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Cattle evolved through the slow process of natural selection until human domestication, which rapidly accelerated their development as an artificially selected species fit less for survival than to satisfy human needs. This lesson focuses on how and why humans have been so successful in selectively breeding cattle to suit these needs, while also exploring the limitations and consequences of this success.
Natural selection describes the process by which organisms best adapted to their environments are the ones that survive and reproduce. The Introductory Activity helps students understand that some traits, such as the ability to digest grass, made certain species more desirable for domestication by humans. The Learning Activities explore how, by nurturing and protecting animals that might not have survived in the wild, human domestication interrupted the process of natural selection. Reproductive success was no longer primarily determined by an animal’s most naturally adaptive survival traits, but rather by its artificially selected traits desirable to humans (primarily milk and meat production). The culminating activity presents a case study in which students consider the consequences of losing the naturally selected attributes of breeds less adapted to domestication.
Students should already be familiar with the concepts of evolutionary adaptation, natural selection, and DNA/genetic engineering.
Students will be able to:
Two to three 45-minute class periods
For each student:
For each group:
For the class:
An Oklahoma State University Web site featuring photos and descriptions of various breeds of cattle from around the world.
A map of the world showing different climate zones.
A Regents’ preparatory Web site featuring a description of how the meaty English Shorthorn cow was selectively bred with the heat-resistant Brahman cow to produce the Santa Gertrudis, a hybrid which possesses the positive characteristics of both parent breeds.
An interactive Web-site which explains how Punnett squares can be used to determine the likelihood that certain traits will be passed on to future generations.
A New York Times article which discusses the pros and cons of increasing hybridization by Bahiman cattle ranchers in Uganda of their native Ankole cattle with Holstein cattle from the United States.
Tell the class to imagine they are a tribe of prehistoric hunter/gatherer nomads living in a savanna (a rolling grassland scattered with shrubs and isolated trees). Tell them that they have several species of animal in their environment which they can attempt to domesticate as a food source to relieve the pressures of the hunt. Divide the class into four groups and distribute the Man's Best Friend Student Organizer to each group. For each question category on the student organizer, ask the class why they think the question is important and relevant. Accept all answers, but encourage an understanding of the following:
Explain that each group will now be assigned one of the four different animal species listed on the student organizer (wild dogs, wild pigs, deer, and cattle) to consider as candidates for domestication. Allow students 5 minutes to complete the organizer, explaining their answers in as much detail as possible. Have each group in turn report back about their assigned species, with the other groups noting all answers on their own organizers. Review all answers, encouraging an understanding of those found on the Man's Best Friend Student Organizer Answer Key. (Note that the Answer Key includes only some of the possible answers; the point of this activity is not to identify each species’ precise biological nature, but rather to get students thinking in terms of traits and which ones humans consider desirable in a domesticated species.)
Based on their student organizer answers and the class’s discussion of them, ask the class to vote on which animal they think offers the most to their hypothetical grassland tribe? (Answers will vary, but cattle will probably be the strongest contender.) Explain that many of their prehistoric ancestors came to the same decision, and that wherever cattle existed throughout the world, they came to be domesticated to provide meat, milk, and other resources. Ask what else cattle have provided for humans. (Leather for clothes and shelter, bones for tools, and dung to fuel fires. Eventually they were also harnessed as draught animals to provide agricultural labor.)
Explain to the class that they’ve been talking about some of the traits that make cattle an attractive option for humans to domesticate. One of those traits was that they can survive and thrive on grass. Ask students can dogs eat grass? (No.) Can humans? (No.) What is it about the cow that allows it to thrive eating nothing more than grass? (Accept all answers.)
Tell the class that they will now be looking at a brief video segment that will help them answer that question. Provide students with a focus for media interaction by asking them to note ways in which a cow’s digestive system differs from their own. Play A Cow’s Digestive System QuickTime Video. Ask students again how a cow’s digestive system differs from their own. (Answers will vary, but should include the fact that cows have developed an extra compartment in their digestive system called a “rumen” containing microbes which help break down the cellulose fibers of grass.) Explain that this digestive system is one specific trait that cows evolved through natural selection that make them well adapted to their natural environment—grassland.
