https://www.reddit.com/r/biology/comments/1aszp8i/vertebrata_and_categorizing_their_classes/
created by JacobAn0808 on 17/02/2024 at 11:16 UTC
3 upvotes, 1 top-level comments (showing 1)
In the sub-phylum 'vertebrata', there are 5 main categories: fish, amphibians, reptiles, birds, and mammals. These categories are used to group the classes, especially for fish. For example, some classes of fish are actinopterygii, angatha, and chondrichthyes, but they can all be grouped as fish. However, for the rest of the 4 categories, they only have 1 class. For example, mammalia for mammals and reptilia for reptiles. Is my understanding of this correct? Thanks for helping.
Comment by ScipioAfricanisDirus at 17/02/2024 at 19:59 UTC
14 upvotes, 3 direct replies
Dividing vertebrata into the five discrete categories you mention is an outdated system and those are really colloquial terms that are a holdover from it. That system, and those five discrete categorizations, do not reflect the biological reality of the evolutionary history or relationships of the members of Vertebrata, so they aren't used by actual biologists.
The system used by biologists to classify organisms is called cladistics, in which organisms are grouped together based on common ancestry into groups called clades. Valid clades are groups of organisms that contain a common ancestor and **all** of that ancestor's descendants. They do not necessarily need to have a rank, though they're often given ranks such as "class" or "family" because that nomenclature is already familiar. Clades are also nested, meaning as the tree of life splits you can divide bigger, more inclusive clades into smaller, less inclusive ones (i.e. cats are members of the larger clade Felidae, and members of Felidae are also members of the larger clade Mammalia, and members of Mammalia are members of the larger clade Vertebrata.).
So what's the issue with the five you mentioned? Well, fish is the most obvious one. Using "fish" as a group isn't biologically informative and doesn't reflect the actual evolutionary history of members of the group. As you pointed out, "fish" would include smaller classes like Actinopterygii and Chondrichthyes, but it would also include the lobe-finned fishes, Sarcopterygii. The problem with that is amphibians, reptiles, birds, and mammals all descend from lobe-finned fishes. We're all sarcopterygians, which are "fish". Yet under that classification system amphibians, reptiles, birds, and mammals are simultaneously members of a group that are categorically fish (Sarcopterygii), but are also categorically *not* fish. That obviously can't be. And you can't just exclude Sarcopterygii from fish to fix that, since sarcopterygians are much more closely related to actinopterygians than actinopterygians are to chondrichthyians. Clearly making a group called "fish" just doesn't reflect the reality of the biological relationships.
There are similar problems for some of the other groups. If you take reptiles to include turtles, snakes, lizards, and crocodiles then birds should also be reptiles, since birds and crocodiles not only share a much more recent common ancestor than crocs and lizards, but their anatomy and physiology reflect that closer relationship on the whole (and most recent evidence shows turtles are also probably more closely related to birds than they are to lizards). Once again, biological reality is at odds with the five traditional groups.
As for how Vertebrata is actually categorized by biologists, there's a lot that could be said. Remember we use clades, which are nested and not necessarily ranked, so there are hundreds if not thousands of recognized named clades that are members of the larger sub-phylum Vertebrata. The wikipedia page for Vertebrata has a series of trees showing the relationships between the major groups[1], and here's another one that's a bit more simplified[2] that does a good job of showing the big picture stuff if the wikipedia one seems like a little too much at once. You'll notice this one does include mammals, reptiles (including birds), and amphibians, which are still considered valid clades in their own right, they're just used in a slightly different way from your original example with their greater evolutionary context taken into account.
1: https://en.wikipedia.org/wiki/Vertebrate#Phylogenetic_relationships
2: https://www.ucl.ac.uk/museums-static/obl4he/vertebratediversity/index.html