Clades within clades
A clade (also known as a monophyletic group) is a group of organisms that includes a single ancestor and all of its descendents. Clades represent unbroken lines of evolutionary descent. It's easy to identify a clade using a phylogenetic tree. Just imagine clipping any single branch off the tree. All the lineages on that branch form a clade. If you have to make more than one cut to separate a group of organisms from the rest of the tree, that group does not form a clade. Such non-clade groups are called either polyphyletic or paraphyletic groups depending on which taxa they include.
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Trees A and B adapted from Lo, E.Y.Y., and M.J. Donoghue. 2012. Expanded phylogenetic and dating analyses of the apples and their relatives (Pyreae, Rosaceae). Molecular Phylogenetics and Evolution 63:230-243.
Trees C and D adapted from Geisler, J.H., and J.M. Theodor. 2009. Hippopotamus and whale phylogeny. Nature 458:E1-E4.
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