Pre AP Biology -EVO 1.2 Classifying Evolutionary Relationships - FRQ Exam Style Questions -New Syllabus

You are surprised because you understand that the class Reptilia, even though it includes turtles, lizards, snakes, and crocodilians, which are descendants of a common ancestor, it does not include all descendants of archosaurs—namely, birds. As such, class Reptilia can only be considered a paraphyletic taxon.

To understand why the friend’s claim is incorrect, we must look at the definitions used in cladistics:

• Monophyletic Group (Clade): A group that consists of a single common ancestor and all of its descendants.
• Paraphyletic Group: A group that consists of a common ancestor and some, but not all, of its descendants.

In traditional taxonomy, “Reptilia” includes animals like crocodiles and dinosaurs. However, modern evolutionary biology has shown that birds (Aves) evolved directly from a specific group of theropod dinosaurs. Because birds are descendants of the same common ancestor as crocodiles and dinosaurs but are traditionally excluded from the class “Reptilia,” the group is considered paraphyletic.

For Reptilia to be a true monophyletic taxon, it would necessarily have to include birds, as they are part of the archosaur lineage. Since the common usage of the term “reptile” usually excludes birds, the claim that it is monophyletic is scientifically inaccurate.