Evolutionary Genetics Lecture 6
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Questions and Answers

What does genetic drift cause the frequency of alleles to do in small populations?

  • Remain constant
  • Fluctuate randomly towards 0 or 1 (correct)
  • Decrease
  • Increase

    • What happens to heterozygosity as the allele frequency approaches 1 (or 0)?

  • Heterozygosity increases.
  • Heterozygosity remains constant.
  • Heterozygosity decreases. (correct)
  • Heterozygosity fluctuates randomly.

    • What is the heterozygosity value when p = 0.5?

  • 0.25
  • 0
  • 0.5 (correct)
  • 1

    • How does genetic drift affect average heterozygosity in populations?

    <p>Decreases average heterozygosity. (B)</p> Signup and view all the answers

    Which of the following assumptions of the Hardy-Weinberg equilibrium is violated when a population is finite in size?

    <p>There are no chance events. (C)</p> Signup and view all the answers

    What is the primary cause of genetic drift in a population?

    <p>Random sampling of alleles during reproduction. (A)</p> Signup and view all the answers

    What is a population bottleneck?

    <p>A sudden decrease in population size. (A)</p> Signup and view all the answers

    What is the most likely outcome of a population bottleneck on genetic diversity?

    <p>Decreased genetic diversity. (D)</p> Signup and view all the answers

    What happens when a mutation arises in a population?

    <p>It is always present in the heterozygous form. (D)</p> Signup and view all the answers

    What happens to the frequency of a rare allele under Hardy-Weinberg equilibrium?

    <p>It remains stable over time. (C)</p> Signup and view all the answers

    The founder effect is a particular form of genetic drift that occurs:

    <p>When a new population is established by a small number of individuals. (D)</p> Signup and view all the answers

    What is the interpretation of the 'solid lines' in the figure related to the human genome project?

    <p>They represent the frequency of each genotype calculated using a regression. (C)</p> Signup and view all the answers

    Which of the following is an example of a founder effect?

    <p>The high frequency of certain genetic disorders in the Amish population. (C)</p> Signup and view all the answers

    Which of the following would be LEAST likely to occur as a result of genetic drift?

    <p>Evolution of an adaptation to a new environment. (C)</p> Signup and view all the answers

    What is the significance of Hardy-Weinberg Equilibrium (HWE)?

    <p>It provides a baseline to compare real populations and assess evolutionary forces. (B)</p> Signup and view all the answers

    Which of the following statements accurately describes how genetic drift affects small populations?

    <p>Genetic drift is more pronounced in small populations due to random fluctuations. (D)</p> Signup and view all the answers

    What is the relationship between the frequency of an allele and its probability of being fixed?

    <p>They are directly proportional. (C)</p> Signup and view all the answers

    What is the maximum value of expected heterozygosity (He)?

    <p>0.5 (B)</p> Signup and view all the answers

    Why does the amount of heterozygosity tend to be maximized when allele frequencies are intermediate?

    <p>Because there is more genetic variation in the population. (D)</p> Signup and view all the answers

    What happens to the frequency of genotypes in a population after one generation of random mating?

    <p>They converge towards the expected frequencies based on Hardy-Weinberg equilibrium. (A)</p> Signup and view all the answers

    What is the probability of an allele being lost in a population bottleneck? (Hint: use the formula provided)

    <p>1 - p (D)</p> Signup and view all the answers

    What is the effect of genetic drift on genetic variation over time?

    <p>Genetic drift decreases genetic variation (B)</p> Signup and view all the answers

    In the context of genetic drift, what is meant by the term "fixation"?

    <p>The process of an allele becoming the only allele present in a population (D)</p> Signup and view all the answers

    How are genetic drift and evolution related?

    <p>Genetic drift is one of the mechanisms for evolution (A)</p> Signup and view all the answers

    Why is genetic drift more pronounced in smaller populations?

    <p>Random events have a larger impact on smaller populations (C)</p> Signup and view all the answers

    What is the relationship between allele frequency and the likelihood of an allele being lost in a bottleneck?

