PPT 13

Hardy-Weinberg Equilibrium

• Gene Pool: The total collection of alleles in a population.

  • Large gene pool: High variability, better survival chances.

  • Small gene pool: Low variability, higher risk of extinction (genetic drift).

Assumptions of Hardy-Weinberg Equilibrium

• Conditions required for a stable gene pool:

  • Large population size.

  • No emigration or immigration.

  • No mutations.

  • Random mating.

  • No natural selection.

• Represents stability; any change usually results in evolution.

Speciation and Allele Frequencies

• Equation: p + q = 1

• Individual scale: Natural Selection

• Population scale: Evolution

• If Hardy-Weinberg equilibrium holds, frequencies of alleles (p and q) remain unchanged.

Genetic Polymorphism

• Definition: Variability within a population.

  • Types:

    • Genetic polymorphism: Variation in alleles (e.g., human height).

    • Balanced polymorphism: Natural selection maintains stable allele frequencies.

      • Example: Sickle cell anemia shows heterozygote advantage.

Modes of Natural Selection

• Selection influenced by environmental pressures, leading to adaptations.

• Darwinian Fitness: Contribution to the next generation's gene pool.

• Types of Natural Selection:

  1. Directional Selection: Favors one extreme phenotype (e.g., Peppered moth during industrial revolution).

  2. Disruptive Selection: Favors extreme phenotypes and selects against intermediates (e.g., Peppered moth in diverse pollution levels).

  3. Stabilizing Selection: Favors intermediate phenotypes (e.g., human birth weight).

Sexual Selection

• Intrasexual Selection: Competition among one sex for mating opportunities.

• Intersexual Selection: One sex (usually females) selects mates based on specific traits.

  • Resulting in sexual dimorphism, where males exhibit showy traits (e.g., bright feathers, larger size).

Speciation Concepts

Morphological Species Concept

• Groups based on similar morphology (anatomy).

• Limitations: Inaccurate and can lead to mistakes (e.g., mimicking species).

Biological Species Concept

• Defines species by their ability to interbreed and produce fertile offspring.

• Emphasizes reproductive isolation and some gene flow within populations.

Types of Speciation

Allopatric Speciation

• Populations separated by geographical barriers leading to divergence.

  • Influenced by genetic drift, mutations, and selection.

Sympatric Speciation

• Occurs in overlapping geographic areas due to:

  • Habitat differences.

  • Host specificity.

  • Chromosomal changes resulting in new species.

Reproductive Barriers

Pre-Zygotic Barriers

• Prevent mating or fertilization:

  • Habitat isolation.

  • Temporal isolation.

  • Behavioral isolation.

  • Mechanical isolation.

Post-Zygotic Barriers

• Occurs after fertilization:

  • Reduced hybrid viability.

  • Reduced hybrid fertility.

  • Hybrid breakdown (F2 generation may be feeble or sterile).

Gradualism vs Punctuated Equilibrium

• Gradualism: Slow evolutionary changes over time.

• Punctuated Equilibrium: Long periods of stability interrupted by shorter bursts of evolutionary change, often due to environmental shifts.

Systematics and Taxonomy

• Phylogenies: Study of evolutionary relationships among organisms.

  • Use of classification (taxonomy) to organize species.

• Hierarchy of classification includes: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

• Binomial nomenclature: Two-part scientific names (e.g., Leopard = Panthera pardus).

Common Ancestry and Cladistics

• Divergent evolution leads to homologous traits; Convergent evolution results in analogous traits.

• Cladistics: Groups nested inside each other based on shared derived traits (e.g., mammals distinguished by hair).

• Branch lengths in phylogenetic trees indicate genetic change and time since divergence.