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Population Genetics

Chapter 27

Definition of Population

  • Population: A group of organisms of the same species residing in the same geographic area with the ability to interbreed with one another.

Definition of Population Genetics

  • Population Genetics: The field of biology aimed at understanding the genetic composition of populations and the reasons for changes in this composition over time.
    • Genetic Variation: Arises from the existence of different alleles at various genetic loci.
    • Example: Shell color polymorphism observed in snails, where traits displaying variation within a population are classified as polymorphic traits.

Polymorphism at the Genetic Level

  • A gene is considered polymorphic if two or more alleles exist within a population.
    • The goal of studying polymorphic genes is to determine their prevalence within populations and connect them to phenotypic functions.

Describing Genetic Structure of a Population

  • Key components to describe include:
    • Phenotype Frequencies: The proportion of different phenotypes in a population.
    • Genotype Frequencies: The proportion of different genotypes in a population.
    • Allele Frequencies: The frequency of specific alleles in the gene pool.

Calculation of Phenotype Frequencies

  • For a hypothetical population of 1000 flowers:
    • White flowers: 200/1000 = 0.2
    • Pink flowers: 500/1000 = 0.5
    • Red flowers: 300/1000 = 0.3

Calculation of Genotype Frequencies

  • Genotype frequencies can be calculated as:
    • For genotype rr: 200/1000 = 0.2
    • For genotype Rr: 500/1000 = 0.5
    • For genotype RR: 300/1000 = 0.3

Calculation of Allele Frequencies

  • Definition: The frequency of a specific allele in a population is calculated by the number of copies of that allele divided by the total number of alleles of the gene present.
  • Example with 2000 alleles total:
    • Frequency of allele r: 900/2000 = 0.45
    • Frequency of allele R: 1100/2000 = 0.55

Relationship of Allele Frequencies

  • The sum of frequencies for all alleles at a polymorphic gene must equal 1.
    • Example: For two alleles r and R, where frequency of R is 0.55:
    • Frequency of r: 1 - frequency of R = 1 - 0.55 = 0.45.

Hardy-Weinberg Equilibrium

  • Definition: A principle predicting that allele and genotype frequencies will remain constant over generations in a population under specific conditions.
    • Assumed conditions:
    • No new mutations.
    • No genetic drift (sampling bias).
    • No migration.
    • No natural selection.
    • Random mating.
    • Relationship of allele and genotype frequencies is given by:
      ext{Frequency of } R^2 + 2Rr + r^2 = 1
  • Deriving the H-W equation involves:
    • R + r = 1 (allele frequencies in the population).
    • Each individual has two allele copies, leading to:
    • (R + r)(R + r) = 1
    • Written as (p + q)(p + q) = 1 where p = frequency of allele R and q = frequency of allele r.

Class Question on Allele Frequencies

  • The sum of all allele frequencies at any polymorphic gene equals:
    • a) Two
    • b) One (Correct Answer)
    • c) The number of alleles at that particular gene
    • d) No clue

Hardy-Weinberg Equation Revisited

  • R^2 + 2Rr + r^2 = 1
    • Represents genotype frequencies:
    • R^2 : frequency of homozygous dominant genotype (RR)
    • 2Rr : frequency of heterozygous genotype (Rr)
    • r^2 : frequency of homozygous recessive genotype (rr).
    • Alternatively expressed using p and q : p^2 + 2pq + q^2 = 1 .

Example Calculations under H-W Equilibrium

  • Given:
    • Allele frequency of r = 0.45
    • If population in Hardy-Weinberg equilibrium:
    • Frequency of genotype rr: 0.45^2 = 0.2025 .

Visualization using Punnett Squares

  • Punnett squares are tools that visualize relationships between allele frequencies and genotype frequencies.

Detailed Frequency Calculations

  • For a population with an allele frequency where R = 0.8 and r = 0.2:
    • R^2 = (0.8)(0.8) = 0.64 (RR genotype)
    • 2pq = 2(0.8)(0.2) = 0.32 (Rr genotype)
    • r^2 = (0.2)(0.2) = 0.04 (rr genotype).

