Breeds are groups of animals within a species with a common ancestry, have more or less similar characteristics and the ability to pass those characteristics on to the next generation in a more or less uniform manner. Members of a breed are more closely related than the average of the species. Strains are groups of animals within a breed that have been selected for certain characteristics and often trace their inheritance to a few particular ancestors. Development of strains may be intentional or may occur simply because of the environment at the location of origin.
Selection can be done within strains or within breeds following the same principles. The key is to have accurate information, some level of selection intensity (ability to be choosey) and some underlying genetic variation so individual and group differences can be utilized. Certain strains may have been developed to have more genetic uniformity as a result of selection. The key and the most difficult aspect of selecting within strains is to have key performance information.