Case Study Conclusion: Flu, from Pigs to You
In April 2009, the world was hit with a swine flu pandemic. The Centers for Disease Control estimates that within that first year, 43 to 89 million people worldwide contracted the swine flu and that it contributed to 8,870 to 18,300 deaths. Some people with swine flu were spared serious complications, such as Eric, who you read about it at the beginning of this chapter. At the time, the swine flu spread rapidly because as a newly evolved viral strain, most people had no natural immunity against it, and the existing flu vaccine could not prevent it. But by November 2009, a swine flu vaccine was developed, and now it is included in the annual flu vaccine in the U.S. By August 2010, the World Health Organization declared the H1N1 swine flu pandemic to be over. The virus is still around, but because of the vaccine and the natural immunity of those who had the virus previously, its infection rate is no longer of pandemic proportions.
The swine flu virus appears to have originated in pigs and later evolved the ability to infect humans. How could this happen? Scientists think that a process called reassortment played a critical role. In reassortment, influenza viruses can exchange genetic material with each other if they have infected the same cells. This creates new combinations of genes, somewhat similar to the genetic mixing that occurs in sexual reproduction when two parents with different genes reproduce with each other. As you know, genes help dictate the characteristics of an organism, or in this case, a virus. Therefore, the production of novel combinations of genes due to viral reassortment can lead to the evolution of new viral characteristics.
In addition to reassortment, influenza viruses have other characteristics that cause them to evolve quickly. In contrast to sexual reproduction, the replication of viruses to produce new “offspring” particles is much more rapid. As you have learned in this chapter, evolution is typically a slow process that takes place over many generations. But if these generations are produced rapidly, as in the case of viruses and bacteria, it speeds the rate of evolution. Additionally, RNA viruses have a very high rate of genetic mutation. The rapid evolution of the influenza virus is one of the reasons why the annual seasonal flu vaccine is not always effective against every strain.
But why did this flu pandemic come from pigs? Pigs are actually an ideal “mixing bowl” for the evolution of influenza viruses because pigs can become infected with influenza viruses from other species, including birds and humans. Therefore, genetic reassortment can occur in pigs between viral strains that normally infect different species. This is what scientists think occurred to produce the 2009 H1N1 swine flu virus. The 2009 H1N1 has gene segments from the birds, human, and two different pig influenza viruses, and is therefore called a “quadruple reassortant” virus. In the case of the 2009 H1N1, this resulted in a new influenza strain that could infect humans, and be passed directly from person to person.
Scientists do not know exactly when and where the 2009 H1N1 evolved, but they think that the reassortment event may have occurred several years prior to the 2009 pandemic. This is based on evidence gathered from “molecular evolution” techniques, which are similar to the molecular clock technique described in this chapter. Influenza viruses are known to mutate at a relatively steady rate. The genetic sequences of the new 2009 H1N1 strain were compared to the sequences in related, older influenza viruses to count the number of new mutations, in order to give an estimate of when the new viral strain evolved.
Probably one of the final events that resulted in the generation of the 2009 H1N1 virus was contact between North American and Eurasian pigs. This is because prior to 2009, there were “triple reassortant” variants of H1N1 with gene segments from a bird, human, and North American pig influenza already in existence. The 2009 H1N1 strain additionally contained gene segments from influenza from Eurasian pigs, resulting in the “quadruple reassortant” virus. Scientists think that contact between pigs from these different regions, through international trade or other methods of contact, could have created this new strain. As you have learned in this chapter, the migration of organisms to new locations as well as contact between different organisms can influence evolution in many ways. Some examples are the migration of ancestral camels throughout the world, the coevolution of flowers and their pollinators, and the “founder effect” of small populations that move to new locations, such as the Amish.
Along with fossils, comparative anatomy and embryology, DNA analysis, and biogeography, evidence for evolution includes direct observation of it occurring. Peter and Rosemary Grant observed evolution occurring in the change in beak size of Galápagos finches. The evolution of the swine flu virus is another example of evolution in action. Evolution is not just a thing of the past — it is an ongoing and important process that affects our ecosystem, species, and even our health. Like viruses, bacteria also evolve rapidly, and the evolution of antibiotic resistance in bacteria is a growing public health concern. You can see that evolution is very relevant to our lives today.
In this chapter, you learned about the theory of evolution, evidence for evolution, how evolution works, and the evolution of living organisms on Earth. Specifically, you learned:
- Darwin’s theory of evolution by natural selection states that living things with beneficial traits produce more offspring than others do. This leads to changes in the traits of living things over time.
- During his voyage on the Beagle, Darwin made many observations that helped him develop his theory of evolution, particularly on the Galápagos Islands.
- Darwin was influenced by other early thinkers, including Lamarck, Lyell, and Malthus. He was also influenced by his knowledge of artificial selection.
- Wallace’s paper on evolution confirmed Darwin’s ideas. It also pushed him to publish his book, On the Origin of Species. The book clearly spells out his theory and provides extensive evidence and well-reasoned arguments to support it.
