Monophyletic, Paraphyletic, and Polyphyletic Groups
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A monophyletic group, or clade, includes an ancestor and all its descendants; a paraphyletic group includes only some; a polyphyletic group excludes the ancestor.
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A group built from reptiles alone, excluding birds descended from the same ancestor, is paraphyletic, omitting some descendants.
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Only a monophyletic group is a true clade — paraphyletic and polyphyletic groupings do not capture one complete branch.