- A All molecular mutations observed between species are sometimes thought to be strongly positively selected by natural selection, with genetic drift playing little meaningful role in fixing sequence differences
- B Neutral theory (Kimura, 1968): most molecular evolutionary changes are selectively neutral, fixed by genetic drift not natural selection; protein and DNA evolve at relatively constant rates (molecular clock)
- C All molecular mutations observed between species are sometimes thought to be uniformly harmful and actively selected against, a description that more closely matches a purifying-selection model than neutral theory
- D Neutral theory is sometimes thought to entirely reject genetic drift as an evolutionary mechanism, when Kimura's theory instead places genetic drift at the center of explaining molecular sequence evolution
Correct answer: B. Neutral theory (Kimura, 1968): most molecular evolutionary changes are selectively neutral, fixed by genetic drift not natural selection; protein and DNA evolve at relatively constant rates (molecular clock)
Explanation: Neutral theory (Motoo Kimura): most amino acid substitutions and DNA sequence changes are neutral (same fitness), fixed by drift. Predicts: (1) Molecular evolution rate is constant (molecular clock); (2) Synonymous (silent) substitutions > non-synonymous; (3) Less functional regions evolve faster. Not all neutral - positive selection shapes adaptive changes; purifying selection removes deleterious mutations. Modern view: nearly neutral theory (slightly deleterious mutations in small populations behave as neutral).
Concept context
Origin of life, Darwin's theory, natural selection, speciation, and evidence for evolution.