Individuals that do not aquire a minimum amount of resources fail to reproduce. Population size can not increase indefinitely. Ceiling on population size is carrying capacity, K. 5. Once K is reached, more offspring will be produced than can be supported by available resources. Some individuals do not obtain enough resources to grow and reproduce.
Some individuals fail in that competition.
Some individuals are better equipped for competition to obtain resources, and they are better equipped because of their genetic makeup. Individuals that are better equipped for the competition to obtain resources are more successful at reproducing. Traits that confer improved competitive ability have greater representation in the next generation. This is evolution.