Charles Darwin's theory of evolution by natural selection is the unifying framework of modern biology. Natural selection operates when heritable variation leads to differential survival and reproduction among individuals in a population. Genetic mutations introduce new variation, while gene flow and genetic drift also shape the diversity of life. The fossil record provides compelling evidence for the evolution of species over geological time. Comparative genomics has confirmed that all life on Earth shares common ancestry through shared DNA sequences. Modern evolutionary theory incorporates genetics, developmental biology, and ecology to explain the remarkable diversity of organisms on our planet.