Breeding, or selective breeding, is the development of new strains of plants and animals with more desirable characteristics, such as higher yields, greater resistance to disease, or suitability to the climate. Breeding has been practiced since prehistoric times – producing our modern domestic animals – but without firm scientific basis until Mendel's theory of genetics. The breeder first decides which traits are to be developed, and observes the range of phenotypes in the breeding population. Discounting variants due to environmental differences, individuals of superior genotype are selected. This genetic variation may occur naturally, or be produced by hybridization, mutations, or recombinant DNA. The selected individuals are used as parent stock for inbreeding to purify the strain.