Answer: Is called natural selection.
Explanation:
According to natural selection, differences in the phenotype of individuals of the same species make some of them have prevalence over the others and thus reproduce more easily.
The phenotype is the set of observable characteristics of an individual. For example, if some individuals from some animal species are taller and thus able to reach food that their pairs can´t, they will survive when food scarce, then they will reproduce and their offsping will have that ability.
