Evolution

Evolution is the change in the heritable characteristics of biological populations over successive generations. It arises from variation among individuals, which can be caused by mutations, genetic recombination, and other sources of genetic diversity. Natural selection, genetic drift, gene flow, and mutation shape how traits change in frequency. Through these processes, populations can become better adapted to their environments, and lineages may diverge to form new species.

Evidence for evolution comes from multiple lines of inquiry. The fossil record shows sequences of organisms and transitional forms, along with dating that places changes in a historical timeline. Comparative anatomy reveals homologous structures indicating common ancestry, while vestigial organs reflect historical shifts in function. Molecular genetics and genomics reveal shared genes and pathways across diverse life. Biogeography and observations from experimental evolution further support the ability of populations to adapt and diverge.

Common descent implies that all living organisms are related through branching lineages. Speciation occurs when reproductive barriers or ecological differentiation reduce or block gene flow, allowing populations to accumulate distinct traits. Modes of speciation include allopatric and sympatric, and adaptive radiations can produce many related species from a single ancestor.

History and synthesis: The idea of evolution by natural selection was proposed in the nineteenth century by Charles Darwin and Alfred Russel Wallace. After the rediscovery of Mendelian genetics, the Modern Synthesis integrated natural selection with genetics, population biology, and paleontology. Contemporary evolutionary biology also incorporates genomics, developmental biology, and new approaches such as evo-devo.

Evolutionary theory provides a framework for understanding the diversity of life, the patterns of adaptation, and the history of life on Earth. It is supported by a broad and continually expanding set of observations, experiments, and models, and remains open to revision in light of new evidence.