Hardy-Weinberg & Genetic Drift

Introduction to Evolution of Populations

  • Key Questions:

    • Ability to calculate allele frequencies from genotype frequencies and vice versa.

    • Interpretation of results from comparisons of observed frequencies with Hardy-Weinberg equilibrium.

    • Definition and explanation of genetic drift.

    • Effects of genetic drift on genetic diversity in small populations.

Identifying Evolution

  • How to Determine If a Population is Evolving:

    • Use the Hardy-Weinberg principle to understand that allele and genotype frequencies remain constant if only Mendelian segregation and recombination occur.

    • Understanding that when genetic recombination is random, the population can be said to be in Hardy-Weinberg equilibrium.

Conditions for Hardy-Weinberg Equilibrium

  • Population in HW Equilibrium is not evolving; must meet specific conditions:

    • No mutation occurs.

    • Random mating is observed.

    • No natural selection is present.

    • No gene flow enters or exits the population.

    • The population must be very large to minimize random fluctuations.

Understanding Proportions of Alleles

  • Pricing in Alleles:

    • Each allele in a population has a specific proportion; a fixed allele has a proportion of 100%.

    • In a simple two-allele system, p represents the frequency of one allele, while q represents the frequency of the other.

    • Assuming random gamete combination, the Hardy-Weinberg Equilibrium allows for calculations of homozygote and heterozygote frequencies.

Calculating Frequencies of Genotypes

  • Genotype Frequencies:

    • Frequency of homozygotes: p² (for one allele) and q² (for another), frequency of heterozygotes: 2pq.

    • The frequencies of homozygotes and heterozygotes must always sum to 1 (or 100%).

    • If observed frequencies of genotypes do not match expected frequencies under Hardy-Weinberg Equilibrium, the population is evolving.

    • Chi-square tests help to determine the deviation from expected frequencies.

Example of Allele Frequencies in Plants

  • Scenario:

    • A plant has two alleles for flower color (focusing on dominant "R" for red and recessive "W" for white).

    • If, in a given population 70% are "R", then 30% are "W". Thus, p = 0.70 and q = 0.30.

Analyzing Next Generation Genotypes

  • Next Generation Analysis:

    • Example with 40 red, 8 pink, and 2 white flowers to assess if evolution is occurring.

    • Understanding that pink flowers are heterozygous resulting from incomplete dominance between red and white alleles.

Heterozygosity in Incomplete Dominance

  • Incomplete Dominance:

    • Pink flowers (intermediate phenotype) result from heterozygous genotypes (RW).

Frequency Analysis of Genotypes

  • Actual Frequencies:

    • Calculate frequencies: F(RR) = 40/50 = 0.80; F(WW) = 2/50 = 0.04; F(RW) = 8/50 = 0.16.

    • Compare to equilibrium frequencies (expected) of 0.49, 0.09, and 0.42.

    • If real frequencies are significantly different, it indicates that the population is evolving.

    • The Hardy-Weinberg equation indicates changes but does not specify which condition is violated.

Factors Influencing Evolution

  • Factors Impacting Evolutionary Processes:

    • Mutations, though rare, can have large impacts when adaptations enhance survival and reproductive success.

    • Non-random mating changes phenotype and genotype frequencies but not significantly allele frequencies.

    • Major influences include:

      • Natural selection

      • Genetic drift (notably in small populations)

      • Gene flow.

Understanding Genetic Drift

  • Genetic Drift Defined:

    • Describes random fluctuations in allele frequencies.

    • In large populations, these fluctuations tend to balance out, whereas in small populations significant effects can be deleterious.

Types of Genetic Drift

  • Founder Effect:

    • Occurs when a new population is established by a very small number of individuals.

    • Example: The Amish population in Pennsylvania that exhibits a specific genetic syndrome due to founder effects.

Bottleneck Effect

  • Bottleneck:

    • Occurs when a population undergoes a drastic size reduction for at least one generation.

    • Example: Northern elephant seals and cheetahs faced severe population declines due to environmental events, losing allele diversity.

Inbreeding and Its Effects

  • Consequences of Inbreeding:

    • Inbreeding is mating between relatives, increasing the expression of deleterious recessive alleles (inbreeding depression).

    • Results in decreased fecundity, increased mortality, and above all, increased disease susceptibility.

    • Example: Woolly mammoths on Wrangel Island suffered from a "genomic meltdown" due to loss of genetic variation, impacting vital genes for reproduction.

Long-Term Impacts of Genetic Drift

  • Recovery and Genetic Variation:

    • Population size can bounce back quickly post-bottleneck or founder events, but regaining genetic variation is slow.

    • Changes in allele frequencies are random; over time can build up due to mutations.

    • Low genetic variation limits populations' ability to adapt to environmental changes and may lead to the fixation of harmful alleles.