    <p>Lower frequency alleles are more likely to be lost in a bottleneck (D)</p> Signup and view all the answers

    What is the main reason why genetic drift is considered a 'random walk'?

    <p>The direction of change in allele frequency cannot be predicted (B)</p> Signup and view all the answers

    Why is the founder effect considered a specific example of genetic drift?

    <p>The founder effect involves a small population migrating (B)</p> Signup and view all the answers

    What is the initial frequency of the bw75 allele in each of the populations?

    <p>0.5 (B)</p> Signup and view all the answers

    What was the sample size (N) maintained in the experiment?

    <p>16 (D)</p> Signup and view all the answers

    How many populations had the bw75 allele fixed after 19 generations?

    <p>28 (C)</p> Signup and view all the answers

    What is the term used to describe the reduction in heterozygosity in a population over time?

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

    What is the relationship between the rate of reduction in heterozygosity and the population size?

    <p>Reduction in heterozygosity occurs more slowly in larger populations. (D)</p> Signup and view all the answers

    Which of the following is NOT a main feature of genetic drift?

    <p>Adaptive evolution driven by environmental pressures (B)</p> Signup and view all the answers

    What is the primary reason for the loss of genetic variation within populations due to genetic drift?

    <p>Random sampling of alleles during reproduction. (A)</p> Signup and view all the answers

    Which of the following is an example of genetic drift?

    <p>A population of fruit flies is accidentally transported to a new island, leading to a founder effect. (A)</p> Signup and view all the answers

    Flashcards

    H-W Assumptions

    A set of conditions for a population in genetic equilibrium.

    Genetic Drift

    Random changes in allele frequencies due to sampling error in finite populations.

    Population Bottleneck

    A sharp reduction in population size leading to decreased genetic diversity.

    Founder Effect

    Genetic variation that occurs when a small group colonizes a new area.

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    Sampling Error

    Random deviations in allele frequencies due to finite samples.

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    Non-representative Alleles

    Alleles in a population that do not reflect the source population's diversity.

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    Porphyria Variegate

    A dominant disorder related to heme synthesis, common in Afrikaners.

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    Random Mating

    Mating without regard to genetic traits, a H-W assumption.

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    Allele frequency

    The proportion of a particular allele among all allele copies in a population.

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    Heterozygosity

    The measure of genetic variation representing the presence of different alleles at a locus.

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    Effect of small populations

    In small populations, allele frequencies can drift dramatically, leading to fixation or loss.

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    Heterozygosity and allele frequency

    Heterozygosity decreases as allele frequency approaches 0 or 1.

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    Drift and variation

    Genetic drift reduces variation even in large populations over time.

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    Founder mutation

    A genetic alteration observed in a population that originates from a small number of ancestors.

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    Bottleneck effect

    A sharp reduction in population size that can lead to a loss of genetic diversity.

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    Fixation of an allele

    When an allele's frequency becomes 100%, meaning all individuals carry that allele, and alternatives are lost.

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    Loss of genetic variation

    When genetic diversity decreases, often due to drift or bottlenecks, leading to reduced adaptability.

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    Random walk (in genetics)

    A metaphor describing how allele frequencies can change randomly, without a set direction.

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    Small population effect on drift

    Small populations experience more significant changes in allele frequency due to genetic drift.

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    Hardy-Weinberg Principle

    States that allele and genotype frequencies in a population remain constant in the absence of evolutionary influences.

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    Genotype Frequency

    The proportion of a particular genotype among all genotypes in a population.

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    Expected Genotype Frequencies

    The predicted proportions of genotypes based on allele frequencies using Hardy-Weinberg equation.

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    Heterozygosity (He)

    A measure of genetic variation calculated as He = 2pq for a locus with two alleles.

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    Mutation and Heterozygosity

    New mutations are typically rare and usually found in a heterozygous state.

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    HWE in Nature

    Hardy-Weinberg equilibrium can be achieved even with some evolutionary influences, due to random mating.

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    Maximum Heterozygosity

    The highest expected heterozygosity (He) occurs when allele frequencies are equal (p=q=0.5).

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    Allele Fixation

    When an allele's frequency reaches 100% in a population.

