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in some instances, humans breed animals that are close relatives to produce desired traits. this is called

Inbreeding

Inbreeding is the process of mating of individuals or organisms that are closely related through common ancestry, as opposed to out breeding, which is the mating of unrelated organisms. Inbreeding is used to concentrate desirable traits. Mild inbreeding has been extensively used in laboratory for  mice and rats.

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".