Evolutionary Genetics Lecture 7: Fitness and Selection
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Questions and Answers

What is the effect of the lethal allele 'l' on the heterozygous genotype in Dawson's 1970 experiment?

  • The heterozygous genotype is lethal.
  • The heterozygous genotype has a higher fitness than the homozygous dominant genotype.
  • The heterozygous genotype has the same fitness as the homozygous dominant genotype. (correct)
  • The heterozygous genotype has a reduced fitness compared to the homozygous dominant genotype.

    • Which of the following best describes the relationship between the lethal allele 'Sa' and the heterozygous genotype in Dawson's experiment?

  • The lethal allele 'Sa' is recessive to the wild-type allele in the heterozygous genotype.
  • The lethal allele 'Sa' exhibits partial dominance in the heterozygous genotype. (correct)
  • The lethal allele 'Sa' has no effect on the fitness of the heterozygous genotype.
  • The lethal allele 'Sa' is completely dominant over the wild-type allele in the heterozygous genotype.

    • What is the significance of using relative fitness in evaluating selection against lethal alleles?

  • Relative fitness is only used to measure the fitness of the homozygous genotypes.
  • Relative fitness is used to calculate the absolute fitness of the genotypes.
  • Relative fitness allows you to predict the exact number of offspring produced by each genotype.
  • Relative fitness allows for a standardized comparison of different genotypes regardless of their absolute fitness. (correct)

    • What does the information about the selection of a beneficial allele in Figure 6.18 reveal about the speed and completeness of allele fixation?

    <p>The speed and completeness of allele fixation are directly proportional to the beneficial allele's expression. (A)</p> Signup and view all the answers

    Which of the following is NOT a correct statement about dominant alleles?

    <p>Dominant alleles are always beneficial to an organism. (A)</p> Signup and view all the answers

    Why is it difficult for new dominant alleles to go to fixation in a population, even if they increase in frequency very rapidly through selection?

    <p>Residual recessive alleles end up ‘hiding’ in the remaining heterozygous individuals. (A)</p> Signup and view all the answers

    In the context of the provided passage, what is the consequence of a recessive allele being 'hidden' in heterozygotes?

    <p>The recessive allele can be passed on to future generations. (D)</p> Signup and view all the answers

    The passage mentions a lab experiment by Dawson (1970) using flour beetles (Tribolium castaneum). What is the main conclusion from this experiment, as mentioned in the passage?

    <p>Recessive lethal alleles can persist in a population even after selection pressure is applied. (C)</p> Signup and view all the answers

    Which of the following statements is TRUE regarding additive alleles?

    <p>Additive alleles have a greater effect on the phenotype when they are present in two copies. (B)</p> Signup and view all the answers

    What does the term 'relative fitness' represent in this context?

    <p>The ability of an individual to survive and reproduce relative to other individuals in the population. (D)</p> Signup and view all the answers

    What is the formula for calculating the frequency of the B1 allele (p) in the next generation (t+1)?

    <p>p(t+1) = (w11p^2 + 1/2 * w122pq) / (w11p^2 + w122pq + w22q^2) (A)</p> Signup and view all the answers

    What is the meaning of the term 'w' in the equation for calculating allele frequency in the next generation?

    <p>The relative fitness of the corresponding genotype. (C)</p> Signup and view all the answers

    If the relative fitness of the B1B2 genotype (w12) is greater than the fitness of the B1B1 (w11) and B2B2 (w22) genotypes, what would you expect to happen to the frequency of the B1B2 genotype over generations?

    <p>It would increase because it has the highest fitness. (C)</p> Signup and view all the answers

    How does selection affect the allele frequencies of the population?

    <p>Selection favors alleles that increase reproductive success, leading to changes in allele frequencies. (A)</p> Signup and view all the answers

    What is the relationship between the mean fitness of the population and the frequency of the B1B2 genotype?

    <p>A higher frequency of the B1B2 genotype contributes to a higher mean fitness, but other genotypes also play a role. (A)</p> Signup and view all the answers

    If the fitness of the B1B1 genotype (w11) is lower than the fitness of the B2B2 genotype (w22), what are the potential consequences for the B1 allele in the population?

    <p>The B1 allele will have a lower frequency in the next generation compared to the current generation. (C)</p> Signup and view all the answers

    What is the significance of the equation for calculating allele frequencies in the next generation (pt+1 and qt+1)?

    <p>This equation demonstrates how selection can lead to changes in allele frequencies. (C)</p> Signup and view all the answers

    Which of the following assumptions of the Hardy-Weinberg principle is violated when natural selection occurs?

    <p>There is no selection. (C)</p> Signup and view all the answers

    What is the most fit genotype in the mouse example?

    <p>B1B1 (C)</p> Signup and view all the answers

    What is the relative fitness (w) of the B1B2 genotype in the mouse example?

    <p>0.75 (B)</p> Signup and view all the answers

    What is the concept of fitness in evolutionary biology?

    <p>The ability to reproduce successfully. (A)</p> Signup and view all the answers

    What is the primary mechanism by which natural selection causes evolutionary change?

    <p>Differences in fitness among genotypes. (C)</p> Signup and view all the answers

    Which of the following is NOT a component of fitness?

    <p>Ability to learn and adapt. (B)</p> Signup and view all the answers

    What is the expected outcome of natural selection on a population over time?

