Simplify the following text for a Grade 7 native English-speaking person.
Original: A polyphyletic group is any group other than a monophyletic group or a paraphyletic group , which like a paraphyletic group contains only some of the descendants of their closest common ancestor , but unlike a paraphyletic group is not characterized by the missing descendants forming one ( or more ) monophyletic groups .
Simplified: If there is a group made of a common ancestor , and all its descendants , that group is monophyletic .