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    Impact of Population Size

    Smaller populations experience quicker loss of genetic variation.

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    Rare Alleles

    Alleles that occur infrequently in a population.

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    Common Alleles

    Alleles that are frequently found in a population.

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    Genetic Divergence

    Increased genetic differences between populations over time.

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    Study Notes

    Evolutionary Genetics: Lecture 6 - Finite Populations and Drift

    • Hardy-Weinberg Assumptions: The Hardy-Weinberg equilibrium describes a theoretical population where allele and genotype frequencies remain constant across generations. It assumes:

      • No selection: All individuals have equal survival and reproduction probabilities.
      • No mutation: Genes do not change from one allele to another.
      • No migration: No genes are added from outside the population.
      • No chance events: Infinite population size (no random sampling effects).
      • Random mating: Mating is random.

    • Finite Populations and Chance Events: Real populations are finite in size, and chance events can significantly alter allele frequencies, disrupting the Hardy-Weinberg equilibrium. This is called genetic drift.

    • Genetic Drift: Random fluctuations in allele frequencies due to chance events in small populations. Allele frequencies in finite populations will not remain constant in proportions, as in infinitely large populations. Drifting can lead to the fixation or loss of certain alleles.

    • Genetic Drift Results from Random Sampling: Random sampling of gametes from a gene pool during reproduction can result in allele frequencies differing from the initial proportions.

      • The larger the sample size (size of the population), the smaller the effect of random variation in the population's allele frequencies.

    • Population Bottlenecks: A sharp reduction in population size that dramatically affects allele frequencies. A bottleneck occurs when a significant portion of a population is lost, and the surviving individuals do not represent the original population's genetic diversity.

      • Example: Northern elephant seals, decimated by hunting in the 1980's.

    • Founder Effect: A type of genetic bottleneck that occurs when a small group of individuals found a new, isolated population. The allele frequencies of the new population may be quite different from the original population from which they emigrated.

    • Founder Effect and Long-Term Effects: Founder effects can lead to a higher frequency of certain genetic diseases in relatively new populations.

      • Example: Afrikaners in South Africa have a higher frequency of porphyria variegate.

    • Population Size and Genetic Drift: Small populations experience more pronounced genetic drift than large populations due to random sampling of alleles. Large populations are less susceptible to random loss of alleles.

    • The Relationship Between Population Size and Genetic Drift: Genetic drift has a stronger impact on small populations. This change is not predicable in small populations and more likely to be random.

    • Rare Alleles and Bottlenecks: Rare alleles are more prone to loss during bottlenecks compared to common alleles. The probability of an allele being lost from a population during a bottleneck is inversely related to population size.

    • Genetic Drift and Evolution: Genetic drift causes changes in allele frequencies and is a mechanism of evolution. It's important to note that drift does cause evolution but is not adaptive (unlike natural selection). It can lead to:

      • Loss of genetic variation, as alleles are lost
      • Greater divergence between isolated populations.

    • Heterozygosity as a Measure of Genetic Variation: Heterozygosity (the probability that an individual carries two different alleles at a given locus) measures genetic variation in a population. Drift tends to reduce heterozygosity. This is because drift will cause populations to lose genetic variation.

    • Heterozygosity and Allele Frequencies: The maximum heterozygosity occurs when the allele frequencies are 0.5 because drift tends to reduce heterozygosity.

    • Relationship between Genetic Drift and Hardy-Weinberg Equilibrium: Genetic drift disrupts the Hardy-Weinberg equilibrium.

    • Measuring Genetic Drift: The experiment conducted by Buri in 1965 showed the effect of genetic drift in Drosophila melanogaster, demonstrating that genetic drift is a powerful mechanism that results in the loss and eventual fixation or loss of specific alleles.

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    Lecture 6: Finite Populations, Drift PDF

    Description

    Explore the concepts of the Hardy-Weinberg equilibrium and genetic drift in finite populations. This quiz delves into the assumptions of the Hardy-Weinberg model and how chance events can significantly impact allele frequencies. Test your understanding of these fundamental principles in evolutionary genetics.

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