Overproduction is a natural phenomenon in which the number of produced offspring is higher than the number of individuals the ecosystem can sustain. Thus, competition for limited resources occurs.
Because of the genetic variation, the organisms within a population have different chances to reproduce. Most often, some organisms have a lower reproductive success than others do.
The number of individuals with a high reproductive success is rather low as few males are very successful at passing on their genetic material.
The number of organisms with no reproductive success is also very low, as these "ill adapted" individuals will disappear from the population extremely quickly.
Genetic variation is the amount of differences between individuals. Thus, no organism taken by itself has any genetic variation.
