SELECTION

  • Drift and Selection

  • Changes in allele frequencies over time.

  • Hardy-Weinberg Theorem (HWE)

  • Assumptions of HWE:

    • Infinite population size.
    • No migration (immigration/emigration).
    • No mutation.
    • No selection.

  • Genotype frequency representation:

    • A1A1 = p²
    • A1A2 = 2pq
    • A2A2 = q²

  • Example: HgbS gene:

    • Heterozygous (AS) allele = malaria resistance.
    • Homozygous (SS) allele = sickle cell disease.

  • Natural Selection

  • Selection affects the phenotype, which is linked to genotype, leading to changes in allele frequencies.

  • Under no selection (HWE), all genotypes have equal chances of passing alleles to the next generation.

  • Under selection, certain genotypes have different chances based on fitness.

  • Fitness

  • Definition: An organism's success in surviving and reproducing enough to contribute offspring to future generations; central to understanding selection.

  • Complicating factors: Fitness is based on overall phenotype, which influences selection.

  • Relative fitness (w):

    • w = 1: No selection, standard HWE.
    • w > 1: Selection favors genotype.
    • w < 1: Selection against genotype.

  • Impact on allele frequencies: Under selection, allele frequencies depend on average fitness and relative fitness of genotypes.

  • Allele Frequency Changes Under Selection

  • The process involves average fitness calculations and adjustments in genotype frequencies.

  • The average excess of fitness informs how allele frequencies change after selection.

  • The strength of selection influences the rate of frequency changes.

  • Drift vs. Selection

  • In small populations, genetic drift often dominates; in larger populations with moderate frequencies, natural selection plays a significant role.

  • Selection can hinder harmful alleles in small populations due to recessive traits being masked.

  • Inbreeding

  • Leads to an increased prevalence of rare genetic disorders, termed inbreeding depression.

  • Example: Isle Royale moose and wolves.

  • Types of Selection

  • Negative Frequency Dependent Selection: Fitness varies with allele frequency—for example, floral color in elderflower orchids influences pollination success based on rarity.

  • Balancing Selection: Maintains genetic diversity. In malaria-prevalent areas, heterozygous (AS) is often the most fit genotype, supporting the survival of diverse alleles.

  • Conclusion

  • Evolution is defined as a change in allele frequencies through time. Both drift and selection shape these changes.

  • The Hardy-Weinberg framework aids in predicting evolutionary dynamics of Mendelian traits.