Are all different taxa true clades?

Not all different taxa are true clades. A clade is a monophyletic group, meaning that it includes an ancestor and all of its descendants. All taxa are groups of organisms, but not all of them are monophyletic. Some taxa are paraphyletic, meaning that they include an ancestor and some, but not all, of its descendants. Other taxa are polyphyletic, meaning that they include organisms from multiple different ancestors.

For example, the taxon "mammals" is a true clade. It includes the ancestor of all mammals and all of its descendants. The taxon "reptiles" is a paraphyletic group. It includes the ancestor of all reptiles and some, but not all, of its descendants. Birds are excluded from this group, even though they evolved from reptilian ancestors. The taxon "invertebrates" is a polyphyletic group. It includes organisms from multiple different ancestors. It includes all animals that are not vertebrates.

It is important to distinguish between monophyletic, paraphyletic, and polyphyletic taxa because they have different implications for our understanding of evolutionary history. Monophyletic taxa represent groups of organisms that share a common ancestor and have a unique evolutionary history. Paraphyletic and polyphyletic taxa do not represent groups of organisms that share a common ancestor and have a unique evolutionary history.