Let population genetics be your guide to evolution

This exercise is a part of Educator Guide: Plague Immunity Left a Lasting Mark / View Guide
Directions for teachers:

Ask students to read the Science News Explores article “Explainer: What are genes?” and use the first set of questions to review some basic genetics concepts.

Then ask students to read the online Science News article “Black Death immunity came at a cost to modern-day health” and work with partners or in small groups to answer the second set of questions, which connect those basic concepts to population genetics and how species evolve. A version of the Science News article, “Plague immunity left a lasting mark,” appears in the November 19, 2022 issue.

Want to make it a virtual lesson? Post the online Science News article to your virtual classroom. Discuss the article and questions with your class on your virtual platform.

An individual’s genes

When we talk about genes and inheritance, we often focus on the individual. These questions will get us started by reviewing some basic concepts.

1. What is DNA and how is it organized? Where did you get your DNA?

DNA is short for deoxyribonucleic acid. It’s a double-helix molecule that carries genetic information from one generation to another. Packaged into chromosomes within cell nuclei, DNA gives cells instructions for building and maintaining life, including making proteins. We get our DNA from our biological parents.

2. What are genes? What are alleles?

Genes are the segments of DNA that provide instructions for making proteins. Alleles are different versions of the same gene, also called variants. We get a version of a gene, called an allele, from each of our biological parents. The alleles can be the same or different, and the allele pairs determine our observable traits.

3. Define the terms genotype and phenotype. How are they related?

A genotype is an organism’s complete set of genes. Phenotype refers to an organism’s observable traits. Phenotype is determined by genotype as well as environmental factors.

4. With a partner, use the Science News Explores article “Explainer: What are genes?”  to create a diagram that shows the relationships among the following terms: DNA, nucleotide, gene, allele, chromosome, nucleus and cell. Note: Some of the terms are linked to Science News Explores “Scientists Say” articles that will give you more information.

Student answers will vary. Here is an example from the National Institutes of Health.

Zooming out to species evolution

When we talk about evolution, we often zoom out from the individual to the species level. These questions will introduce the study of population genetics as a way of bridging the two.

1. Give an example of a population, then come up with a biological definition for population that includes the terms “individual” and “species.”

Student answers will vary. All the people within a city or all the trout in a lake are examples of populations. A population is a group of individuals of the same species that live among each other and interbreed.

2. Based on your understanding of genetics, your definition of population and the story you read, how would you define population genetics? What do population geneticists try to understand?

Population genetics is the study of what genes and gene variants, or alleles, are present in a population and how they and their distributions change over time.

3. What factors could change what gene variants, or alleles, are present within a population and their frequency?

Student answers should touch on topics that relate to natural selection (factors that make individual more fit based on environmental factors, food supply, predators, parasites, disease, etc.), sexual selection (non-random mating), mutation (changes in alleles due to random changes in DNA), genetic drift (natural disasters or other random events that affect the population) and gene flow (the introduction of new individuals with different genes into the population).

4. What was the Black Death? According to the article, how did it influence the frequency of alleles within the human population in Europe? How does the change relate to a factor you listed in your answer to the previous question?

From 1346 to 1350, the bacterium Yersinia pestis killed roughly one-third of the human population in Europe — a wave of bubonic plague known as the Black Death. After the Black Death, the frequency of an allele of the immune-related gene ERAP2 increased drastically. People with that allele were more likely to survive and so pass it on to their offspring, increasing its frequency in the population. It’s an example of natural selection, since people with the allele were more fit to survive the disease.

5. Define adaptation and evolution in the context of the article. How are they related?

Adaptation refers to a trait that helps an organism or a group of organisms survive in its environment. In this case, in the context of disease, the beneficial allele became an adaptation that spread through the population. When the frequency of the allele in the population changed over time through natural selection, the population was evolving.

6. Draw a diagram that represents the change in relative frequency of the standard ERAP2 gene and its variant (the beneficial allele) in the European population before and after the Black Death.

Student answers will vary. Generally, before the Black Death, the frequency of the standard ERAP2 gene should be higher than the frequency of the beneficial allele within the population. After the Black Death, the allele frequency should be higher than the frequency of the standard ERAP2 gene. 

7. What are the advantages of this adaptation? Are there disadvantages? How could the disadvantages cause the population to evolve in a different direction, and what would that look like for allele frequency?

Advantages include more individuals who are less likely to die from infections of the bacterium Yersinia pestis. Disadvantages include possible increased risk for developing immune-related conditions or diseases. In the absence of disease risk from Y. pestis, if the immune-related diseases make people less fit for survival or affect their ability to mate, the standard gene (or another variant) could become a beneficial adaptation, decreasing the frequency of the once-beneficial allele.