Selective Breeding Description: This learning experience is designed to help students understand changes in traits can take place over several generations through natural occurrence and selective breeding. Time Frame: 1 lesson (45 minutes) Materials: Selective breeding Passages (included in the blackline masters section for this learning experience) Background Information for the Teachers During the learning experience, each student group will read a different passage about selective breeding. For thousands of years, humans have selectively bred certain organisms to achieve specific traits. Breeders look for the organisms’ “superior” traits and keep careful records of parent-offspring relationships. Then the breeders selectively crossbreed those3 organisms with the desired traits. Advance Preparation: Make a copy of the Selective Breeding Passages and cut each passage apart so that each group of 2-3 students can have a different passage. Procedures: 1. Place students in groups of two or three. 2. Give each group a different selective breeding passage to read. Have each group give a brief summary of the passage to the class. 3. Have each group brainstorm the similarities and differences between the plants or animals in their selective breeding passages. These similarities and differences may include plant or animal, natural occurrence or selective breeding, desirable or undesirable outcomes, found in the United States or worldwide, found recently or in the past, advantages, uses, etc. 4. Have the class construct a comparison matrix that lists the similarities and differences among all of the examples presented in the selective breeding passages. Such a chart might look like a table with the list of organisms in rows and columns for things like Plant or animal? Naturally occurring or selectively bred? Trait Worldwide or present in one region only When trait first appeared Advantage of trait Disadvantage of trait 5. Have students explain their understanding of selective breeding, including reasons for using selective breeding to achieve desired outcomes. Selective Breeding Passages The following 10 passages provide examples of selective breeding. Make a copy of the passages and cut them apart so that each group of 2-3 students can have a different passage. 1. Bananas Historians think that bananas originated in Malaysia and were taken form there to India. The earliest documented mention of bananas is in Indian texts from around 600 BCE. Through trade, bananas made their way to the west coast of Africa, where they grew well in the tropical climate. In the 1500s, bananas were taken to the West Indies and Central America, but they did not arrive in the U.S. until the late nineteenth century. There are many varieties of bananas. Bananas originally had large seeds and were not very tasty. The sweet yellow seedless bananas that are popular in the U.S. are descended from bananas that have been selectively bred for thousands of years. Another popular kind of banana is the plantain, which is starchy and is usually cooked before eaten. Because the popular yellow bananas are seedless, they have to be cultivated from root cuttings. This means that they have very little genetic diversity, which makes them vulnerable to disease. For example, the Gros Michel type of banana was once grown widely in South America, but it was virtually destroyed by disease and has been replaced by the Giant Cavendish type. The Giant Cavendish is also susceptible to disease, and scientists disagree about whether the banana as we know it can continue to exist. 2. Cats Historians know cats had been domesticated in Egypt by about 1500 BCE, though cats show up in Egyptian sacred art dating back to almost 2500 BCE. Egyptians probably originally domesticated cats because there were good at protecting granaries from rodents. Cats appear in Greek and Chinese art from around 500 BCE, so historians know that domestic cats were in those regions by that time. Cats had made it to England as early as the 900s. Domestic cats can be of a specific breed or a “mix” of two or more breeds. There are between 30 and 40 officially recognized breeds of cat. Different breeds are roughly the same size and shape, though they can have different colors and lengths of fur and different tails, ears eye coloring and markings. The original wild cats domesticated in Egypt were probably dark-colored with stripes, like today’s tabby cats. When cats moved to different regions, their decendants often gradually changed appearance. Some of today’s breeds of cats, like the Siamese, Manz and Norwegian Forest Cat, came from these cats with regional differences. Other breeds are the result of selective breeding by people. In those cases, people either bred cats for a specific appearance or found a genetic mutation they wanted to replicate. For example, the American Curl breed, which has folded ears, came from a spontaneous genetic mutation. These cat breeds are relatively new varieties that were developed by breeders who selected those cats with mutations and bred them specifically for those desired traits. 