People have been choosing and changing genes since the beginning of human history. Farmers did it when they saved seeds from the best plants in one year's crop to plant in the following year. Ranchers did it when they let only their strongest and most productive animals mate and bear offspring. Men and women did it when they chose mates who looked energetic and healthy. To be sure, none of these people had ever heard of a gene. All they knew was that off- spring tended to be like their parents. By selecting parents, people could eventually, over many generations, produce offspring with features they wanted.

Natural Selection

In the mid-nineteenth century, British naturalist Charles Darwin revolutionized biology by saying that nature did the same thing plant and animal breeders did. Living things of the same type, or species, were similar to each other, Darwin said...