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9.1: Case Study: Everyday Evolution

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    Case Study: Flu, from Pigs to You

    One night in April 2009, Mateo woke up soaked in sweat. He had a fever of 102.4 °F, chills, an intense headache, and body aches. He soon develops a sore throat and a bad cough. The next day he felt so sick and exhausted that he could hardly get out of bed, and his fever and other symptoms lasted for days. Clearly, this was not just a mild cold virus — Mateo most likely had influenza, commonly known as the flu.

    Sow and piglets
    Figure \(\PageIndex{1}\): Sow and five piglets

    While watching TV as he recovered in bed, Mateo saw a news report about a new “swine flu” strain of the influenza virus that was spreading in people throughout North America, particularly in Mexico. It was called the swine flu because scientists thought it most likely originated in pigs, based on similarities in its genetic sequence with viruses that infect pigs. However, contact with pigs was not necessary for people to catch swine flu. This version seemed to spread directly between people, similar to the typical seasonal flu virus.

    Mateo’s symptoms were similar to those described in the news report on swine flu. Although he was beginning to recover, he saw that others were not so lucky. Many people with swine flu developed severe pneumonia, and some even died. Because this was a new strain of flu virus that was significantly different than the previous seasonal flu viruses, the existing flu vaccine was largely ineffective against swine flu. Therefore, the only way to try to prevent infection by the swine flu virus was to limit exposure to it, including avoiding contact with people with the flu and using good hand washing practices. The news report showed people in Mexico wearing masks as they went about their daily lives, to try to prevent exposure to the virus.

    By June 2009, Mateo was back to normal, but many other people worldwide were not. Within just a few months, the swine flu had spread from North America to over 70 countries and territories throughout the world. The World Health Organization declared the spread of swine flu to be a pandemic, meaning that a significant portion of the world’s population was infected. In September 2009, over 99% of the influenza viruses circulating in the U.S. were the swine flu strain, which is also known as the 2009 H1N1 virus. If you had the flu in the U.S. during this time period, chances are high that it was the swine flu.

    Swine Flu Masked Train Passengers in Mexico City
    Figure \(\PageIndex{2}\): Train passengers in Mexico City in April 2009, wearing masks to protect against the swine flu epidemic.

    How could a new viral strain like this emerge so suddenly? And how could it change from infecting pigs to infecting humans? This is an example of evolution in action. You may think of evolution as something that occurred in the distant past, for instance, how humans evolved from earlier primates. But evolution is occurring all the time. As you will learn in this chapter, evolution is the process by which characteristics of biological entities, such as living organisms or viruses, change over time. Evolution can occur very slowly or more quickly, but it is particularly rapid in viruses and bacteria. In the Case Study Conclusion for this chapter, you will learn specifically about how the 2009 H1N1 virus evolved from a virus that infects pigs to one that infects humans.

    Chapter Overview: Biological Evolution

    In this chapter, you will learn about the theory of evolution, evidence for evolution, how evolution works, and the evolution of living organisms on Earth. Specifically, you will learn about:

    • Darwin’s theory of evolution by natural selection and how he developed this theory.
    • Evidence for the theory of evolution from fossils, DNA, and observations of living organisms.
    • Microevolution, which is an evolution that occurs over a relatively short period of time within a population.
    • How allele frequencies in a population change due to the forces of evolution, which include mutation, gene flow, genetic drift, and natural selection.
    • Macroevolution, which is an evolution that occurs at or above the species level. This includes the generation of new species and coevolution between species.
    • Influences on the timing of macroevolution.
    • The tools used by scientists to study evolution including the fossil record, methods of establishing the age of fossils, and molecular clocks based on DNA or amino acid sequences.

    As you read this chapter and learn more about evolution, think about the following questions about the swine flu virus.

    1. Viruses can replicate quickly. Why does this contribute to their rapid rate of evolution?
    2. Mutation plays an important role in the evolution of viruses. How does mutation relate to evolution?
    3. One of the reasons why the 2009 H1N1 swine flu virus evolved is that different types of influenza viruses can exchange genetic material with each other if they infect the same host, in a process called reassortment. Why might this lead to a new strain of influenza virus with different characteristics? How is this similar to the genetic variation produced by sexual reproduction?
    4. It is thought that contact between North American and Eurasian pigs, possibly through international trade, may have contributed to the evolution of the swine flu virus. What are some other examples in which the movement of organisms or contact between organisms has contributed to evolutionary changes?


    1. Sow and five piglets by Scott Bauer, U.S. Department of Agriculture, public domain via Wikimedia Commons
    2. Masked Train Passengers by Eneas De Troya from Mexico City, México, CC BY 2.0 via Wikimedia Commons
    3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0

    This page titled 9.1: Case Study: Everyday Evolution is shared under a CK-12 license and was authored, remixed, and/or curated by Suzanne Wakim & Mandeep Grewal via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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