Neutral theory challenges the idea that natural selection drives all evolutionary changes. It proposes that most molecular changes are selectively neutral, with genetic drift as the primary force. This concept revolutionized our understanding of molecular evolution.

Evidence for neutral theory includes constant rates of amino acid substitutions across species and high levels of polymorphism in natural populations. It contrasts with the selectionist view, which emphasizes natural selection as the main driver of molecular evolution.

Neutral Theory Foundations

Principles of neutral theory

• Motoo Kimura proposed in 1968 challenged prevailing view of natural selection driving all evolutionary changes
• Most molecular changes selectively neutral mutations have no effect on fitness neither advantageous nor deleterious
• Genetic drift primary force driving molecular evolution random changes in allele frequencies over time
• Mutation-drift equilibrium balances new mutations and loss of existing variants
• Molecular clock hypothesis posits relatively constant rate of molecular evolution over time
• Effective population size influences fixation rate smaller populations experience faster genetic drift

Genetic drift in neutral theory

• Random sampling of alleles between generations leads to changes in allele frequencies
• Fixation of neutral mutations process variant becomes only allele in population
• Loss of genetic variation drift reduces diversity over time
• Founder effects and population bottlenecks enhance impact of drift on molecular variation (island colonization)
• Stronger drift effect in smaller populations (endangered species)
• Interaction with mutation rate balances loss of variation due to drift

Evidence and Comparisons

Evidence for neutral theory

• Constant rate of amino acid substitutions across species (hemoglobin)
• High levels of polymorphism in natural populations (fruit flies)
• Correlation between evolutionary rate and generation time (mice vs elephants)
• Synonymous mutations occur more frequently than nonsynonymous ones
• Molecular evolution studies in various organisms (Drosophila, humans)
• DNA sequencing advances provide comprehensive data to test theory

Neutral theory vs selectionist view

• Neutral theory:
  • Most molecular changes due to genetic drift
  • Selection plays minor role in molecular evolution
  • Emphasizes importance of random processes
  • Predicts higher levels of polymorphism
  • Relatively constant rate of evolution
• Selectionist view:
  • Natural selection primary driver of molecular evolution
  • Adaptive changes common at molecular level
  • Emphasizes role of environmental pressures
  • Predicts lower levels of polymorphism due to selective sweeps
  • Variable rate of evolution depending on selective pressures
• Nearly neutral theory compromise incorporates slightly deleterious mutations
• Implications for speciation and adaptation:
  • Neutral theory emphasizes non-adaptive processes
  • Selectionist view focuses on adaptive divergence