8-Genetics and the Evolution of phenotypes

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.