Home Page Textbooks Prentice Hall Biology (California) Section 16.2: Evolution as Genetic Change

Prentice Hall Biology (California)

1st Edition

Kenneth R. Miller, Levine

ISBN: 9780132013529

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Section 16.2: Evolution as Genetic Change

Exercise 1

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Solution 3

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Natural selection is the process of how different organisms adapt to changes in the environment. It is an evolutionary process where the genes of organisms change, thus their physical characteristics (phenotypes) as well.

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An example of a single-gene trait is the color of the skin of the frogs. Mutations to the gene that determines the skin color will lead to changes in phenotypes making them survive or perish in the environment.

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Natural selection on a single-gene trait can change the allele frequencies, leading to evolution.

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See explanation.

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Natural selection affects a single-gene trait by changes in allele frequencies and, thus, to changes in phenotype frequencies. Also, the relative fitness of the phenotypes and thereby produce one of three types of selection: directional, disruptive, and stabilizing selection.

Exercise 2

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Three modes of natural selections are directional selection where one extreme trait is favored over others, disruptive selection where two extreme traits are favored over average trait, and stabilizing selection where average trait is favored over extreme traits. Disruptive selection leads to two distinct phenotypes.

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See explanation.

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Polygenic traits are traits that are controlled by multiple genes, thus resulting in a range of phenotypes. Polygenic traits are influenced by natural selection in three distinct patterns.

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One pattern of natural selection in polygenic traits is **directional selection**. This occurs when one extreme phenotype is favored over the other phenotypes in the polygenic trait.

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Another pattern of natural selection in polygenic traits is **stabilizing selection**. This occurs when the intermediate phenotype is favored over the two extreme phenotypes in the trait.

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A third pattern of natural selection in polygenic traits is **disruptive selection**. This occurs when the two extreme phenotypes are favored over the intermediate phenotype. This also leads to the selection of *two distinct phenotypes*, one for each extreme.

Exercise 3

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Genetic drift describes changes in the allele frequency of a population. Small populations are most susceptible because individual that carries an allele may leave more offspring than the other individuals just by chance, thus changing the frequency of alleles in the next generation.

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See explanation.

Exercise 4

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Hardy-Weinberg equilibrium states that allele frequency of a population remains constant when all the following conditions are met: large population size, no mutation, no gene flow, no natural selection, and random mating.

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See explanation.

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The Hardy-Weinberg principle states that a population is said to be in Hardy-Weinberg equilibrium if the relative frequencies of alleles in the gene pool remain constant over time.

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In order for a population to be in Hardy-Weinberg equilibrium, the forces that drive evolution must be absent. These forces include mutations, natural selection, and nonrandom mating.

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If the driving forces of evolution are present, the population would begin to evolve as the allele frequencies change over time. In this case, it is no longer in Hardy-Weinberg equilibrium.

Exercise 5

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Directional selection involves individuals with a high fitness, where the range of phenotypes is larger because more reproduction and survival ability is present. Disruptive selection also involves individuals with high fitness, where the individuals act against individuals of an intermediate type. Therefore, disruptive selection creates two distinct phenotypes.

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Direction selection and disruptive selection are two patterns of natural selection that occur in polygenic traits. They both change the frequencies of phenotypes exhibited by these traits.

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Directional selection favors only one extreme phenotype over the other phenotypes along the range of a polygenic trait. This causes a population to exhibit an extreme trait more frequently.

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In contrast, disruptive selection favors two extreme phenotypes over the intermediate phenotype of a polygenic trait. This would cause a population to exhibit two distinct extreme phenotypes.

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Section 16.2: Evolution as Genetic Change

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