Natural Selection and Fitness

Natural selection is the systematic change in allele frequencies that results when genotypes differ in fitness — their relative contribution of offspring to the next generation. Unlike drift, selection acts in a consistent direction, increasing the frequency of alleles associated with higher reproductive success and decreasing those that lower it.

Definition

Natural selection is the change in allele or genotype frequencies across generations caused by consistent differences in fitness among genotypes, where fitness is the relative contribution of a genotype's bearers to the gene pool of the next generation.

Scope

The entry defines fitness and the selection coefficient, distinguishes the main modes of selection (directional, stabilising, balancing), and outlines how signatures of selection are detected in genomes. It is a conceptual and methodological topic within population genetics, presented as reference material rather than clinical guidance.

Core questions

• How do differences in fitness change allele frequencies over generations?
• What are the principal modes of selection and how do they differ in effect?
• How can the genomic signatures of past and ongoing selection be detected?

Key concepts

• Fitness
• Selection coefficient
• Directional selection
• Stabilising selection
• Balancing selection (including heterozygote advantage)
• Selective sweep
• Genomic signatures of selection

Key theories

Selection as an evolutionary force

Genotypes that contribute disproportionately to future generations have their alleles increase in frequency; the strength of this directional change is captured by the selection coefficient and was formalised in the population-genetic models of the modern synthesis.

Mechanisms

When genotypes differ in survival or reproduction, the alleles they carry are transmitted to the next generation in unequal proportions, so their frequencies change systematically; the per-generation strength of this change is summarised by the selection coefficient. Directional selection drives a favoured allele toward fixation, stabilising selection maintains an intermediate optimum, and balancing selection — for example heterozygote advantage — keeps multiple alleles in the population. A strong, recent sweep of a favoured allele leaves detectable footprints in surrounding variation, such as extended haplotype homozygosity, which genome scans use to identify loci under selection.

Clinical relevance

Several medically relevant alleles are maintained or have spread through selection — the classic example being heterozygote advantage for some haemoglobin and other variants in regions with endemic malaria — and genome scans for selection help explain why certain alleles are common in particular populations. It describes evolutionary processes shaping variation and is not a basis for individual diagnostic or treatment decisions.

Epidemiology

Balancing selection driven by infectious-disease pressure is one explanation for the elevated frequency of certain disease-associated alleles in specific regions, and genome-wide scans have identified many loci bearing signatures of recent positive selection in human populations.

History

Darwin's idea of differential reproductive success was given a quantitative, gene-level form by Fisher, Haldane, and Wright during the modern synthesis of the 1920s and 1930s, who modelled how selection coefficients change allele frequencies. With genome-wide data, attention turned to detecting the molecular footprints of selection, and studies such as Voight and colleagues' map of recent positive selection illustrate how these signatures are now identified empirically.

Debates

How pervasive is positive selection in the human genome?

Genome scans differ in how many loci they flag as recently selected and in how confidently sweeps can be distinguished from demographic effects such as bottlenecks, so the overall extent of recent positive selection remains debated.

Key figures

• Charles Darwin
• Ronald A. Fisher
• J. B. S. Haldane
• Sewall Wright
• Jonathan Pritchard

Seminal works

• wright-1931
• voight-2006

Frequently asked questions

What does fitness mean in population genetics?

Fitness is the relative contribution a genotype makes to the gene pool of the next generation, reflecting differences in survival and reproduction; genotypes with higher fitness pass on their alleles more often.

How is natural selection different from genetic drift?

Selection changes allele frequencies systematically and directionally according to fitness differences, whereas drift changes them randomly because populations are finite. Both act on allele frequencies but only selection is consistently directional.