In: Biology
Answer: 1.polyphyletic taxa and paraphyletic taxa
Taxon is a group of organisms that is given a nomenclature that is formal and scientific. Some taxa (singular: Taxon) may contain organisms that are immediately related by a common ancestor and others may lack a relation through common ancestry. For greater accuracy on evolutionary history, the taxon must contain organisms that are related by an immediate common ansestor.
Monophyletic taxa has organisms that are related by an immediate ancestor. In other words, monophyletic taxa contain organisms that are descended from a single common ancestor. This makes monophyletic taxa an excellent candidate for evolutionary studies.
Both polyphyletic and paraphyletic taxa doesn't contain groups of species that are related by an immediate common ancestor. To be more clear, a paraphyletic taxa doesn't contain all the descents of the immediate common ancestor. Where as in the case of polyphyletic taxa, the organisms don't share a common ancestor.
Hence, both polyphyletic and paraphyletic taxa may invite problems or troubles if used for evolutionary studies.