How are birds reptiles? Paraphyly expanded

Have you ever heard that birds are reptiles? We can explain this concept through the lens of paraphyly and monophyly.

If we look at our vertebrate phylogeny, you may notice that Crocodiles are not listed within reptiles. This is a bit misleading as by all intents and purposes, we humans informally classify Crocodiles as reptiles. A more accurate phylogeny may then list the reptiles tip as “Non-crocodilian reptiles” or leave a note in the caption.

However, also notice that crocodiles are sister to the birds. This is a true relationship we can see not only in their shared genetic material, but in their shared morphological similarities. Each has a 4-chambered heat, one way airflow for their respiratory system, and exhibit parental care!

If we were to create a reptiles group using the commonly held understanding that crocodiles are reptiles, and birds are not, we would end up with the following paraphyletic group.

When we describe life, monophyletic groups are preferred. This is because monophyletic groups provide clear boundaries for what makes any particular tip a part of a group, they are easier to define without excluding groups arbitrarily, and they represent a shared evolutionary history by way of the common ancestor. For these reasons, we avoid paraphyly for describing groups whenever possible.

If we were to create a monophyletic group that contains all the reptiles including crocodiles, our circle would need to contain the birds by the definition of monophyly. Thus, the birds are also reptiles.

Remember, many of these groups were defined hundreds of years before DNA was even known. Superficially looking at crocodiles and the other reptiles makes it incredibly easy to lump them as a single group. It also makes sense why we would incorrectly assume birds are their own distinct lineage, far removed from the crocodiles. While a case could be made to simply exclude Crocodiles from reptiles and refer to them as their own distinct group sister to birds, it would make no meaningful difference!

Nothing changes about their biology, their evolutionary history, or their real relationship to other tips on the tree just because we call them in a different group. As far as anyone can tell, the birds do not seem to care if they are considered reptiles or not. In fact, the birds don’t even know they are called birds!

Ultimately, the labels we put on biota are for us. While we may now look at birds and reptiles slightly differently, that is because of the evolutionary relationships between one another. Not because of the name we applied to their group.