The primary factor that leads to evolution in populations is natural selection. Natural selection occurs when certain traits increase an organism's chances of survival and reproduction in a specific environment, allowing these favorable traits to be passed on to future generations. Over time, this process can lead to adaptations that enhance the fitness of a population. The interplay of genetic variation, environmental pressures, and reproductive success shapes the evolutionary trajectory of species, making natural selection a cornerstone of the theory of evolution and a fundamental concept in biology.