Simplify the following text for a Grade 8 native English-speaking person.
Original: Scientific classification aims to group species together such that every group is descended from a single common ancestor . A polyphyletic group can be `` fixed '' either by excluding clades or by adding the common ancestor .
Simplified: A polyphyletic ( Greek for `` of many races '' ) group is one whose members ' last common ancestor is not a member of the group .