Variation and Inheritance
Members of any given species are seldom exactly the same, either inside or outside. Organisms can vary in size, coloration, ability to fight off diseases, and countless other traits. Such variation is often the result of random mutations, or "copying errors," that arise when cells divide as new organisms develop.
When organisms reproduce, they pass on their DNA--the set of instructions encoded in living cells for building bodies--to their offspring. And since many traits are encoded in DNA, offspring often inherit the variations of their parents. Tall people, for example, tend to have tall children.