Tell students that this digestive system, which cows evolved to survive in grasslands, also helped make them highly suitable for human domestication, because they can convert grass (which humans cannot digest) into meat and milk (which we can). Explain that domesticated animal species benefit from human resources including food, protection from predators, and medical care. Ask students how they think domestication affects the process of natural selection in cows. (Accept all answers, but explain that by providing food and protection for animals that might not have been able to survive in the wild, humans are interfering with the “survival of the fittest” principle of natural selection.) What is now replacing natural selection as the determinant of reproductive success for any given domesticated cow? (Its possession of attributes desirable to humans.) Explain that this process of selection, determined by human agency rather than nature, is known as “artificial selection.”
Tell the class that they will now be watching a segment describing some specific traits humans have artificially selected in cows to give them certain desirable attributes. Provide a focus for media interaction by having the class write down the traits that humans seek in breeding a good beef cow. Play the Desirable Breeding Traits in Cattle QuickTime Video. Pause the segment after the words “…they’ve been selected by the butcher, in essence.” Have the class offer their responses to the focus question. (Meat cattle are bred to have short legs and a stocky build with large quantities of meat.) Provide a focus for media interaction by having the class write down the traits that humans seek in a good dairy cow. Play the segment through to the end and then ask students to respond to the focus question. (Dairy cows are bred to have good udders, well-placed teats, well-set legs, open ribs—all attributes that help it produce more milk.)
Tell students they will now be taking a closer look at one particular breed similar to those they have just seen in the video segment. Provide a focus for media interaction by asking where the Shorthorn comes from and what makes it a valuable breed of cow. Have them go to the Breeds of Livestock Web site and use the menu at the left of the screen to select the Shorthorn. Have students read the first two paragraphs. Ask students to respond to the focus question. (It originated in England, and is valued because it puts on a lot of meat and produces a lot of milk.) Explain that for centuries the Shorthorn has been artificially selected for these traits. Ask students if they would consider these the most important traits in any cow. (Accept all answers, but most will say they are.)
Tell the class that they will now be watching another video segment featuring cattle that have evolved through a combination of artificial and natural selection to fit some other criteria. Provide a focus for media interaction by asking what other characteristics besides meat and milk volume might be desirable to breed into cattle? Play the Different Breeds of Cattle QuickTime Video. (Answers should include an ability to withstand cold, quality of milk/dairy produced, and adaptability to a variety of environments.)
On a world climate map (available for download), have students locate England. (You might want to have students compare the climate map with a political map.) Ask students to describe the climate in England. (Mild and temperate.) Now ask students to find India on the world climate map. What do they think the climate is like there? (Hot and tropical.) Do they think cattle in India are any different from the Shorthorn? (Accept all answers.) Return to the Breeds of Livestock Web site and use the menu at the left of the screen to select the Brahman. Divide the class into their four groups again. Instruct the groups to read the two introductory paragraphs and the “Physical Characteristics” section of the Web page. Provide them with a focus for media interaction by asking each group to write down the traits of the Brahman that have enabled it to survive and thrive in India’s hot climate. After allowing 10 minutes for completion of this task, have each group report back on one heat resistant trait they found. (Answers should include hair coat, skin pigmentation, loose skin, sweating ability, and internal body heat.)
Explain that while all cows evolved the ability to survive on grass, over time different breeds have evolved additional adaptive attributes that have helped them to survive and thrive in their particular climates and environments. Ask students if they think that Shorthorn cattle would thrive in India. (No.) Why not? (Coming from a more temperate climate, Shorthorns had no need to develop the Brahman’s adaptations to a tropical climate.)
Ask students if they think that the Brahman’s heat-adaptive traits have come at the expense of any other attributes. (Answers should reference the “body heat” bullet point, which states the Brahman produces—i.e. disperses—waste heat from feed “at the expense of growth and milk production.”) What does a cow’s growth mean for the human who raises it? (More meat.) Ask students if they think that less meat per cow is necessarily a problem for Indian cattle raisers? (Answers might mention the fact that cows are sacred to Hindu Indians, who do not slaughter them for food, or that a leaner, but healthier cow is still more valuable in this environment than a large one that gets weak and sick in extreme temperatures.)
