Individuals are not all the same because of differences in their genes. So there is variation within populations of organisms.
Most organisms produce more young than can survive to adulthood.
There are no enough resources for organisms to live longer.
The limited supply causes competition between species.
Those individuals with variation that are better adapted will be most likely to survive and the poorly adapted ones will die.
Individuals that survive may breed to reproduce and pass their advantageous variation/ characteristics to their offspring.