The individual organism that make up a population vary in the traits they posses, such as their size and shapes.
Genetic that lead to structural, passed on to offspring
The differences are, at least in part, passed from parent to offspring.
EX. Tall parents tend to have tall offspring
Some individuals are more successful at surviving and reproducing than others
The successful individuals are not merely lucky, instead, they succeed because of the variant traits that they have inherited and will pass onto their offspring