Natural Selection: One Example.
Populations are never identical; there are differences among individuals. When individuals with certain traits leave more offspring than those without them, the average over the population changes: the population evolves.
Assume a population of giraffes with the same number of long-necked and short-necked individuals. The short-necked giraffes will face a disadvantage when reaching for leaves from the trees.
Long-necked giraffes will leave more offspring who will inherit their long-neck genes. Eventually, the number of long-necked giraffes will be greater than the number of short-necked ones.
As time goes by, all short-necked giraffes will disappear The whole population will have long necks, just as it happens today.