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Ch. 22 - Evolution by Natural Selection
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All textbooksFreeman 8th EditionCh. 22 - Evolution by Natural SelectionProblem 13

Chapter 21, Problem 13

Over half of the world’s human population lives in cities. Are species adapting to life in these novel urban environments? A global team of researchers (including Tiffany Longo, Jesse Bragger, and Summer Shaheed, shown in in the photo) used white clover as a study system to find out. They measured the production of a compound called hydrogen cyanide (HCN) in rural and urban clover plants. Production of HCN deters herbivores and increases tolerance to drought but has a cost: Clover must expend energy to produce HCN.

The production of HCN is a heritable Mendelian trait controlled by just two unlinked genes (Ch. 14, Section 14.2; clover plants with at least one dominant allele produce HCN, but plants that are homozygous recessive for both genes produce no HCN). Apply Darwin’s four postulates to explain natural selection for low HCN production in a population of clover living in a city with few herbivores and ample water.

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1
Identify the variation in the trait within the population: In this case, the trait is the production of hydrogen cyanide (HCN) in clover plants. Some clover plants produce HCN due to having at least one dominant allele, while others do not produce HCN because they are homozygous recessive for the genes controlling HCN production.
Determine if the trait is heritable: According to the information provided, the ability to produce HCN is a heritable trait controlled by two unlinked genes. This means that the trait can be passed from parents to offspring.
Assess the differential survival or reproduction associated with the trait: In an urban environment with few herbivores and ample water, the production of HCN may not be as advantageous. Plants that produce HCN expend energy to do so, which could be a disadvantage if there are few herbivores to deter and water is not a limiting factor.
Conclude how the trait's frequency might change over time: If producing HCN is costly and does not provide a significant survival advantage in the urban environment, natural selection may favor clover plants that produce less or no HCN. Over time, the frequency of the alleles for low or no HCN production could increase in the urban clover population.
Apply Darwin’s four postulates to the scenario: Variation in the trait exists; the trait is heritable; the trait influences reproductive success (less energy expenditure on HCN production could mean more resources for growth and reproduction); and thus, the trait's frequency may change over generations due to natural selection favoring plants with low or no HCN production in this specific environment.
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Related Practice
Textbook Question

The geneticist James Crow wrote that successful scientific theories have the following characteristics: (1) They explain otherwise puzzling observations; (2) they provide connections between otherwise disparate observations; (3) they make predictions that can be tested; and (4) they are heuristic, meaning that they open up new avenues of theory and experimentation. Crow added two other elements of scientific theories that he considered important on a personal, emotional level: (5) They should be elegant, in the sense of being simple and powerful; and (6) they should have an element of surprise. How well does the theory of evolution by natural selection fulfill these six criteria?

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Open Question

Over half of the world’s human population lives in cities. Are species adapting to life in these novel urban environments? A global team of researchers (including Tiffany Longo, Jesse Bragger, and Summer Shaheed, shown in in the photo) used white clover as a study system to find out. They measured the production of a compound called hydrogen cyanide (HCN) in rural and urban clover plants. Production of HCN deters herbivores and increases tolerance to drought but has a cost: Clover must expend energy to produce HCN.

Compare how evolution by inheritance of acquired characters and the theory of evolution by natural selection would explain the observation that HCN production is often high in rural environments with many herbivores and low in urban environments with few herbivores.

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Textbook Question

Most mice living on the mainland of Florida are brown, but the mice that live on the sand dunes of the barrier islands have white fur. The differences in color are heritable, determined by genes such as MC1R. It is intuitive that the light color of beach-dwelling mice is an adaptation for blending into their environment—and thus evading predators. What is an evolutionary adaptation? a. a trait that improves the fitness of its bearer, compared with individuals without the trait b. a trait that changes in response to environmental influences within the individual's lifetime c. the ability of an individual to adjust to its environment d. a trait that an individual wants so that it can survive

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Open Question

Over half of the world’s human population lives in cities. Are species adapting to life in these novel urban environments? A global team of researchers (including Tiffany Longo, Jesse Bragger, and Summer Shaheed, shown in in the photo) used white clover as a study system to find out. They measured the production of a compound called hydrogen cyanide (HCN) in rural and urban clover plants. Production of HCN deters herbivores and increases tolerance to drought but has a cost: Clover must expend energy to produce HCN.

This study was conducted by a network of 287 scientists who sampled 110,019 white clover plants (Trifolium repens) in 160 cities globally. Why do you think they chose to study white clover instead of other plants like red maples, coconut palms, or redwoods? What’s one advantage and one disadvantage of conducting such a large collaboration?

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Open Question

Over half of the world’s human population lives in cities. Are species adapting to life in these novel urban environments? A global team of researchers (including Tiffany Longo, Jesse Bragger, and Summer Shaheed, shown in in the photo) used white clover as a study system to find out. They measured the production of a compound called hydrogen cyanide (HCN) in rural and urban clover plants. Production of HCN deters herbivores and increases tolerance to drought but has a cost: Clover must expend energy to produce HCN.

The researchers measured the frequency of clover plants producing HCN along a transect from each city center out to its rural environment. The graph shows one example of results from Münster, Germany: Anna Bucharova and Christian Lampei sampled 765 plants in 40 populations along a 9.87 km transect. What does the data reveal? What was the prediction of the null hypothesis?


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Open Question

Over half of the world’s human population lives in cities. Are species adapting to life in these novel urban environments? A global team of researchers (including Tiffany Longo, Jesse Bragger, and Summer Shaheed, shown in in the photo) used white clover as a study system to find out. They measured the production of a compound called hydrogen cyanide (HCN) in rural and urban clover plants. Production of HCN deters herbivores and increases tolerance to drought but has a cost: Clover must expend energy to produce HCN.

When a statistical test was used to compare the trend in the data to the prediction of the null hypothesis, the 𝑃 value (see BioSkills 3) was <0.001 for Münster, Germany. Does this result increase or decrease your confidence in the result? Explain.

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