Mutation, Selection, and Genetic Drift
Allele frequencies change over generations through a small set of forces: mutation supplies new variants, natural selection sorts them by fitness, migration moves them between populations, and random drift shuffles them by chance.
Definition
Mutation, selection, and genetic drift are the principal forces of microevolution, respectively introducing, sorting, and randomly altering allele frequencies in a population from one generation to the next.
Scope
This topic covers mutation as the ultimate source of variation, the modes and quantitative description of natural selection through fitness and selection coefficients, the effect of dominance on selection, mutation-selection balance, genetic drift and effective population size, the founder effect and bottlenecks, and the neutral theory of molecular evolution. It treats the dynamics of single loci under these forces; the equilibrium baseline they perturb is covered under Hardy-Weinberg.
Core questions
- How does natural selection change allele frequencies, and how is its strength quantified by fitness?
- Why is genetic drift stronger in small populations, and what is effective population size?
- How does mutation-selection balance maintain deleterious alleles at low frequency?
- What does the neutral theory claim about most molecular variation?
Key concepts
- Mutation as the source of new alleles
- Fitness, selection coefficients, and modes of selection
- Genetic drift and effective population size
- Founder effect and population bottlenecks
- Mutation-selection balance and the neutral theory
Mechanisms
Selection acts by differential survival and reproduction tied to genotype, shifting frequencies deterministically in large populations; drift acts through random sampling of gametes, with variance inversely related to effective population size; mutation continually regenerates variants, and the balance between mutation input and selective removal sets the equilibrium frequency of deleterious alleles.
Clinical relevance
These forces explain why some deleterious alleles persist in human populations, why isolated or recently founded populations show elevated frequencies of particular disease alleles, and how pathogens and cancers evolve resistance under selective pressure.
History
Fisher, Wright, and Haldane laid the mathematical foundations of selection and drift in the 1920s and 1930s; Wright emphasized drift and the shifting-balance theory, and Kimura's neutral theory in the 1960s and 1970s argued that most molecular variation is selectively neutral and governed by drift, sparking a lasting debate.
Debates
- The relative roles of selection and neutral drift
- The neutralist-selectionist debate asks whether most molecular variation is effectively neutral and shaped mainly by drift, as Kimura argued, or whether selection pervasively molds the genome; modern genomic data support a nuanced mixture rather than either extreme.
Key figures
- Ronald Fisher
- Sewall Wright
- Motoo Kimura
- J. B. S. Haldane
Related topics
Seminal works
- fisher1930
- kimura1983
Frequently asked questions
- What is the difference between natural selection and genetic drift?
- Natural selection changes allele frequencies systematically according to how genotypes affect survival and reproduction, whereas genetic drift changes them randomly through the chance sampling of which individuals reproduce, an effect strongest in small populations.
- What is effective population size?
- It is the size of an idealized population that would experience the same amount of genetic drift as the real population; it is usually smaller than the actual head count because of unequal sex ratios, variation in family size, and past bottlenecks.