For example, all organisms descended from an ancestor that had jaws constitute a large clade; smaller clades later developed 4 limbs, milk, etc.

Note:

• All animals with milk have jaws (and 4 limbs, vertebrae).

• Only animals that have jaws can produce milk.

• Thus:
  1. you cannot find an animal with milk but no jaws
  2. an animal without jaws cannot produce milk