For thousands of years, we've produced new varieties of cultivated plants and nearly all domestic animals—including horses, cats, and cows—by selectively breeding for particular traits. Long before Europeans came to the New World, Native Americans had selectively bred teosinte (tee oh SIN tee), a wild grass native to central Mexico, to produce corn, a far more productive and nutritious plant. Figure 15–2 shows both plants. Corn is now one of the world's most important crops. There are two common methods of selective breeding—hybridization and inbreeding.

FIGURE 15–2 Corn From Teosinte Modern corn was selectively bred from teosinte at least 6000 years ago. During its domestication, corn lost the ability to survive in the wild but gained valuable agricultural traits. For example, the hard case around the kernel disappeared over time, leaving the rows of soft corn kernels we enjoy today. Observe What other differences can you see between the two plants?

Hybridization American botanist Luther Burbank may have been the greatest selective breeder of all time. During his lifetime (1849–1926), he developed more than 800 varieties of plants. As one of his tools, Burbank used hybridization, crossing dissimilar individuals to bring together the best of both organisms. Hybrids—the individuals produced by such crosses—are often hardier than either of the parents. Many of Burbank's hybrid crosses combined the disease resistance of one plant with the food-producing capacity of another. The result was a new line of plants that had the traits farmers needed to increase food production. Figure 15–3 shows a type of peach developed using Burbank's methods.

Inbreeding To maintain desirable characteristics in a line of organisms, breeders often use a technique known as inbreeding. Inbreeding is the continued breeding of individuals with similar characteristics. The many breeds of dogs—from beagles to poodles—are maintained using this practice. Inbreeding helps ensure that the characteristics that make each breed unique are preserved. Although inbreeding is useful in preserving certain traits, it can be risky. Most of the members of a breed are genetically similar, which increases the chance that a cross between two individuals will bring together two recessive alleles for a genetic defect.

In Your Notebook Compare and contrast hybridization and inbreeding.

FIGURE 15–3 Selectively Bred Fruit Luther Burbank used hybridization—a form of selective breeding—to develop a variety of plants. These July Elberta peaches, Prunus persica, are among his most successful varieties.

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