DEVELOPMENTAL BIOLOGY
Developmental biology is the study of the processes and patterns by which organisms grow and change from the fertilized egg (zygote) to the fully formed organism. Shared embryological characteristics point to a common ancestry, whereas differing characteristics indicate a more distant relationship.

The discovery of controlling genes (e.g., Hox genes), which regulate major developmental processes (including the number, placement, and form of limbs), allow us to see how major evolutionary changes can occur as a result of minor genetic mutations. Consider the question "How did snakes become limbless?" It appears that mutations in Hox genes are involved in snake evolution. Snakes probably lost their forearms first. In pythons, remnants of hindlimb bones are embedded in their bodies.