To understand the pattern of genetic variation among humans.
By the time students reach high school, they have been introduced to basic concepts in genetics. They know that cells contain genetic information that, when translated by the cell, instructs it to make proteins, a process that underlies human appearance, health, and behavior. Students at the high-school level also know that genes and environment interact to affect the ways that traits are expressed (their phenotypes), and that every person inherits a unique combination of genes from mother and father.
Within this topic of genetics, students need repeated exposure to the concepts of natural selection and adaptation to reach understanding. Students at the high-school level may hold the misconception that people choose traits to fit into their environment and that adaptations acquired during a lifetime can be passed on to the next generation. They also may misconstrue the concept of genetic variation, thinking that populations of humans vary dramatically from each other in genetic terms.
This lesson is designed to reinforce the concept that biological change occurs at the cellular and molecular levels and that, despite the changes introduced by natural selection over thousands of years, the DNA of any two people in the world is 99.9 percent alike. In other words, all people belong to one species, one human race. The lesson should help students understand furthermore that within that small percentage of DNA that varies among humans, as much variation can be found between two people from the same ancestral population as between two people who are completely unrelated and live across the globe from each other. This information can be used to help deconstruct the notion of separate biological races.
Before guiding your students through this lesson, explore the interactive, Nowhere to Hide. Also practice the remarks you will make to transition between the motivational activity and the main activity, based on the Race and Human Variation resource.
This lesson requires you to carefully guide students through the distinction between the scientific concept that human genetic diversity has occurred through evolution and two other concepts that have been disproven. The population concept is biologically valid but has been disproven by DNA evidence. Because it is not a valid concept, class time should not be devoted to more than the superficial explanation provided in the resource. The essentialist concept is not scientifically based and so cannot be supported or refuted with evidence. Furthermore, it has been used to defend racist thinking by suggesting that groups of people differ based on “essential” characteristics that never change within each group through the generations, and that certain “essential” characteristics can make one group superior to another. Be prepared to convey to students that the resource does not support essentialism and, in fact, debunks it.
To begin this lesson, students should use their Genetic Variation with the One Human Race student esheet to go to and play Nowhere to Hide, an activity that simulates natural selection, and to read its “learn more” section. They can do this task independently, in small groups, or, if you have a computer projector, as a whole class. Students should answer the observation questions on their Genetic Variation within the One Human Race student sheet; this will prepare them to participate in class discussion of the questions.
After students have interacted with Nowhere to Hide, conduct a discussion of these questions:
- When the forest is green, which color prey (bugs) are most often picked off by their predators (birds)?
(The orange bugs are most often eaten.)
- When the forest is discolored by orange factory smoke, which color prey are most often picked off by predators?
(The green bugs are most often eaten.)
- When the predators have eaten all the prey of one color, does the other color disappear forever?
- In a green forest, which bugs are best adapted to their environment, that is, which bugs have the best chances of surviving to the age of reproduction?
(The green bugs, because their color camouflages them, which makes them harder to be seen by their prey. The orange bugs are more likely to be seen and eaten by their predators, reducing their chances of living long enough to reproduce.)
- When the forest is completely discolored by factory smoke, why do the green bugs never completely disappear?
(The genes for green color may be recessive. When two orange bugs mate that each carry the recessive trait for green, that combination can sometimes appear in offspring, who will be green. Alternatively, new genetic mutations can reintroduce the green coloration.)
- Let’s say that some of the green bugs living in a green forest migrate to a new forest near a factory. In several generations, what will probably be the primary color of the population of bugs that stays behind? In several generations, what will probably be the primary color of the population of bugs that moves to the new forest?
(The population that stays in the original forest will probably remain primarily green, while the population in the new forest will probably adapt through natural selection to become primarily orange.)
- How does this activity coincide with what you understand about natural selection and how natural selection plays a role in evolution?
(Student answers should reflect an understanding of Benchmark (5F/H3): “Natural selection provides the following mechanism for evolution: Some variation in heritable characteristics exists within every species; some of these characteristics give individuals an advantage over others in surviving and reproducing; and the advantaged offspring, in turn, are more likely than others to survive and reproduce. As a result, the proportion of individuals that have advantageous characteristics will increase.”)
