Natural Selection
The evolutionary mechanism by which heritable traits that improve survival and reproduction become more common in a population over generations.
Natural selection is the process, first formulated jointly by Charles Darwin and Alfred Russel Wallace in 1858 and presented in Darwin's On the Origin of Species (1859), by which populations change over generations in response to their environment. It is not a force that strives toward goals; it is the statistical consequence of three facts: individuals in a population vary, some of that variation is heritable, and individuals with certain variants leave more surviving offspring.
Because more offspring are produced than can survive, individuals compete for limited resources. Those whose heritable traits make them better at surviving and reproducing — better at finding food, evading predators, resisting disease, or attracting mates — contribute more copies of their genes to the next generation. Over many generations, the advantageous variants become more common and the population becomes adapted to its environment. Adaptation is therefore a cumulative, blind, and gradual process: it has no foresight, and it can only work with the variation that already exists.
Classic examples include the darkening of peppered moths during the industrial revolution as soot-covered trees favored the melanic form, and the evolution of antibiotic resistance in bacteria, where each dose of an antibiotic selects for resistant mutants — resistance spreads within years, not millennia. The three requirements (variation, heritability, differential fitness) are precisely the conditions under which allele frequencies change, which is the definition of evolution in population genetics.
The genetic basis of variation is Mendelian inheritance: genes are passed in discrete units, mutations create new alleles, and recombination reshuffles them. Natural selection acts on phenotypes — the visible expression of genotypes — so its target is the organism, but its effect is on gene frequencies. It is one of several evolutionary mechanisms — mutation, gene flow, and genetic drift also change allele frequencies, with drift dominating in small populations — but it is the only one that produces adaptation. The modern synthesis of the 20th century unified Darwinian selection with Mendelian genetics and population mathematics, and natural selection remains the central explanatory principle of biology.
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adaptation biology darwin evolution
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