3. Corn In a process that took about seven thousand years, Native Americans bred grain from scrawny weeds into ears of corn that people could really sink there teeth into. Starting with a wild grass called teosinte, farmers in ancient Mexico chose the plants with the most grains for replanting. Native American farmers stuck with this strategy of artificial selection, and by the time Europeans arrived in the New World, the farmers had developed ears of corn that look very much like the corn we roast or boil or pop or grind into cornmeal. However, farmers found the selectively bred corn could be infected with viruses that made it unhealthy. When corn plants became sick, they no longer grew as big as they should, and they yielded very little if any grain. Breeding healthy plants didn’t help, since those offspring were also vulnerable to corn viruses. Scientists searched for an answer to this problem, and they found it in a Mexican field. They discovered a descendant of the old teosinte weed that was immune to many corn viruses. By adding specific genes from their discovery to high-yielding varieties of corn, biologists developed virus-resistant strains of high-yielding corn. 4. Dogs Domestic dogs are all descended from the gray wolf. People and dogs have been living together for at least 12,000 years. By around 6,500 years ago, humans had managed to develop five different kinds of dogs, which included mastiffs (for guarding people and property), hounds (for hunting), and herd dogs. People still use dogs for those purposes; however the majority of dogs in the U.S. are not working dogs but are pets. Though there is great variety in dog appearance – from tiny dogs that can fit in your hand to huge dogs that can be taller than some sixth-graders – all dogs are the same species. There are about 400 recognized breeds of dogs in the world, though not all countries recognize all breeds. The American Kennel Club recognizes about 150 breeds, divided into seven categories: sporting, hound, working, terrier, toy, non-sporting, and miscellaneous. People have selectively bred dogs for a long time, but in the past 200 years, people have selectively become more interested in selectively breeding certain breeds or lines of dogs. For example, dogs in the “toy” category – such as the Chihuahua or the Pekinese – were bred to be pets. Other small dogs, like the Dachshund or the Fox Terrier, were bred to fit into burrows or other small places so they could kill badgers and rats. Though people own all kinds of dogs just as pets, purebred dogs and dogs of mixed breed retain the instincts of their ancestors. For example, Collies and other herding dogs kept as pets sometimes try to herd children or other animals. 5. Goldfish Most fish experts think that the goldfish is a very close relative or a descendant of the Crucian carp. Carp are naturally a silvery color, but a genetic mutation can cause individual carp to be more yellow or orange. With selective breeding of the yellow and orange carp, several varieties of goldfish were created. It is likely that goldfish were first bred selectively in China during the Sung dynasty (960 – 1279). Eventually, goldfish were exported to Japan, then to Europe, and finally to the Americas. In each of those places, people continued selectively breeding goldfish to produce different appearances. Now there are about 125 different breeds of goldfish; they can be of various colors and have different markings, tails, fins, eye types and head shapes. Because many people have released their pet goldfish into ponds, goldfish libe in the wild in the U.S. Goldfish are freshwater fish, and in the wild they tend to be greenish-brown or gray. When colorful pet goldfish are released, their descendants actually lose the genetic mutations and once again become greenish-brown or gray and grow larger. 6. Miniature Horses A miniature horse is not just a pony or a young horse but a separate breed of horse. At least as far back as the seventeenth century in Europe, people bred miniature horses as pets and for use as workhorses. The first miniature horses were probably selectively bred from a mix of larger horses and ponies. Because they were small and very strong, miniature horses were used for mine work in some European countries, including England. When miniature horses were first brought to the U.S. in the late 1800s, they were used in Appalachian cola mines and people continued to use them in coal mines until the 1950s. People have continues to selectively breed miniature horses as pets and as show horses. Today, there are many recognized breeds of miniature horses. The American Miniature Horse Association, founded in 1978, has a set of standards that define the breed called the American Miniature Horse. Other breeds of miniature horses were developed in other countries; for example, in the early twentieth century the Falabella family in Argentina selectively bred a line of miniature horses that is called the Falabella Miniature Horse. This breed is standardized by the Asociación de Criadores de Caballos Falabella. 