Recent advances in animal breeding theory and its possible application in aquaculture

The genetic change of the population mean of a quantitative trait is the outcome of the action of two antagonistic forces: artificial or natural selection and genetic drift. The latter may generate inbreeding depression and erosion of the genetic variance, both for production and fitness traits. Inbreeding changes are cumulative and inversely proportional to the effective population size. To keep undesirable inbreeding effects in check, a minimum number of selected parents larger than that required to produce the desired offspring number should be used. Moreover, the variability of selection response is also inversely proportional to the effective size. Thus, a reasonable chance of success of achieving some response can only be attained by further increasing the effective size. In parallel, the magnitude of short-term selection response depends linearly on the accuracy of selection. Efficient selection methods rely on the use of family information through BLUP evaluation. This procedure induces a closer genetic relationship between selected individuals, increasing the rate of inbreeding. Therefore, a compromise should be reached by using optimizing strategies considering: (i) the number of selected individuals and their respective contribution of offspring; (i¡) the selection criterion used; and (iii) the mating system imposed. When members of a family must be reared together until they reach sufficient size to be individually identified, family information may also be of use to detect indirect genetic effects on growth rate shared by all members of a family.