A monophyletic group (clade)
contains all organisms descended from a common ancestor, with shared traits.
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:
- you cannot find an animal with milk but no jaws
- an animal without jaws cannot produce milk