- Fossils provide a window into the past and are evidence for evolution. Scientists who find and study fossils are called paleontologists.
- Scientists compare the anatomy, embryos, and DNA of living things to understand how they evolved. Evidence for evolution is provided by homologous and analogous structures.
- Biogeography is the study of how and why plants and animals live where they do, which provides additional evidence for evolution. On island chains, such as the Galápagos, one species may evolve into many new species to fill available niches. This is called adaptive radiation.
- Peter and Rosemary Grant re-studied Galápagos finches. During a drought in the 1970s, they were able to directly observe evolution occurring.
- Microevolution refers to evolution that occurs over a relatively short period of time within a population. Macroevolution refers to evolution that occurs at or above the level of species as the result of microevolution taking place over many generations.
- The population is the unit of evolution, and population genetics is the science that studies evolution at the population level. A population’s gene pool consists of all the genes of all the members of the population. For a given gene, the population is characterized by the frequency of different alleles in the gene pool.
- There are four forces of evolution: mutation, which creates new alleles; gene flow, in which migration changes allele frequencies; genetic drift, which is a random change in allele frequencies that may occur in a small population; and natural selection, in which allele frequencies change because of differences in fitness among individuals.
- New species arise in the process of speciation. Allopatric speciation occurs when some members of a species become geographically isolated and evolve genetic differences. If the differences prevent them from interbreeding with the original species, a new species has evolved. Sympatric speciation occurs without geographic isolation first occurring.
- Coevolution occurs when interacting species evolve together. An example is flowering plants and their pollinators.
- Darwin thought that evolution occurs steadily and gradually. This model of evolution is called gradualism. The fossil record better supports the model of punctuated equilibrium. In this model, long periods of little change are interrupted by bursts of relatively rapid change.
- The fossil record is the record of life on Earth as reconstructed from the discovery and analysis of fossils. It is one of the most important tools in the study of evolution, but it is incomplete because fossilization is rare. To be added to the fossil record, fossils must be dated using relative or absolute dating methods.
- Molecular clocks are additional tools for reconstructing how life on Earth evolved. Molecular clocks use DNA or protein sequences to estimate how much time has passed since related species diverged from a common ancestor.
- The geologic time scale is a timeline of Earth's history. It divides Earth's chronology into smaller units of time such as eons and eras that are based on major changes in geology, climate, and living things.
Chapter Summary Review
1. Data from Peter and Rosemary Grant’s study on the evolution of beak size in Galápagos finches is shown above. The top graph (1976) shows the distribution of beak size in the population before a drought, and the bottom graph (1978) shows beak size after the drought. The drought reduced seed availability. Finches with big beaks can crack open and eat seeds of all sizes, while finches with small beaks can only crack open and eat small seeds. Answer the following questions about this data.
a. How was the average beak size affected by the drought? Although scientists would calculate this mathematically, you may answer just based on your observation of the graphs.
b. Explain how natural selection and the “struggle for existence” likely changed the beak size in this population.
c. Is this an example of microevolution or macroevolution? Explain your answer.
d. Explain why variation is important for evolution by natural selection, using the data above as a specific example.
e. What do you notice about the distribution of beak sizes in the 1978 graph — are all the beaks one size? If not, why not?
f. Is the change in beak size shown here an example of stabilizing selection, disruptive selection, or directional selection?
2. Which of the following is an example of macroevolution?
C. Structures that become larger in a population
D. A and B
3. Speciation is:
A. The movement of a species to a new niche
B. An evolution that occurs within a species
C. The evolution of a new species from an existing species
D. The development of analogous structures
4. True or False. An individual’s genotype is known as their gene pool.
5. True or False. New species can evolve without geographic separation.
6. True or False. In punctuated equilibrium, the periods of relatively little evolutionary change are shorter than the periods of dramatic change.
7. Describe one example of a major environmental change that influenced the evolution of life on Earth. This change could include climate change, geologic change, change in existing species, change in the atmosphere, etc.
8. Explain why mass extinction events often cause rapid evolutionary changes afterward.
9. Choose one. Species with homologous structures are (more/less) likely to be closely related than species with analogous structures.
10. Explain why the fossils of extinct animals provide evidence for evolution.
11. Which of the following is an example of evolution by natural selection?
A. Humans breeding dogs for certain characteristics
B. Bats developing wings as an adaptation for flight
C. A and B
D. None of the above
12. Compare and contrast Darwin’s theory of evolution by natural selection and Lamarck’s idea of inheritance of acquired characteristics.
13. Explain how microevolution and macroevolution relate to each other.
14. The fact that embryonic humans have gill slits is evidence for:
B. Evolution of analogous structures
C. Common ancestry of vertebrates
D. Gene flow
15. The study of allele frequencies in a group of the same species in the same time and place is known as _________ genetics.
16. Explain how biogeography can be used to study adaptive radiation.