Evolution
The process by which populations of organisms change over generations, primarily through natural selection acting on inherited variation.
Evolution by natural selection is the central organising principle of biology, formalised by Charles Darwin and Alfred Russel Wallace in the mid-19th century. It explains the diversity, complexity, and adaptation of life through three observations and one conclusion: individuals vary, traits are inherited, and resources are limited — therefore variants better suited to local conditions tend to leave more offspring, and populations change over generations.
Modern evolutionary biology integrates Darwin's framework with genetics, molecular biology, and paleontology, creating the Modern Synthesis and its successors.
Evolution is supported by overwhelming evidence: the fossil record, comparative anatomy, biogeography, embryology, and especially DNA sequence comparisons across species. It is also directly observed in real time — bacteria evolving antibiotic resistance, insects developing pesticide tolerance, finches' beaks shifting with environmental conditions.