Polyphyletic Taxa.
Polyphyletic taxa are those that aren't monophyletic. They could however be paraphyletic. In order to explain what is meant by any of this, we should backtrack a little.
A classification that reflects phylogeny is the ideal of systematists. It is just like a family tree: organisms are placed on it according to their descent. This ideal is hard to attain, partly because we do not know enough, and partly because the results of strictly phylogenetic classification can be counter-intuitive. Still, it is possible to classify a great many organisms according to descent, and when we do so, the animals on a given branch of the tree are all descendants of one ancestral line. They are therefore a monophyletic group (or taxon).
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