Genes, Phenotypes, and the Environment

Genotypes, Phenotypes, and Environments

  • Phenotypic variation doesn't always stem from genetic differences.
  • Phenotype = Genotype + Environmental interactions.

Phenotypic Plasticity

  • Definition: Changes in an organism's phenotype due to environmental interactions, despite a fixed genotype.
  • All organisms display some degree of this plasticity.
  • Encompasses all environmentally-induced changes (morphology, physiology, behavior, life history), regardless of permanence.

Reaction Norms

  • Definition: The pattern of phenotypic expression of a genotype across a range of environments.
  • Describes how a single genotype maps to various phenotypes as the environment shifts.
  • Each combination of phenotypic trait, genotype, and environmental variable can result in a different reaction norm.
  • Useful for comparing responses of related species or different genotypes to environmental variation.

Polyphenism

  • Definition: Discrete phenotypes arising from a single genotype.
  • A special case of phenotypic plasticity.
  • An "all-or-nothing" phenotypic change in response to an environmental cue.
  • Examples:
    • Seasonal changes
    • Predation pressure
    • Sex determination
    • Resource availability

Acclimatization

  • Short-term (hours, days, weeks) adjustments within an individual to a new environment.
  • Distinct from adaptations, which occur over many generations.
  • Generally reversible upon return to the original environment.
  • Example: High altitude acclimatization:
    • Lower atmospheric oxygen concentrations.
    • Increased red blood cells and hemoglobin.
    • Increased respiratory rate.
    • Increased heart rate.

Evolvability of Phenotypic Plasticity

  • The specific phenotypes themselves aren't heritable.
  • However, the ability to express different phenotypes in response to the environment can enhance fitness.

Biological Robustness

  • Definition: The stability of biological systems when faced with genetic or environmental changes.
  • Biological systems are modular, redundant, and heterogeneous.
  • Properties increasing robustness:
    • Genetic redundancy
    • Structure of the genetic code
    • Protein structural robustness
    • Gene expression robustness
    • Developmental patterning robustness

Canalization

  • Definition: The ability of an organism to produce the same phenotype despite environmental or genetic disturbances during development.

Canalization and Evolutionary Capacitance

  • Evolutionary capacitance: Canalization may facilitate evolvability by allowing mutations to accumulate without affecting the phenotype (hidden from selection).
  • This accumulated genetic diversity can be revealed through environmental change or molecular switches.
  • C. H. Waddington's epigenetic landscapes explain how cells can have different fates despite sharing a common genome.

Epigenetics

  • Definition: A stably heritable phenotype resulting from changes in a chromosome without alterations in the DNA sequence.
  • The study of heritable traits not linked to changes in DNA sequences.
  • Influenced by environmental or developmental factors affecting gene expression.

Nature vs. Nurture

  • Nature:
    • Genes and hereditary factors.
    • Physical appearance.
    • Personality characteristics.
  • Nurture:
    • Environmental variables.
    • Childhood experiences.
    • How we were raised.
    • Social relationships.
    • Surrounding culture.

Epigenetic Mechanisms

  • Cell fate is determined not only by DNA sequence but also by factors influencing gene expression.
  • Mechanisms:
    • Histone modification
    • DNA (or RNA) methylation
    • RNA-based mechanisms