Resources are limited. More individuals than environment can support leads to struggle for existence. Only a fraction of offspring survive. No two individuals are exactly alike. Even identical twins differ slightly, e.g., weight, etc.
Much variation is inheritable.
Survival is not random but depends in part on inherited characteristics that suit them better to an environment. Unequal ability of some to survive and reproduce leads to gradual change in a population. B. Natural selection is the differential success in reproduction. Think of the variation in dog breeds. May be the most variable species on earth.