A monophyletic group (clade)
contains all organisms descended from a common ancestor, and share a trait.
For example, all organisms descended from an ancestor that had
constitute a large clade; smaller clades later developed 4 limbs, milk, etc.
- All animals with milk
- Only animals that have jaws
can produce milk.
- Thus, you cannot find an animal with milk but no jaws, and an animal without jaws cannot produce milk.