The answer is natural selection.
Natural selection is the differences in survival and reproduction as the consequence of differences in phenotypes. According to Darwin's theory of evolution, natural selection is the only factor that affects a frequency of different variations and it is the only mechanism of evolution. In natural selection, genotype variations that will increase the chance of survival and reproduction of some organism are preserved and will be inherited. Darwin's theory suggested that there are variations among individuals in a population and that these variations will help individuals to survive.