Explain that there are many hot weather countries that both produce and eat beef. Ask students if they know which country produces the most beef. (Brazil.) Have students find Brazil on the climate map. What is the climate like there? (Largely hot, tropical, rainforest.) Ask students what they would do if they were tropical cattle ranchers like the Brazilians who want more meat on their cows? (Accept all answers, but encourage the idea of selectively breeding a good “meat” cow with a good “heat” cow.)
Go to the “Genetic Engineering” website. Have students read the “Selective Breeding” section. Provide a focus for media interaction by asking them to define “hybridization.” (It involves the selective breeding of two individuals with different desirable traits to produce offspring with a combination of both desirable traits.)
Explain that there are approximately 1.3 billion cattle in the world today—approximately twice the number that existed 40 years ago. Ask the class if they can think of any another species that has doubled its population over the same period. (Humans.) Ask what they think the link might be. (Humans breed more cattle to meet their growing need for food.) Explain that those 1.3 billion cattle are increasingly dominated by a relatively few breeds and hybrids that have been selected by humans for their superb meat and milk production traits. Ask students if they think that selectively breeding hybrid cattle can produce “a perfect cow.” (Accept all answers. Explain to students that they will be looking at an article that discusses some of the limitations of artificial selection.)
Tell students that they will now be reviewing the science of heredity which underpins artificial selection by playing the Punnett Squares interactive. Ask for a volunteer to review what Punnett squares are. (Diagrams used to predict the outcome of particular genetic crosses by plotting different combinations of dominant and recessive maternal and paternal “alleles”, or chromosonal codes.) Provide them with a focus for media interaction by asking them to write down their answers to the following questions:
Call on students to provide answers to the Punnett Squares questions. If necessary, reviewing the interactive with the class to ensure that all students understand how Punnett squares work. Explain that the class will now be using Punnett squares to simulate a simplified dihybrid cross-breeding of meaty Shorthorn cattle and heat-resistant Brahman cattle.
Divide the class into four groups. Distribute the Cattle Breeding Student Organizer to each group, and project a copy of it from your computer. Tell the groups that they will now be completing the “1st Generation” section of their organizers using the genotypes for Shorthorn cattle (MMhh, where “M” is the dominant allele for meatiness) and for Brahman cattle (mmHH, where “H” is the dominant allele for heat resistance). Allow groups five minutes for completion of this task.
Call on one group to give their answers, noting them in the projected organizer. (All boxes should be filled in with “MmhH.”) Explain that this result indicates a 100% likelihood that cattle of this first generation will carry dominant alleles for both meatiness (M) and heat-resistance (H), as well as recessive alleles for both these attributes (m and h respectively).
Explain that one generation of successfully bred cattle are only useful for the lifespan of that generation (a lifespan often cut short by the butcher), and that successful long-term cattle breeding happens over many generations. Now have the groups complete the “2ndGeneration” box of the “Cattle Breeding” student organizer. Explain that both parents of their 1st generation have gone to the slaughterhouse at this point, so the class must breed their 1st generation cattle (i.e. mating two mMHh genotype cattle to each other). Ask students to predict if they think cattle of this 2nd generation will be more, less, or equally likely to carry dominant traits for meatiness and heat resistance than the 1st generation (Accept all answers.) Allow groups five minutes for completion of this task.
Call on each group to give their answers for one row of the chart, correcting them if necessary and noting all answers in the projected worksheet. Ask if the cattle of this 2nd generation are more or less likely to carry dominant alleles for both meatiness and heat-resistance than the 1st generation? (Less. There is only a 9/16 chance that these cattle will be both meaty and heat-resistant; there is a 3/16 chance that they will be heat-resistant but lack meatiness; there is also a 3/16 chance that they will be meaty but can’t stand the heat; and there is a 1/16 chance that a cow of this generation will carry neither of the dominant alleles.)
