Evolution of Phenotypic Traits

• Phenotypic Variation: Differences in observable (phenotypic) traits within a population.

• Examples of traits: Different breeds of dogs:

  • Bloodhound

  • French Bulldog

  • Boston Terrier

  • Chow Chow

Natural Selection and Real-Time Evolution

• Case Study: Biston betularia (Peppered Moth):

  • Melanic Form: Discovered in 1848, related to industrial pollution.

  • Mutation around 1819 led to dark variants.

  • By late 1800s, melanic moths nearly replaced the typical coloration due to natural selection.

• Graph Overview: Moth frequency correlating with smoke pollution levels from 1960 to 1980.

Ecological Influences on Phenotype

• Oldfield Mice (Peromyscus polionotus):

  • Color Variation:

    • Beach habitats favor white mice; inland favors brown mice.

    • Populations mostly isolated and color reflects their habitat.

    • Color is genetically influenced by several alleles.

Case Study: Soapberry Bugs

• Variation based on food sources:

  • Differences in host plants (native vs. introduced).

  • Beak Length Variation:

    • Native: Average beak length = 6.68 mm

    • Introduced: Average beak length = 9.32 mm

  • Significance: Adaptations to different pod radii of host plants.

Components of Phenotypic Variation

• Types of Variance:

  • Phenotypic variance: Total variance due to both genetic and environmental differences.

  • Genetic variance (Vg): Variance attributed to genetic differences.

  • Environmental variance (Ve): Variance due to environmental influences.

  • Additive genetic variance: Some genetic differences have a larger impact on phenotype.

Phenotype vs. Genotype

• Definitions:

  • Phenotype: Expressed physical characteristics of an organism.

  • Genotype: Genetic makeup of an individual.

• Polygenic Traits:

  • Traits influenced by multiple genes, leading to continuous variation (e.g., human height, color of eyes).

Understanding Variation Potential

• Factors Influencing Variation:

  • Magnitude of selective pressure and availability of genes determine potential for variation.

Continuous vs. Discontinuous Variation

• Continuous Variation: Traits that show a range (e.g., height).

• Discontinuous Variation: Categorical traits influenced by a single gene (e.g., flower color).

Selection and Variance

• Not all traits are equally variable; heritability affects selection potential.

Heritability in Evolution

• Heritability refers to the proportion of phenotypic variance attributable to genetic variance.

  • Example: Milk production traits in cows based on genetic differences.

Selection Dynamics

• Responses to Selection:

  • Directional Selection: Shifts the frequency of phenotypes in a particular direction (e.g., beak size in finches).

  • Stabilizing Selection: Favors average phenotypes; reduces variance.

  • Disruptive Selection: Favors extreme phenotypes over average.

Maintenance of Variation in Populations

• Variables Influencing Populations:

  • Mutation, gene flow, and balances between selection and variation loss.

Correlated Evolution of Traits

• Changes in one character often correlate with changes in another (e.g., body size and fecundity).

  • Types of Correlation: Phenotypic, environmental, genetic.

Gene Interactions

• Epistasis: Interaction between alleles where the effect of one is dependent on the presence of another.

• Latent Variation: Traits that are hidden and may reappear in future generations based on genotype combinations.

Linkage Equilibrium vs. Disequilibrium

• Equilibrium: Alleles at different loci are independent and assort randomly.

• Disequilibrium: Alleles are non-randomly associated due to linkage, affecting inheritance patterns.

Conclusion

• Understanding gene interactions and selection processes is crucial for grasping evolution and phenotypic diversity.