Molecular Evolution
Molecular evolution studies how DNA, RNA, and protein sequences change over evolutionary time, and the relative roles of mutation, drift, and selection in shaping them.
Definition
Molecular evolution is the study of the rates, patterns, and processes of change in biological macromolecules, principally nucleic acid and protein sequences. It seeks to explain observed sequence variation and divergence in terms of mutation, genetic drift, and natural selection.
Scope
This area covers the processes governing change in molecular sequences: the neutral and nearly neutral theories that frame much molecular change as drift, the detection of adaptive selection in genes and proteins, the rates and patterns of substitution, and the evolution of whole genomes including their size, structure, and content.
Sub-topics
Core questions
- What fraction of molecular change is driven by drift versus selection?
- How do substitution rates vary among sites, genes, and lineages?
- How can adaptive evolution be detected in molecular sequences?
- How do genome size, structure, and gene content evolve?
Key theories
- Neutral theory of molecular evolution
- Most molecular substitutions and much standing variation are selectively neutral and governed by drift, making the neutral model the essential null hypothesis for molecular evolution.
- Nearly neutral theory
- Slightly deleterious mutations behave as effectively neutral or are removed depending on effective population size, linking substitution rates to demography and refining the neutral framework.
Mechanisms
Molecular evolution proceeds by the fixation of mutations over generations. Neutral mutations fix at a rate equal to the mutation rate, independent of population size, while deleterious mutations are usually removed and beneficial ones occasionally swept to fixation by selection. Substitution rates vary with functional constraint, so synonymous sites and pseudogenes evolve faster than constrained coding sites. Comparisons of synonymous and nonsynonymous substitution rates, and contrasts of polymorphism with divergence, allow inference of selection. At the genome scale, processes such as gene duplication, transposable-element activity, and rearrangement shape size, structure, and gene content.
Clinical relevance
Molecular-evolutionary analysis identifies functionally important and rapidly evolving regions in pathogens and host immune genes, guides the interpretation of conservation in disease-associated sequences, and tracks the evolution of resistance and virulence.
History
Molecular evolution emerged in the 1960s with protein and then DNA sequence comparisons, the molecular clock, and Kimura's neutral theory, which sparked the neutralist-selectionist debate. Ohta's nearly neutral theory refined it, and from the genomic era the field expanded to whole-genome and population-scale analyses of selection.
Debates
- Neutralist versus selectionist views
- How much molecular variation and divergence reflects neutral drift versus positive and balancing selection has been the central debate of the field since the 1960s and is now tested with genome-scale data.
Key figures
- Motoo Kimura
- Tomoko Ohta
- Masatoshi Nei
- Carl Woese
Related topics
Seminal works
- kimura1983
- ohta1973
- saetreRavinet2019
Frequently asked questions
- Does the neutral theory deny natural selection?
- No. The neutral theory accepts that selection drives adaptation, but argues that the majority of molecular substitutions and polymorphisms are selectively neutral and governed by genetic drift.
- Why do some parts of the genome evolve faster than others?
- Because they are under weaker functional constraint; sites where changes have little or no effect on fitness, such as synonymous positions and pseudogenes, accumulate substitutions faster than functionally important sites.