Ask students which of their 2nd generation genotypes they would breed to create a 3rd generation of cattle with the greatest likelihood of as being both meaty and heat-resistant? (Answers will vary, but a cow with the MMHH genotype will be an obvious choice, with cattle having either the mMHH or MMHh genotypes being the next best options.) Explain that for the purposes of this exercise, the class will breed an MMHH genotype cow with an MMHh genotype cow to produce a 3rd generation. Have groups complete the “3rd Generation” box of the “Cattle Breeding” student organizer. Allow groups five minutes for completion of this task.
Call on each group to give their answers for one row of the chart, correcting them if necessary and noting all answers in the projected worksheet (Cattle Breeding Student Organizer Answer Key is provided). Ask students what the likelihood is of their 3rd generation carrying dominant traits for both meatiness and heat-resistance? (100%.) How might they breed further generations in which these qualities would be guaranteed? (Students should understand that breeding MHHM genotypes to other MHHM genotypes will produce MHHM genotypes 100% of the time.)
Explain to students that traits that might not appear particularly desirable to human breeders may nevertheless be very important to the health of the species as a whole. To continue the with hypothetical example of the students’ cattle breeding, tell the class that the recessive gene for heat resistance (h)—one that they’ve been actively breeding out of their herd—has just been discovered to provide natural resistance to a certain tropical disease. What percentage of their 3rd generation can be expected to lack resistance to this disease? (50%.) What percentage would lack resistance if it were the recessive gene for meatiness that provided resistance? (100%.) Explain that this is a very simplified way of understanding the risks and limitations of selective breeding. By artificially selecting certain qualities and attributes useful to humans, we also risk losing naturally selected attributes necessary for a species’ health and survival.
Tell students that they will next be looking at a real-world case study involving artificial and natural selection. As homework, assign the students to read the article “A Dying Breed” which discusses the increasing hybridization by Bahiman cattle ranchers in Uganda of their native Ankole cattle with Holstein cattle from the United States. Tell them to consider the strengths and weaknesses of each breed, and the various needs of the Bahiman population. Have them take notes with the Traits of Ankole and Holstein Cattle Student Organizer.
At the beginning of the next class session, project a blank student organizer and call on individual students for the answers they recorded in their student organizer. For each question, encourage classroom discussion and consensus before moving on (Traits of Ankole and Holstein Cattle Student Organizer Answer Key is provided).
Ask the class which breed is the most fit for survival in Uganda? (Answers will vary—accept all. Some students will point out that the Ankole is much better adapted to Uganda’s environment and the Bahiman’s traditional nomadic life; others will point out that the more productive and domesticated Holsteins make much more money for Uganda’s impoverished people.)
1. Divide the class into the same four groups they were in for the introductory activity. Explain to the class that they will again be pretending they are a grassland people—this time the Bahimans in Uganda about whom they’ve just read. Distribute the Finding the Balance Student Organizer. Assign each group one of the following socio-economic groups:
2. Explain that each group will be given ten minutes to collaboratively determine whether or not to cross-breed their Ankole cattle with Holstein (made possible in this scenario through an international aid organization) according to the needs and desires of their assigned Bahiman group. (Explain that there are no “right” answers in this exercise, aside from the need to justify each breeding plan with evidence from the article and the lesson generally; each group’s plan will reflect different and complex cultural, social, and economic priorities that each group must balance as they see fit.)
3. After allowing ten minutes for the completion of this task, have each group report its answers, having the other groups record them on their own organizer. Encourage discussion of commonalities and alternate viewpoints. Explain to students that no amount of natural or artificial selection will ever produce a “perfect” cow, only different breeds that are more or less suited to the often conflicting demands of their natural environment and the needs and desires of humans (Finding the Balance Student Organizer Answer Key is provided).
Have student find pictures of different cattle breeds at the "Breeds of Cattle" Web site and instruct them to draw hypothetical images of what hybrids between them might look like. Alternatively, you could have students create "mash-up" hybrid images using Photoshop or other digital imaging software.
Have students research the climates and environments of different regions of the world and predict which traits cattle living there might have developed. These predictions can then be compared with the actual breeds of each region.
Ask a cattle rancher or dairy farmer to come in and discuss what makes a good beef or milk cow. Alternatively, schedule a field trip to a local cattle ranch or dairy farm.