Evolution explains how populations of living things change over generations and how Earth's enormous diversity of life arose.
Evolution is a central idea in biology. It describes changes in inherited traits within populations over many generations. Scientists study evolution using fossils, anatomy, genetics, observations of living organisms, and experiments. Natural selection is one important mechanism, but mutation, genetic drift, and movement of genes between populations can also influence how populations change.
Individuals in a population differ, and some of those differences can be passed from parents to offspring.
Learn MoreInherited traits that improve survival or reproduction can become more common over generations.
Learn MorePopulations can become better suited to particular environments as useful inherited traits accumulate.
Learn MoreFossils, DNA, comparative anatomy, and observed change all help scientists reconstruct life's history.
Learn MoreSpecies can share ancestors, forming branching patterns of relatedness across the tree of life.
Learn MoreEvolution can be observed in real time in rapidly reproducing organisms and changing populations.
Learn MoreMembers of the same species are not genetically identical. Differences can arise from mutations, from the reshuffling of genes during reproduction, and from other biological processes. Some traits are strongly influenced by genes, while others also depend on nutrition, temperature, experience, or other environmental conditions. Evolution depends on inherited variation because only differences that can be passed to later generations can consistently change a population's genetic makeup over time.
Natural selection occurs when individuals with certain inherited traits tend to leave more surviving offspring than others in a particular environment. Over many generations, those traits can become more common in the population. Natural selection does not plan ahead or produce perfect organisms. A trait is useful only in relation to a specific environment, and a feature that is helpful in one place or time may be less useful when conditions change.
An adaptation is an inherited feature that has become common because it improves survival or reproduction in a particular setting. Examples include camouflage, specialized teeth, resistance to certain diseases, or behaviors that help animals find food. Adaptations develop across generations in populations rather than appearing because an individual organism suddenly needs them. Many organisms also carry traits that are neutral, inherited from ancestors, or useful only under certain conditions.
Scientists use many independent lines of evidence to study evolution. Fossils show that different organisms lived at different times in Earth's history and reveal transitional forms linking major groups. Comparative anatomy shows shared structural patterns, while embryology can reveal similarities in early development. DNA provides especially detailed evidence because closely related species usually share more genetic similarities than distantly related species. These different forms of evidence can be compared with one another to test evolutionary explanations.
Evolutionary relationships are often represented as branching trees. When two species share a recent common ancestor, their branches join closer to the tips of the tree; species whose common ancestor lived much earlier join farther back. Scientists build these trees using fossils, physical features, and genetic data. The branches do not form a ladder from "lower" to "higher" life. Modern species have all been evolving for the same amount of time since their shared ancestors, but along different paths.
Evolution is not limited to the distant past. Scientists can observe it directly, especially in organisms that reproduce quickly. Bacteria can evolve resistance to antibiotics, insects can become resistant to pesticides, and populations of plants or animals can shift as climates and habitats change. These examples help researchers study how mutation, selection, migration, and chance influence populations in real time. Understanding evolution is important in medicine, agriculture, conservation, and many other fields of science.