The term describing the gradual adaptation of a species to its environment over generations is called evolution. This process occurs through mechanisms such as natural selection, genetic drift, and gene flow, leading to changes in the traits of populations over time. As environmental pressures act on organisms, those with advantageous traits are more likely to survive and reproduce, passing these traits on to their offspring. Ultimately, evolution results in a population's increased fitness and adaptation to specific ecological niches, contributing to the biodiversity observed in different ecosystems.