What is the name for the gradual change in species over time?

Evolution is the gradual change in species over time, driven by shifts in heritable characteristics within populations. In modern biology, evolution is commonly described as a change in allele frequencies—the proportions of different gene versions—in a population across generations.

Evolution does not mean that individual organisms change their inherited traits because they need to. Instead, populations evolve as processes such as mutation, natural selection, genetic drift, and gene flow alter which traits become more or less common. Sexual reproduction also reshuffles existing genetic variation.

Charles Darwin’s On the Origin of Species, published in 1859, provided a powerful explanation for evolution through natural selection. Evidence now comes from fossils, comparative anatomy, embryology, biogeography, and molecular biology. DNA comparisons can reveal relationships among species even when fossils are incomplete.

Evolution is not necessarily a march toward greater complexity or perfection. Some organisms become simpler, retain similar forms for long periods, or adapt to changing conditions in different ways. “Adaptation” is a trait shaped by evolutionary processes, while “evolution” is the broader population-level change. “Transmutation” is an older term, not the standard modern answer.