The primary pressure on species to change over time is natural selection. Natural selection is the process by which certain traits become more or less common in a population over time as a result of the differential survival and reproduction of individuals with those traits. It occurs because not all individuals in a population can survive and reproduce, and those that do so successfully pass their traits on to their offspring. Over many generations, this can lead to changes in the traits and characteristics of a species, allowing it to better adapt to its environment and increasing its chances of survival.