Genetics and Natural Selection

Darwin's Contribution to Natural Selection

• 1835: Charles Darwin's visit to the Galapagos Islands begins his observations.

• 1838: Darwin theorizes that certain individuals possess characteristics providing a competitive advantage.

Darwin's Theory of Natural Selection

• Traits are passed from parents to offspring (like begets like).

• There are chance variations among individuals.

• More offspring are produced than can survive.

• Individuals with advantageous traits have a higher chance of survival.

Gregor Mendel's Experiments

• Mendel studied various traits in pea plants:

  • Flower color, flower position, seed color, seed shape, pod shape, pod color, stem length.

• Example data:

  • Parental generation: YY (yellow) and GG (green) alleles.

  • F1 generation consists of all yellow axial flowers (YG).

  • F2 generation shows variability in traits suggesting dominant and recessive alleles.

Variation Within Populations

• Experiment on P. glandulosa transplanted at different elevations:

  • Null hypothesis: No genetic differences among populations.

  • Results indicated genetic differences: variations affected growth in gardens (lowland, mid-elevation, and alpine).

Hardy-Weinberg Principle

• States that in a random mating population without evolutionary influences:

  • Allele frequencies remain constant.

  • Equation: p² + 2pq + q² = 1.0

Calculating Gene Frequencies

• Example breakdown of allele frequencies:

  • SS (homozygous dominant, 81%), SA (heterozygous, 18%), AA (homozygous recessive, 1%).

  • Frequency of S allele: SS + 1/2 SA = 0.90.

  • Confirmation using gene frequency equation leads to a total of 1.0.

Conditions for Hardy-Weinberg Equilibrium

• Required conditions include:

  • Random mating.

  • No mutations.

  • Large population size.

  • No immigration/emigration.

  • Equal reproductive success across genotypes.

The Process of Natural Selection

• Natural selection affects population fitness positively or negatively:

  • Mutation creates variation; unfavorable mutations are selected against.

  • Types of selection include:

    • Stabilizing Selection: favors average phenotypes; reduces extremes.

    • Directional Selection: favors one extreme phenotype.

    • Disruptive Selection: favors both extreme phenotypes, leading to increased diversity.

Stabilizing Selection in Ural Owls

• Egg size selection in Ural owls shows stabilizing selection:

  • Higher hatching rates for eggs around the average size of 42.45 cm³.

  • Extreme egg sizes (both smaller and larger) correlate with lower hatching success.

Directional Selection in Soapberry Bugs

• Observed adaptation of soapberry bugs with longer beaks on larger fruit species.

• This illustrates rapid adaptation to environmental changes.

Genetic Drift and Chance Effects

• Genetic drift causes changes in allele frequencies in small populations due to chance events.

  • Larger populations tend to have higher genetic diversity compared to smaller populations.

Genetic Variation in Island Populations

• Generally, island populations show lower genetic variation compared to mainland populations:

  • Genetic diversity on the mainland is typically higher due to larger demographic sizes and varied environments.

Evolution and Agriculture

• Evolutionary principles applied in agriculture lead to domestication:

  • Comparison of wild and domesticated soybean traits.

• GMOs involve specific modifications to a plant's DNA for beneficial traits.

  • These traits often include disease resistance or yield improvements.

Unintended Evolutionary Consequences of Agriculture

• Example of Johnsongrass populations' response to glyphosate herbicide:

  • Populations exposed to glyphosate develop resistance over time, contrasting with unexposed populations.