Population genetics

Introduction

• The evolution of populations is a key area of study in understanding biodiversity and adaptation.

Microevolution

• Definition: Microevolution is defined as a change in allele frequencies in a population over generations.

• Key Questions: What factors cause microevolution?

Genetic Variation

• Key Concepts:

  • Individual genotypes lead to variations in phenotypes.

  • Questions on heritability:

  • Is all phenotypic variation heritable?

  • Can natural selection act on non-genetic variation?

Types of Variation Within a Population

• Discrete Characters: Traits categorized as "either-or" (e.g., blood type).

• Quantitative Characters: Traits that vary along a continuum (e.g., height, beak depth).

  • Mendelian genetics focused on discrete traits.

  • Darwin observed continuous variation in traits such as beak depth in finches.

Quantifying Genetic Variation

• Gene Variability: Average percentage of loci that are heterozygous in a population.

  • Example: Drosophila species are heterozygous for approximately 1,920 out of 13,700 loci.

• Nucleotide Variability:

  • Often doesn't lead to phenotypic differences:

  • Introns: Silent mutations that do not affect coding sequences.

  • Types of Mutations: Include base-pair substitutions, insertions, deletions, etc.

Sources of Genetic Variation

• New Alleles: Most mutations can be harmful; however, some persist in heterozygotes.

• Neutral Variation: Not all amino acid changes are detrimental.

• Heritability: Only mutations in the germline are heritable.

• Gene Duplication: Important for genome expansion.

• Sexual Reproduction: Enhances genetic variation through processes like crossing over, independent assortment, and random fertilization.

Hardy-Weinberg Principle

• Use: Helps to test if a population is evolving.

• Definition: A population that does not evolve meets the Hardy-Weinberg conditions.

Defining a Population

• Definition: A population is a localized group of individuals capable of interbreeding and producing fertile offspring.

• Gene Pool: All alleles for all loci within a population.

  • A locus is termed fixed if all individuals are homozygous for the same allele.

Allelic Frequencies

• Each allele has a specific frequency within the population.

• For diploid organisms, the total number of alleles at a locus is double the number of individuals.

Hardy-Weinberg Equilibrium

• Equation: If there are two alleles, let p and q denote their frequencies; then $p + q = 1$.

• Expression of Equilibrium: Frequencies of alleles and genotypes remain constant under specific conditions:
  $p^2 + 2pq + q^2 = 1$.

Example Application of Hardy-Weinberg

• Phenylthiocarbamide (PTC) tasting in a class: 100 students; genotypes break down as follows:

  • 50 students (TT), 20 students (Tt), 30 students (tt).

• Frequency of Recessive Allele Calculation:

  1. Frequency of tt (recessive phenotype); denote as $q^2 = 0.30$ yielding $q = 0.60$.

  2. The additional calculations for the frequency of dominant allele (p) and heterozygotes (2pq).

Conditions for Hardy-Weinberg Equilibrium

• These ideal conditions rarely occur in nature and include:

  • No mutations.

  • Random mating.

  • No natural selection.

  • Infinitely large population size.

  • No gene flow.

Natural Selection and Evolution

• Natural Selection: The mechanism by which certain alleles inherit advantages leading to increased reproductive success.

Genetic Drift

• Definition: Unpredictable changes in allele frequencies across generations.

  • Tends to reduce genetic variation and can lead to loss of alleles.

Effects of Genetic Drift

• Founder Effect: A few individuals isolate from a larger population, resulting in different allele frequencies.

• Bottleneck Effect: Sudden sharp reduction in population size, impacting the gene pool.

Gene Flow

• Definition: Movement of alleles between populations (fertile individuals or gametes).

  • May reduce genetic differences or introduce alleles that decrease fitness.

Natural Selection as a Driving Mechanism

• Adaptive Evolution: Enhances organism-environment match, often increasing allele frequencies that confer survival advantages.

• Relative Fitness: Contribution made by an individual to the next generation's gene pool relative to others.

Modes of Selection

• Directional Selection: Favors one extreme of the phenotypic range.

• Disruptive Selection: Favors both extremes of the phenotypic range.

• Stabilizing Selection: Favors intermediate variants, acting against phenotypic extremes.

Sexual Selection

• Definition: A form of natural selection aimed at reproductive success, resulting in sexual dimorphism.

• Types:

  • Intrasexual Selection: Competition among individuals of one sex for mates.

  • Intersexual Selection: Mate choice based on specific traits.

Preservation of Genetic Variation

• Mechanisms:

  • Balancing selection: Maintains stable frequency of two or more phenotypic forms.

  • Heterozygote advantage: Heterozygotes display higher fitness than homozygotes.

  • Frequency-dependent selection: Phenotypic fitness declines if too common in a population.

Neutral Variation

• Variation that does not confer significant selective advantages or disadvantages, prevalent in noncoding DNA regions.

Limitations of Natural Selection

• Natural Selection can only act on currently existing variations; it cannot create perfect organisms due to constraints and compromises. Adaptations are a result of historical and environmental interactions.