Book cover for Biology

Biology

Sylvia S. Mader, Michael Windelspecht

ISBN #9780078024269

12th Edition

687 Questions

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153,501 Students Helped

Homework Questions

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

This chapter emphasizes that evolution is the change in allele frequencies within populations over time. By using the Hardy–Weinberg equilibrium as a baseline, we can measure microevolution and identify the influence of various evolutionary mechanisms such as genetic drift, gene flow, mutation, nonrandom mating, and natural selection. Particularly, natural selection can occur in multiple forms affecting trait distribution, and the heterozygote advantage plays a crucial role in preserving genetic diversity within populations.

Learning Objectives

1

Explain how populations evolve through changes in allele frequencies over time.

2

Describe the Hardy–Weinberg equilibrium and its underlying assumptions as a baseline for microevolution.

3

Differentiate among the evolutionary mechanisms including genetic drift, gene flow, mutation, nonrandom mating, and natural selection (stabilizing, directional, and disruptive selection).

4

Understand the concept of heterozygote advantage and its role in maintaining genetic diversity within populations.

Key Concepts

CONCEPT

DEFINITION

Evolution

A change in allele frequencies within a population over time.

Hardy–Weinberg Equilibrium

A principle stating that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of evolutionary influences, given conditions such as no mutation, no migration, large population size, random mating, and no selection.

Genetic Drift

Random fluctuations in allele frequencies that occur in small populations.

Gene Flow

The transfer of alleles or genes from one population to another through migration.

Mutation

Any change in the DNA sequence that can introduce new alleles into a population.

Nonrandom Mating

Mating that does not occur by chance, which can affect allele frequencies.

Natural Selection

The differential survival and reproduction of individuals due to differences in phenotype, which can include various forms such as stabilizing, directional, and disruptive selection.

Heterozygote Advantage

A situation where heterozygous individuals have higher fitness than homozygous individuals, contributing to the maintenance of genetic diversity despite selection pressures.

Example Problems

Example 1

Assuming a Hardy-Weinberg equilibrium, 21% of a population is homozygous dominant, 50% is heterozygous, and 29% is homozygous recessive. What percentage of the next generation is predicted to be homozygous recessive? a. 21% b. 50% c. 29% d. 42% e. 58%

Example 2

A human population has a higher than usual percentage of individuals with a genetic disorder. The most likely explanation is a. mutations and gene flow. b. mutations and natural selection. c. nonrandom mating and founder effect. d. nonrandom mating and gene flow. e. All of these are correct

Example 3

The offspring of better-adapted individuals are expected to make up a larger proportion of the next generation. The most likely explanation is a. mutations and nonrandom mating. b. gene flow and genetic drift. c. mutations and natural selection. d. mutations and genetic drift

Example 4

When a population is small, there is a greater chance of a. gene flow. b. genetic drift. c. natural selection. d. mutations. e. sexual selection

Example 5

Which of the following cannot occur if a population is to maintain an equilibrium of allele frequencies? a. People leave one country and relocate in another. b. A disease wipes out the majority of a herd of deer. c. Members of an Indian tribe allow only the two tallest people in the tribe to marry each spring. d. Large black rats are the preferred males in a population of rats. e. All of these are correct
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Step-by-Step Explanations

QUESTION

How do you calculate the expected genotype frequencies using the Hardy–Weinberg principle given allele frequencies?

STEP-BY-STEP ANSWER:

Step 1: Identify the allele frequencies. Let p be the frequency of one allele and q be the frequency of the other allele (with p + q = 1).
Step 2: Calculate the expected genotype frequency for the homozygous dominant genotype as p^2.
Step 3: Calculate the expected genotype frequency for the heterozygous genotype as 2pq.
Step 4: Calculate the expected genotype frequency for the homozygous recessive genotype as q^2.
Final Answer: The expected genotype frequencies are p^2 for homozygous dominant, 2pq for heterozygotes, and q^2 for homozygous recessive.

Hardy–Weinberg Equilibrium

QUESTION

How can you determine which type of natural selection (stabilizing, directional, or disruptive) is acting on a population based on changes in trait distribution?

STEP-BY-STEP ANSWER:

Step 1: Observe the distribution of traits in the population and identify any shifts or patterns.
Step 2: Determine if extreme traits are being selected against (stabilizing), if there is a shift towards one extreme (directional), or if both extremes have advantages over intermediate traits (disruptive).
Step 3: Compare the survival and reproduction rates associated with different trait values.
Final Answer: By analyzing the trait distribution and corresponding fitness outcomes, you can classify the type of natural selection acting on the population.

Natural Selection

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Common Mistakes

  • Assuming that Hardy–Weinberg equilibrium conditions are always met in natural populations.
  • Confusing microevolution (small changes in allele frequencies) with macroevolution (large-scale changes over time).
  • Misinterpreting the role of heterozygote advantage by assuming it applies universally without context.
  • Overlooking the fact that multiple evolutionary mechanisms can act simultaneously on a population.
  • Believing that natural selection always favors a single, optimal phenotype without considering variations like disruptive or stabilizing selection.