Importance of H-W Equilibrium Assessments

  • H-W equilibrium states allele and genotype frequencies do not change due to the following:
    • No new mutations.
    • No genetic drift.
    • No migration between populations.
    • No natural selection.
    • Random mating.
  • Challenge: Evaluate whether a population is in H-W equilibrium using the equation.

Observational Data vs. Expected Frequencies

  • Calculated expected genotype frequencies based on allele frequencies:
    • RR = R^2 = (0.55)^2 = 0.3025
    • rr = r^2 = (0.45)^2 = 0.2025
    • Rr = 2Rr = 2(0.55)(0.45) = 0.495 (summary of all genotypes represented in the population).

Chi-Square Test for H-W Equilibrium Verification

  • Chi-square can assess deviations from expected frequencies to determine if the population is in H-W equilibrium.
  • Degrees of Freedom: Calculated as #genotypes - #alleles.
  • If the null hypothesis is failed to be rejected, the population is considered to be in HWE.

Role of Genetic Variation in Space and Time

  • The potential for change in genetic structure is critical for:
    • Adaptation to environmental changes.
    • Conservation efforts.
    • Maintenance of biodiversity.
    • Its relation to disease phenotypes.

Significance of Genetic Variation

  • Genetic variation impacts:
    • Disease susceptibility
    • Survival capacity
    • Lack of variation leads to possible extinction.

Factors Causing Changes in Genetic Structure

  • Variations can occur through:
    • Mutation: Random alterations in DNA forming new alleles. An ultimate source of genetic diversity.
    • Migration: Genetic exchange across populations due to the movement of fertile individuals or gametes, creating genetic flow.
    • Natural Selection: Influences variability due to differences in offspring contribution based on survival and reproduction, adapting populations over time.
    • Genetic Drift: Random alterations in allele frequencies due to chance, especially significant in smaller populations.
    • Non-random Mating: Alterations in genotype distributions arising from mate selection processes based on specific traits.

Mutation

  • A spontaneous change in DNA resulting in new alleles at random.
  • It is the foundation for all genetic variation in populations.

Migration (Gene Flow)

  • The transfer of genetic material between populations due to the movement of breeding individuals or their gametes.
  • Example: Migration impacts are illustrated through distinct populations facing differing selective pressures.

Natural Selection Explained

  • Certain genotypes yield more offspring through enhanced fitness, which modifies allele frequencies over generations, allowing adaptation.
  • Resistance to Antibiotics Case Study: Observes the gradual population shifts in allele frequencies related to treatment over generations.

Breed Patterns Over Generations due to Natural Selection

  • Population studies display shifting allele frequencies over successive generations due to consistent selective pressures.

Genetic Drift

  • A random process affecting allele frequency changes independent of natural selection.
    • More pronounced in smaller populations leading to either fixation (all members possess the allele) or loss (absence of the allele).

Effects of Bottleneck and Founder Events on Genetic Drift

  • Bottleneck Effect: A drastic reduction in population size resulting in a loss of genetic diversity.
  • Founder Effect: Occurs when a small number of individuals start a new population, leading to variations influenced by the limited genetic pool.

Simulation of Genetic Drift Impact

  • Data indicates fluctuations in allele frequency are more extreme in smaller populations compared to larger groups where frequencies stabilize.

Demographic Stochasticity's Role

  • Refers to unpredictable variations in growth rates across generations due to individual-level survival and reproduction discrepancies.
  • Highlights factors affecting demographic outcomes, even within genetically uniform populations, culminating significant effects in small populations.

Sources of Demographic Stochasticity

  • Variability in fecundity and mate acquisition.
  • Environmental factors impacting survival outcomes.

Interaction of Evolutionary Forces

  • Mutation adds genetic variation, while genetic drift can diminish it. Natural selection might retain or diminish variation, and migration can augment genetic diversity.
  • The overall rate of allele frequency changes is contingent upon factors such as population size, natural selection strength, migration rates, and mutation frequency.