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19.E: The Evolution of Populations (Exercises)

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    19.1: Population Evolution

    Initially, the newly discovered particulate nature of genes made it difficult for biologists to understand how gradual evolution could occur. But over the next few decades genetics and evolution were integrated in what became known as the modern synthesis—the coherent understanding of the relationship between natural selection and genetics that took shape by the 1940s and is generally accepted today.

    Review Questions

    What is the difference between micro- and macroevolution?

    1. Microevolution describes the evolution of small organisms, such as insects, while macroevolution describes the evolution of large organisms, like people and elephants.
    2. Microevolution describes the evolution of microscopic entities, such as molecules and proteins, while macroevolution describes the evolution of whole organisms.
    3. Microevolution describes the evolution of organisms in populations, while macroevolution describes the evolution of species over long periods of time.
    4. Microevolution describes the evolution of organisms over their lifetimes, while macroevolution describes the evolution of organisms over multiple generations.


    Population genetics is the study of:

    1. how selective forces change the allele frequencies in a population over time
    2. the genetic basis of population-wide traits
    3. whether traits have a genetic basis
    4. the degree of inbreeding in a population


    Which of the following populations is not in Hardy-Weinberg equilibrium?

    1. a population with 12 homozygous recessive individuals (yy), 8 homozygous dominant individuals (YY), and 4 heterozygous individuals (Yy)
    2. a population in which the allele frequencies do not change over time
    3. p2 + 2pq + q2 = 1
    4. a population undergoing natural selection


    One of the original Amish colonies rose from a ship of colonists that came from Europe. The ship’s captain, who had polydactyly, a rare dominant trait, was one of the original colonists. Today, we see a much higher frequency of polydactyly in the Amish population. This is an example of:

    1. natural selection
    2. genetic drift
    3. founder effect
    4. b and c


    Free Response

    Solve for the genetic structure of a population with 12 homozygous recessive individuals (yy), 8 homozygous dominant individuals (YY), and 4 heterozygous individuals (Yy).

    p = (8*2 + 4)/48 = .42; q = (12*2 + 4)/48 = .58; p2 = .17; 2pq = .48; q2 = .34

    Explain the Hardy-Weinberg principle of equilibrium theory.

    The Hardy-Weinberg principle of equilibrium is used to describe the genetic makeup of a population. The theory states that a population’s allele and genotype frequencies are inherently stable: unless some kind of evolutionary force is acting upon the population, generation after generation of the population would carry the same genes, and individuals would, as a whole, look essentially the same.

    Imagine you are trying to test whether a population of flowers is undergoing evolution. You suspect there is selection pressure on the color of the flower: bees seem to cluster around the red flowers more often than the blue flowers. In a separate experiment, you discover blue flower color is dominant to red flower color. In a field, you count 600 blue flowers and 200 red flowers. What would you expect the genetic structure of the flowers to be?

    Red is recessive so q2 = 200/800 = 0.25; q = 0.5; p = 1-q = 0.5; p2 = 0.25; 2pq = 0.5. You would expect 200 homozygous blue flowers, 400 heterozygous blue flowers, and 200 red flowers.


    19.2: Population Genetics

    Individuals of a population often display different phenotypes, or express different alleles of a particular gene, referred to as polymorphisms. Populations with two or more variations of particular characteristics are called polymorphic. The distribution of phenotypes among individuals, known as the population variation, is influenced by a number of factors, including the population’s genetic structure and the environment.

    Review Questions

    When male lions reach sexual maturity, they leave their group in search of a new pride. This can alter the allele frequencies of the population through which of the following mechanisms?

    1. natural selection
    2. genetic drift
    3. gene flow
    4. random mating


    Which of the following evolutionary forces can introduce new genetic variation into a population?

    1. natural selection and genetic drift
    2. mutation and gene flow
    3. natural selection and nonrandom mating
    4. mutation and genetic drift


    What is assortative mating?