Once you have discussed these questions with your class, lead into the next section by saying:
This game presents a simplified visual demonstration of natural selection at work. It can help you understand how human genetic variation has also emerged over generations, through natural selection, as populations of humans adapted to various environments. This genetic variety is, however, greatly outweighed by the genetic similarity of all humans to each other.
Recall that every one of you has inherited some entirely new combinations of existing genes or mutations of genes in reproductive cells. Some of these changes do not make you discernibly different from any other human, whereas other changes have distinctive effects. Only mutations that appear in your reproductive cells (germ cells) can create the variation that will be inherited by your offspring. Such changes in reproductive cells may be transmitted to one or more of the children you may have, and your grandchildren and other descendants may also transmit them further down the family line. In the very rare situation in which a genetic change offers an advantage to survival, your offspring may live to the age of reproduction in greater numbers, generation by generation, than the offspring of others without that genetic change. The new genetic combination may gradually become common within the population of your descendants. Other genetic combinations will remain in the population; they will die out completely only if they offer a disadvantage to survival.
But remember also that each of you has three billion base pairs of DNA. The amount of variation among humans constitutes only perhaps one-tenth of a percent of that three billion. Thus DNA similarity totally swamps DNA differences.
In this lesson, you will study how human variation has been introduced into our species over time and across the globe. Then you will select a particular trait variation to explore.
Now students should use their student esheet to visit Race and Human Variation, which is a section of the website, Race: Are We So Different? This section explains how genes vary within humans, with the greatest variety occurring within the population of Africa, where humans have lived longest. Students also should read the National Geographic resource, The Human Journey: Migration Routes, to supplement their understanding of how the human gene pool expanded as humans migrated across the globe.
Note that this resource contrasts the scientific concept of human genetic diversity with two other concepts, population biology and essentialism. As noted in the Planning Ahead section, the resource, Race: Are We So Different emphasizes that both of these concepts are invalid—the former because it is disproven by DNA evidence and the latter because it is nonscientific so cannot be either supported or refuted by experimental evidence.
Assign students to use their student esheet to review this short resource and to answer the questions that appear on the student sheet. After they have completed the task, discuss these questions:
- Why does the continent of Africa contain the most diversity among humans?
(It contains the most diversity among humans because the human species first emerged in Africa and has lived there for the greatest number of generations, providing the most time for genetic variations to be introduced.)
- Do people indigenous to Africa have some genetic variations that are not found elsewhere in the world? Explain your answer.
(Yes. Only part of the African population migrated out of the continent, so only part of the human variation introduced over generations within Africa has been carried elsewhere.)
- Do Asians and Europeans possess any variations not found among indigenous Africans? Explain your answer.
(Yes. Some genetic mutations or new combinations have been introduced into the human species among populations after they had migrated out of Africa).
- If the “Out of Africa” map were to include North and South America, what color dots (representing genetic variation) might you expect to see on those continents?
(Dots of all colors, since peoples of this continent have emigrated to North and South America from nearly everywhere else on the globe, bringing with them their genetic variations.)
Ask students to select a particular human trait from this list:
- Chromosomal abnormalities
- Blood types
- Skin tone
- Hair color
- Heterochromia iridum (differently colored eyes)
- Male-pattern baldness
- Tongue dexterity (ability to roll or point the tongue)
- Susceptibility to a particular disease (e.g., sickle cell disease, hypertension, age-related macular degeneration)
Students should use the Internet and other sources to research the trait in terms of human variation across the globe. Instruct them to use the Genetic Variation for a Human Trait Project Sheet to compile their information.
Online resources students can use to find a trait for their research include:
- Help Me Understand Genetics
- Heredity and Traits
- Health Connections
- Understanding Gene Testing
- Specific Genetic Disorders
In its handbook Help Me Understand Genetics, The National Institutes of Health’s Library of Medicine provides an overview of the genetic science that helps explain human variation within the single species.
An explanation for why humans originating in Africa migrated out of the continent in successive waves over history is presented in the radio report, What Drove Early Man Across Globe? Climate Change.
National Geographic’s Genographic Project has used advanced DNA analysis to explore the question of where in Africa humans originated and the patterns by which they spread across the world. Students can check out the Genographic Project’s News page to find up-to-date reports on how the project is contributing evidence to confirm patterns of human migration, human diversity, and relatedness.