship is not unambiguous. Statistical analysis of the zootechnical register data was carried out on more than 26 thousand pigs that were born within 23 years at an experimental farm (Novosibirsk Province), being the regional authorized Landrace breed nucleus. Ancestors of the formed population were delivered from Latvia in the early 1960s. They were the elite young animals estimated on a standard complex of selection traits. The entire period of existence of population the formation of its breeding nuclear was carried out on a complex of traits according to existing Instruction for estimation of breeding value of pigs. As the Instruction didn’t contain standards on a large newborn weight, the selection on the specified trait wasn’t made. The changes of statistical parameters of the newborn weight in piglets were estimated for each year of the observation, because the analyzed livestock was not a model laboratory population, but a typical breeding commercial herd in which continuous variability of a trait is interfaced to continuous «sliding» alternations of generations and the variability of age structure. During long-time analysis of the dynamics of this unselected trait, the directional (moving) selection on genotypes for the loci controlling growth rate of pigs in ontogenesis (in pre- and post-natal periods) is revealed. In the populations where such selection works, the newborn weight of piglets can be used for forecasting pig weight during the postnatal period. Application of «parent-descendant» regression on the newborn weight allowed to estimate the duration of a population gene pool adaptation to new environment. It appeared that process of adaptation lasted nearly two decades that maked five full changes of the generations. In the same population at the same time the stabilizing selection optimized an individual animal weight at birth, cutting both minimum and maximum values. The described mechanism includes cyclic vector changes towards driving selection against the stabilizing selection vectors and thus maintains the population polymorphism on loci which control prenatal growth and large weight in the newborns. An observed unevenness of wavy change of these cycles should be also noted.