Evolution is the change in heritable characteristics of populations across generations. In modern biology, evolution explains how populations become genetically different over time and how the diversity of life has arisen through descent with modification. Evolution occurs because hereditary variation exists within populations and because processes such as mutation, natural selection, genetic drift, gene flow and recombination alter the frequencies of genetic variants across generations.
Evolution applies to populations rather than individual organisms changing their inherited genetic makeup during their own lifetimes. An individual can develop, acclimatise or change physiologically in response to its environment, but such changes are not necessarily evolutionary unless they influence heritable variation across generations. Evolution therefore connects genetics, reproduction, ecology, development, palaeontology and the history of life.
Charles Darwin and Alfred Russel Wallace independently developed natural selection as a mechanism explaining adaptation and biological diversity in the nineteenth century. Darwin published On the Origin of Species in 1859, presenting extensive evidence that species change and share common ancestry and arguing that natural selection can produce adaptation. Gregor Mendel's earlier work on inheritance was initially not fully integrated with Darwin's ideas, but twentieth century genetics provided the mechanism linking hereditary variation with evolutionary change. The modern evolutionary synthesis combined Mendelian genetics with natural selection, population genetics and other biological disciplines. Later advances in molecular biology, genomics, developmental biology and evolutionary ecology have expanded evolutionary theory while retaining common descent and population level genetic change as central principles.
Mutation introduces new genetic variation by altering DNA sequences, although most mutations have neutral or harmful effects and a smaller proportion can be beneficial in particular environments. Recombination during sexual reproduction reshuffles existing genetic variants, producing new combinations in offspring. Natural selection occurs when heritable differences influence survival or reproductive success, causing some variants to become more common over generations. Selection does not operate according to conscious purpose or foresight.
It results from differences in reproductive contribution under particular environmental conditions. A trait that is advantageous in one environment can be disadvantageous in another. Evolution therefore has no predetermined destination, and adaptation reflects historical interactions between organisms and their environments rather than movement towards perfection.
Genetic drift is another major evolutionary mechanism. It involves random changes in allele frequencies, particularly in small populations. A rare allele can become common or disappear simply because of chance sampling during reproduction. Bottlenecks occur when population size is sharply reduced, potentially causing substantial loss of genetic diversity. Founder effects occur when a small group establishes a new population carrying only a subset of the original population's genetic variation.
Gene flow occurs when individuals or gametes move between populations and transfer genetic variants. These processes can increase or decrease genetic differences among populations and operate alongside mutation, recombination and natural selection. Evolutionary change is therefore produced by multiple interacting mechanisms rather than natural selection alone.
Evolution produces adaptation but adaptation and evolution are not synonymous. Adaptation refers to a heritable characteristic that increases fitness in a particular environment as a result of evolutionary processes, whereas evolution includes any change in heritable population characteristics, including changes that are neutral or harmful. Genetic drift, for example, can change allele frequencies without producing an adaptive advantage.
Sexual selection is a form of selection in which traits affect mating success, sometimes producing characteristics that increase reproductive success despite imposing survival costs. Artificial selection occurs when humans influence reproduction, as in selective breeding of crops and domesticated animals. These processes demonstrate that evolutionary change can occur through different routes and that natural selection is one mechanism within the broader evolutionary framework.
Evolution also explains the origin of biological diversity through speciation. When populations become sufficiently isolated genetically or reproductively, they can diverge into distinct species. Geographic isolation can reduce gene flow, allowing mutation, selection and drift to produce differences. Ecological specialisation, behavioural divergence, chromosome changes and sexual selection can also contribute to reproductive isolation. Over longer periods, evolutionary divergence can produce genera, families and other taxonomic groups. The branching pattern of evolutionary history is represented by phylogenetic trees, which depict hypothesised relationships among lineages. Common ancestry means that all living organisms are connected through an immense history of descent, although the precise relationships among some lineages remain subjects of ongoing research.
Evidence for evolution comes from many independent scientific fields. The fossil record documents sequences of organisms through geological time and includes transitional forms showing combinations of ancestral and derived characteristics. Comparative anatomy reveals homologous structures inherited from common ancestors. Comparative embryology can reveal shared developmental patterns, although modern developmental genetics provides more precise evidence. Molecular biology demonstrates similarities in DNA, RNA and proteins among related organisms, while shared genetic features can provide strong evidence of common ancestry. Biogeography shows that geographic distributions often correspond to historical isolation and evolutionary relationships. Direct observations include measurable changes in populations, such as shifts in antibiotic resistance in bacteria and changes in traits under documented selective pressures.
Evolutionary theory has major practical significance. Understanding evolution helps explain antibiotic resistance, pesticide resistance, emerging pathogens, agricultural breeding, conservation genetics and ecosystem change. In medicine, evolutionary principles help researchers understand how tumour cell populations can evolve resistance to treatments and how infectious agents can change through mutation and selection. In conservation, maintaining genetic diversity can increase the capacity of populations to respond to environmental change. In agriculture, evolutionary knowledge informs breeding and management of pests.
Evolution also provides the unifying framework connecting diverse biological disciplines because the similarities among living organisms, their differences and their historical relationships can be understood through common descent and modification. Evolution is not a claim that individual organisms deliberately transform into other kinds of organisms; it is a population and lineage process operating over generations through changes in heritable biological variation.
