Natural Selection

Natural selection is the differential survival and reproduction of individuals due to differences in heritable phenotype, and it is the only evolutionary mechanism that consistently produces adaptation.

Definition

Natural selection is the process by which heritable traits that increase an organism's relative reproductive success become more common in a population over generations. It requires variation in a trait, heritability of that variation, and a consistent association between the trait and fitness.

Scope

This topic covers the logic and modes of natural selection: the conditions required for selection to occur, the distinction between directional, stabilizing, and disruptive selection, the meaning of fitness and the selection coefficient, and the special cases of frequency-dependent and sexual selection.

Core questions

• What three conditions must hold for natural selection to drive evolutionary change?
• How do directional, stabilizing, and disruptive selection differ in their effects on trait distributions?
• How is the strength of selection quantified through the selection coefficient and fitness?
• When does selection fail to maximize population fitness, as under frequency dependence or genetic constraints?

Key theories

Darwinian natural selection

Given heritable variation in fitness, traits associated with higher reproductive success spread, producing organisms apparently designed for their environments without any designer.

Fisher's fundamental theorem of natural selection

The rate of increase in mean fitness of a population equals its additive genetic variance in fitness, formalizing how selection acts on heritable variation.

Mechanisms

Selection acts whenever genotypes differ in their expected contribution of offspring to the next generation. The change in allele frequency per generation is governed by the selection coefficient and the dominance relations among alleles. Directional selection shifts the trait mean, stabilizing selection reduces variance around an optimum, and disruptive selection favors extremes. Frequency-dependent selection makes a genotype's fitness depend on its commonness, which can maintain polymorphism. Selection cannot create variation; it can only sort among the variants mutation and recombination supply.

Clinical relevance

Natural selection drives the evolution of drug resistance in pathogens and cancer cells and of pesticide and herbicide resistance in pests and weeds, making its dynamics central to medicine and agriculture.

History

Darwin and Wallace independently formulated natural selection in 1858, with Darwin's On the Origin of Species (1859) providing the full argument. Fisher's 1930 fundamental theorem placed selection on a rigorous quantitative footing within the modern synthesis, linking the rate of adaptation to additive genetic variance.

Debates

Units and levels of selection

Whether selection acts primarily on genes, individuals, or higher-level groups remains debated, with most adaptations explained by individual-level selection but kin and multilevel selection invoked for cooperation.

Key figures

• Charles Darwin
• Alfred Russel Wallace
• Ronald A. Fisher

Seminal works

• futuyma2017
• fisher1930
• ridley2004

Frequently asked questions

Does natural selection act for the good of the species?

Generally no. Selection favors traits that maximize the relative reproductive success of individuals (or genes), even when those traits do not benefit the population or species as a whole.

Can selection create new traits from nothing?

No. Selection only sorts among existing heritable variation; the new variation it acts on originates from mutation and is reshuffled by recombination.