Paraphyletic vs. Polyphyletic — What's the Difference?

Paraphyletic groups are defined by the inclusion of a common ancestor and some, but not all, of its descendants, making the group somewhat incomplete from an evolutionary perspective, whereas polyphyletic groups consist of various organisms that do not share an immediate common ancestor within the group, often grouped together based on similar features that evolved independently.

In a paraphyletic group, members are more closely related to each other than to any outside the group due to a shared common ancestor, even if not all descendants are included. In contrast, polyphyletic groups might include species that appear similar due to convergent evolution but are genetically and evolutionarily distinct.

The concept of a paraphyletic group often arises when a single lineage within a larger group evolves significantly different traits, leading to its classification as a separate group, thereby making the original group paraphyletic. On the other hand, polyphyletic groups are often identified by shared characteristics that are not inherited from a common ancestor within the group, but rather due to similar selective pressures causing convergent evolution.

Paraphyletic groupings can lead to more natural and evolutionary meaningful classifications in some contexts by highlighting transitional forms and evolutionary pathways, whereas polyphyletic groupings can be misleading by grouping together organisms that do not share a recent common ancestor, potentially obscuring true evolutionary relationships.

While paraphyly is often seen as a byproduct of evolutionary processes where not all descendants of a common ancestor are similar enough to be grouped together, polyphyly is usually the result of convergent evolution where unrelated lineages develop similar traits independently, making polyphyletic groups less cohesive from an evolutionary standpoint.