    <p>A decrease in genetic variability. (B)</p> Signup and view all the answers

    What information is needed to predict the change in allele frequency from one generation to the next due to selection?

    <p>Both the initial allele frequencies and the relative fitnesses of the genotypes. (B)</p> Signup and view all the answers

    In the shrimp population, what is the frequency of the 'aa' genotype after one round of selection?

    <p>0.16 (C)</p> Signup and view all the answers

    Given that the survival rate of 'aa' genotype shrimp is 60%, what is their absolute fitness?

    <p>0.6 (A)</p> Signup and view all the answers

    If a genotype has a lower survival rate compared to other genotypes in the population, will its relative fitness be greater than 1, less than 1, or equal to 1?

    <p>Less than 1 (A)</p> Signup and view all the answers

    What does the term 'allele frequency' refer to?

    <p>The proportion of a specific allele in a population's gene pool (D)</p> Signup and view all the answers

    If a population exhibits a high frequency of a specific allele, what can we infer about that allele?

    <p>It is likely advantageous to the population's survival (A)</p> Signup and view all the answers

    Natural selection acts directly on which of the following?

    <p>Phenotype frequency (A)</p> Signup and view all the answers

    Study Notes

    Evolutionary Genetics: Lecture 7 - Fitness and Selection

    • Hardy-Weinberg Equilibrium Assumptions: These assumptions are crucial for understanding when evolution is NOT occurring in a population.
      • No selection, individuals have equal survival and reproduction probabilities.
      • No mutation: genes remain unchanged between generations.
      • No migration: genes aren't added from outside the population.
      • No chance events: the population size is infinitely large.
      • Random mating: individuals choose mates randomly.

    Fitness

    • Fitness: The expected reproductive success of an organism with a specific genotype or phenotype.
    • Components of Fitness:
      • Survival to reproductive age.
      • Mating success.
      • Fecundity (fertility).
    • Genotype Fitness: Calculated as the product of survival probability and average number of offspring.

    Fitness and Selection

    • Initial Allele Frequencies: Examples provided show initial frequencies of B₁ and B₂ alleles (e.g, B₁ = 0.6; B₂ = 0.4).
    • Number of Zygotes: The diagram shows the distribution of different genotypes (e.g, B₁B₁, B₁B₂, B₂B₂) among zygotes.
    • Selection: The process where individuals with certain genotypes have higher/lower survival rates and produce more/fewer offspring compared to other individuals.
    • Calculation of relative fitness (w): The fitness of a genotype standardized by comparison to other genotypes in the population; it is calculated by dividing the fitness of a given genotype by the fitness of the most fit genotype in the population.
    • Changes in allele frequencies after selection: After a period of selection, the allele frequencies change from original values. (e.g., B₁=0.675; B₂ = 0.325)

    How Selection Causes Change in Genotype Frequencies

    • Initial Allele Frequencies: In example populations, start with allele frequencies like B₁ = 0.6 and B₂ = 0.4 in Hardy-Weinberg Equilibrium.
    • Survival Rates: Different genotypes (e.g., B₁B₁, B₁B₂, B₂B₂) have different survival rates.
    • Genotype Frequencies: The table shows initial and post-selection genotype frequencies.
    • Frequency at reproduction: The calculated frequency of genotypes after selection for reproduction.
    • Relative Fitness and Scaling: Frequencies need scaling to take into account the overall proportion of the population that survives to reproduce.

    Practice Problem: Shrimp Example

    • Genotype Frequency: Provided frequencies of AA, Aa, and aa genotypes in a population.
    • Relative Fitness: Given that AA and Aa have 100% survival while aa have a 60% survival rate, the relative fitness values are 1 for AA & Aa and 0.6 for aa.
    • Calculating Frequencies After Selection: Frequencies are scaled to account for survivors after environmental pressures.

    Relationships Among Alleles

    • Additive Alleles: Alleles yield twice the effect when present in two copies.
    • Dominant Alleles: Dominant alleles mask the expression of recessive alleles in heterozygotes.

    Fixation of Dominant Alleles (Why it's difficult)

    • Residual Recessive Alleles: The presence of remaining heterozygotes with recessive alleles can hide the presence of the recessive allele in the population.

    Demonstration of Directional Selection

    • Lethal Alleles: Selection examples focused on alleles creating lethal conditions.
    • Genotypes in the Population: Examples were used with all heterozygotes.

    Beneficial Selection

    • Effect on Allele Frequencies: Beneficial alleles tend to increase in frequency within a population over time.
    • Types of Alleles: The impact of selection on different types of alleles (additive, dominant, recessive) was illustrated in the graphs.

    Key Points

    • Selection: Selection occurs when individuals with different genotypes have different fitness levels.
    • Relative Fitness: Use relative fitness to compare genotypes based on their reproductive success.
    • Frequency Change: Genotype frequencies change according to their relative fitness as one generation passes to another.
    • Genotypes After Selection: Calculate and predict genotype frequencies after selective events.
    • Beneficial Allele Fixation: Explains how quickly and completely a beneficial allele might be fixed depending on its expression compared to the alternative allele.

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    Lecture 7 Fitness and Selection - Biology 206

    Description

    This quiz covers key concepts from Lecture 7 of the Evolutionary Genetics course, focusing on fitness and selection mechanisms. It explores the Hardy-Weinberg equilibrium assumptions and delves into the components of fitness, including survival, mating success, and fecundity. Test your understanding of how these concepts apply to evolutionary processes.

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