Artificial Selection and Selective Breeding

Core Idea

Artificial selection, or selective breeding, happens when people choose which plants or animals get to reproduce based on traits they want. Over many generations, the chosen traits become more common and more pronounced in the population. Dog breeds and crop plants were shaped this way, starting from wild ancestors. Artificial selection works like natural selection, except that people decide which traits are favored, and it only works on traits that vary and are passed from parents to offspring.

How It's Best Learned

Show pictures of broccoli, cabbage, kale, cauliflower and Brussels sprouts and reveal that they all come from one wild mustard species. Then run a simple simulation: start with a population of paper "plants" of different heights, let students choose the tallest to "breed" each round, and track the average height. Compare the results with a round in which the environment, not the students, picks survivors.

Common Misconceptions

• Breeders can create any trait they want. (Selection can only work with variation that already exists in the population or that arises by mutation; it cannot produce a trait from nothing.)
• Selective breeding is the same as genetic engineering. (Selective breeding chooses which individuals mate; genetic engineering changes DNA directly in the laboratory.)
• Traits favored by breeders always help the organism survive. (Many bred traits, such as very short snouts in some dog breeds, can harm the animal's health and would be selected against in the wild.)

Explainer

A Chihuahua and a Great Dane look nothing alike, yet both are dogs, and both descend from wolves. How did one species turn into hundreds of different-looking breeds? The answer is artificial selection.

Every population has variation. Some wolves were calmer around people, some puppies were smaller, some plants had bigger seeds. Many of these differences are inherited, passed from parents to offspring through their genes. When people choose which individuals get to breed, they decide which of these inherited traits get passed on. If a breeder lets only the smallest dogs have puppies, and then lets only the smallest of those puppies breed, the dogs get smaller generation after generation.

Crops show the same process. The wild mustard plant Brassica oleracea grows along European coasts. Farmers who favored plants with big leaves eventually produced kale. Those who favored a tight cluster of leaves at the top produced cabbage. Those who favored large flower clusters produced broccoli and cauliflower. All of these vegetables are still one species. Corn, or maize, came from a wild grass called teosinte in what is now Mexico. Teosinte has a tiny ear with a few hard kernels. Over thousands of years, farmers who saved seed from the best plants turned it into the large, many-kerneled ears we eat today.

Charles Darwin paid close attention to breeders. The first chapter of On the Origin of Species, published in 1859, is about variation in domesticated animals and plants, including the many breeds of pigeons. Darwin argued that if people could change a species so much in a few thousand years, nature could do the same over much longer times. In nature, the "breeder" is the environment. Individuals whose traits help them survive and reproduce leave more offspring. Darwin called this natural selection.

Artificial selection has limits. It can only work with traits that already vary and are inherited. It is also different from genetic engineering, in which scientists change an organism's DNA directly. Finally, traits people want are not always good for the organism. Some dog breeds have such short snouts that they have trouble breathing, and some crop plants could not survive without farmers to plant and protect them.

Artificial selection shows, on a timescale people can watch, how choosing which individuals reproduce changes the traits of a population.