The term for the gradual change in characteristics of a species is "evolution." Evolution is the process through which populations of organisms change over generations in response to environmental pressures, genetic variations, and natural selection. This concept, first systematically articulated by Charles Darwin in the 19th century, explains how species adapt and diversify over time. Modern evolutionary biology incorporates genetics and molecular biology to further understand these processes. Evolutionary mechanisms include mutation, gene flow, and genetic drift, all contributing to the complexity of life on Earth and explaining the diversity of species found today.