Evolution
Evolution is the change in the inherited traits of a population over many generations, explaining the diversity and shared ancestry of life.
Definition
Evolution is the way living things change over many generations, with small changes adding up until new kinds of living things appear, and it happens to groups over time rather than to one individual within its lifetime, so a single fish never turns into a frog even though a population of fish-like ancestors can give rise to amphibians over millions of years. More formally, evolution is change in the heritable characteristics of populations over successive generations, driven by natural selection acting on variation from mutation and sexual reproduction, plus genetic drift and migration, and it is supported by evidence from fossils, comparative anatomy, embryology, DNA similarities, and directly observed change in living populations, since all living things share common ancestors. At the deepest level, evolution is change in allele frequencies within populations and the resulting divergence of lineages, from microevolution within species to macroevolutionary patterns such as speciation, adaptive radiation, and extinction; the modern synthesis unites Darwinian selection with Mendelian genetics, molecular phylogenetics reconstructs relationships from DNA, and common descent is supported by nested hierarchies of shared derived traits, a universal genetic code, and molecular fossils such as pseudogenes and endogenous retroviruses.
Example
Fossils show that whales descended from four-legged land animals whose front legs gradually became flippers and whose back legs disappeared, and the human arm, a bat wing, a whale flipper, and a cat's foreleg all share the same set of bones arranged the same way, evidence of a shared ancestor; humans and chimpanzees share about $98$ to $99$ percent of their DNA sequence. Richard Lenski's long-term experiment has tracked $12$ populations of E. coli through more than $75{,}000$ generations since 1988, and one population evolved the novel ability to metabolize citrate in the presence of oxygen after a series of earlier mutations, much as antibiotic and pesticide resistance are evolution observed on a scale of years.
Key Insight
Evolution is both a fact, since populations observably change, and a theory, the well-tested explanation of how, and in science theory means a broad explanation supported by evidence rather than a guess. As Dobzhansky wrote, nothing in biology makes sense except in the light of evolution, the single framework that explains antibiotic resistance, the fossil record, genome structure, and why the same bones appear in a hand and a wing.