    1. when individuals mate with those who are similar to themselves
    2. when individuals mate with those who are dissimilar to themselves
    3. when individuals mate with those who are the most fit in the population
    4. when individuals mate with those who are least fit in the population


    When closely related individuals mate with each other, or inbreed, the offspring are often not as fit as the offspring of two unrelated individuals. Why?

    1. Close relatives are genetically incompatible.
    2. The DNA of close relatives reacts negatively in the offspring.
    3. Inbreeding can bring together rare, deleterious mutations that lead to harmful phenotypes.
    4. Inbreeding causes normally silent alleles to be expressed.


    What is a cline?

    1. the slope of a mountain where a population lives
    2. the degree to which a mutation helps an individual survive
    3. the number of individuals in the population
    4. gradual geographic variation across an ecological gradient


    Free Response

    Describe a situation in which a population would undergo the bottleneck effect and explain what impact that would have on the population’s gene pool.

    A hurricane kills a large percentage of a population of sand-dwelling crustaceans—only a few individuals survive. The alleles carried by those surviving individuals would represent the entire population’s gene pool. If those surviving individuals are not representative of the original population, the post-hurricane gene pool will differ from the original gene pool.

    Describe natural selection and give an example of natural selection at work in a population.

    The theory of natural selection stems from the observation that some individuals in a population survive longer and have more offspring than others: thus, more of their genes are passed to the next generation. For example, a big, powerful male gorilla is much more likely than a smaller, weaker one to become the population’s silverback: the pack’s leader who mates far more than the other males of the group. Therefore, the pack leader will father more offspring who share half of his genes and are likely to grow bigger and stronger like their father. Over time, the genes for bigger size will increase in frequency in the population, and the average body size, as a result, grow larger on average.

    Explain what a cline is and provide examples.

    A cline is a type of geographic variation that is seen in populations of a given species that vary gradually across an ecological gradient. For example, warm-blooded animals tend to have larger bodies in the cooler climates closer to the earth’s poles, allowing them to better conserve heat. This is considered a latitudinal cline. Flowering plants tend to bloom at different times depending on where they are along the slope of a mountain. This is known as an altitudinal cline.

    19.3: Adaptive Evolution

    Fitness is often quantifiable and is measured by scientists in the field. However, it is not the absolute fitness of an individual that counts, but rather how it compares to the other organisms in the population. This concept, called relative fitness, allows researchers to determine which individuals are contributing additional offspring to the next generation, and thus, how the population might evolve.

    Review Questions

    Which type of selection results in greater genetic variance in a population?

    1. stabilizing selection
    2. directional selection
    3. diversifying selection
    4. positive frequency-dependent selection


    When males and females of a population look or act differently, it is referred to as ________.

    1. sexual dimorphism
    2. sexual selection
    3. diversifying selection
    4. a cline


    The good genes hypothesis is a theory that explains what?

    1. why more fit individuals are more likely to have more offspring
    2. why alleles that confer beneficial traits or behaviors are selected for by natural selection
    3. why some deleterious mutations are maintained in the population
    4. why individuals of one sex develop impressive ornamental traits


    Free Response

    Give an example of a trait that may have evolved as a result of the handicap principle and explain your reasoning.

    The peacock’s tail is a good example of the handicap principle. The tail, which makes the males more visible to predators and less able to escape, is clearly a disadvantage to the bird’s survival. But because it is a disadvantage, only the most fit males should be able to survive with it. Thus, the tail serves as an honest signal of quality to the females of the population; therefore, the male will earn more matings and greater reproductive success.

    List the ways in which evolution can affect population variation and describe how they influence allele frequencies.

    There are several ways evolution can affect population variation: stabilizing selection, directional selection, diversifying selection, frequency-dependent selection, and sexual selection. As these influence the allele frequencies in a population, individuals can either become more or less related, and the phenotypes displayed can become more similar or more disparate.