Identifying Polyphyletic Groups

Polyphyletic groups are groups that contain multiple tips, but not all of their common ancestors.

Let’s take our vertebrate phylogeny and create a polyphyletic group for all tips that live primarily in water. Based on exhaustive study, we determine our tips should include the fishes, the amphibians, and the crocodiles. We may include the common ancestor for the fishes and the amphibians as it likely lived primarily in water. However, we may decide to not include any other ancestors.

This results in a clear polyphyletic group! Polyphyletic groupings are generally avoided in evolutionary thinking as they are imprecise descriptions of evolution. However, they can still have their uses!