7. Rice Humans have selectively bred rice for many purposes. For example, in the 1950s, scientists tried to address a world hunger crisis by breeding strains of rice plants that would produce more rice for people to eat. The new rice plants plus the increased use of fertilizers produced more rice, but the plants were tall and the large amount of rice at the top of the plants made them fall over in heavy rains or high winds. Scientists wanted to breed a strain of rice plant that would produce a large amount of rice but would not grow so tall. In 1966, they crossed two kinds of plants – Peta, a tall rice plant from Indonesia that was resistant to pests and Dee-gee-woo-gen, a short rice plant from Taiwan. The resulting strain of rice plant, called IR8, was able to withstand rain and wind. IR8 also grew fast, so it could be harvested and replanted several times a year, which meant that a single rice paddy could produce much more rice. IR8 plants and later strains that were made from IR8 need to be fertilized a lot, so now scientists are trying to breed a strain of rice that is less dependent on fertilizers. 8. Senepol Cattle Selective breeding of cattle has produced more than two hundred separate breeds. People have bred different kinds of cattle for different reasons: as a source of beef, as a source of milk or as a draft animal (pulling carts or plows or doing other heavy work). Some kinds of cattle were bred to be suited for more than one of these things. Scientists disagree about whether modern cattle all descended from the aurochs, a kind of cattle that is now extinct. Some believe that modern cattle are a subspecies of the aurochs, while other believe that modern cattle are just a relative of the aurochs but not the same species. One type of selectively bred cattle is the Senepol. In the 1800s, a cattle rancher on the Caribbean island of St. Croix began importing N’Dama cattle from Senegal, West Africa, because that type of cattle could best survive the heat and insects on the island. However, the rancher’s son later decided that the cattle reproduced too slowly and that if they could mature earlier they could be more profitable. He imported some Red Poll cattle from England and began breeding them with the N’Dama cattle, eventually developing the breed that is now called Senepol. 9. Strawberries Strawberries were probably used in Ancient Rome mainly for medicinal purposes rather than for food. They were popular in Europe as medicines for digestive problems or sunburns for some time. But around the twelfth century, Saint Hildegard von Binger said that strawberries were tainted because they grew close to the ground. Historians do not know for sure if this caused people to lose interest in strawberries, but they do know that strawberries fell out of general flavor for a few hundred years. In the early 1500s, a French explorer in the Americas found a larger kind of strawberry and took it back to France, where it was crossbred with some of the European strawberries. This produced sweet, juicy strawberries similar to the ones we eat today. People were pleased with the taste of the strawberries, and they have continues to crossbreed other kinds of strawberries to improve their flavor. 10. White Tigers There are five subspecies of tigers in the wild – Bengal, Sumatran, Amur, South Chinese, and Indochinese – and all of them are endangered species. All five types of tigers are different shades of orange with black stripes. But Bengal tigers, which are usually orange, sometimes produce offspring with a genetic mutation that causes them to be white with dark brown stripes, or black with yellow or white stripes. Although they are white, these tigers are not albinos. White tigers are very rare, and currently the only known white tigers are in captivity. Most – and perhaps all – of the white tigers in the United States are descendants of a female white tiger named Mohini. Zoos and conservatories have bred white tigers from that one female, often by mating siblings, cousins, and even parents with their own offspring. Because of this excessive inbreeding, white tigers in the U.S. may suffer from health problems like poor posture, kidney problems, weak immune systems and eye troubles. Despite the risk of such health problems, some zoos still breed white tigers. They know that people will visit zoos particularly to see white tigers and many audiences feel circus acts are even more spectacular with white tigers. Being able to sell white tigers for around $60,000 per animal also motivates owners to keep breeding these cats.