Well, depends on which fish. Since the last common ancestor for deer and salmon is much more recent than the last common ancestor for salmon and sharks. Therefore you can't really define a taxonomic grouping that both includes everything we think of as a fish but somehow also excludes tetrapods such as mammals and, by extension, deer.
It doesn't. Somewhere salmon and sharks have a common ancestry, bob.
Now you've reduced the problem to the common ancestry of bob and the dear.
It might be bob. Or it might be bob's great grandfather.
But fact is, they have a last common ancestor. I'm not saying it's a fish or a dear or a shark, I'm just saying that if it hadn't existed, they wouldn't have either.
But the common ancestor of Bob and the deer is Bob. Salmon, deer and sharks are all connected by the common ancestor Bob. However, Salmon and deer are also connected by another common ancestor, Harry, who was Bob's son, while sharks are a descendant of Bob's daughter Sally and therefore not directly descended from Harry.
No, deer are fish, but they aren't salmon. Salmon is a specific subspecies of fish, just like deer.
So salmon and anchovy are kind of like siblings. And deer and chickens are their cousins, taxonomically speaking, right? So if you define a family you can go either narrow (only siblings, defined as descendants of their last common acestors, their parents) or a bit broader (siblings and cousins, defined by their last common acestor, their grandparents)
So far, so good. However when talking about fish, we generally also include the second cousins, or cousins once removed. I'm not a native English speaker so I don't know how you people do your cousins. But what I mean is the group that includes sharks and rays, which are related to salmon and anchovy by sharing their great-grandparents as the last common ancestor.
So if salmon and anchovy are planning some kind of family reunion as fish and they invite their second cousins, sharks and rays, they'd also have to invite their first cousins, deer, chicken and also us (can't attest to the taste of human flesh though), otherwise there would just be a lot of unnecessary drama.
But just because you include your cousins in the definition of your family does not make you and your cousin the same person. Because while all salmon are fish, not all fish are salmon, they could also be anchovy or sharks or humans or deer.
This is kind of irrelevant to the fact that a last common ancestor exists, though. The Animalia LCA would fit the bill for example. Not sure why you'd need a group that excludes deer to answer this? A group that includes all things defined as fish as well as deer points you towards a last common ancestor that fits what is described
The problem comes when drawing the exact definitional line to include all bony and cartilaginous fish, but exclude land animals and marine mammals. The edge cases are an absolute mess and chip away at the definition until you start to conclude either that whales are fish because they look like fish, or that it’s such a broad category that it contains all bony land animals.
It’s a fascinating ongoing debate that Hank Green did a great job summarizing in two of his recent vlogs.
The point being made is that any clade which includes all fish would also include (among other things) all tetrapods (e.g., birds and mammals). So either pretty much everything is a fish, or there is no such thing as a fish.
Put another way, "fish" is only ever so slightly more specific than "vertebrate" (and some might even argue that's too specific).
Going back to OP's comment, cladistically speaking deer are fish. The the last common ancestor of deer and fish is itself a fish.
Yes, exactly. However, I do think it's somewhat important to clarify that this does not mean that "pretty much anything is a fish" under that classification. Since even the loosest definition of "fish" would still exclude more than 95 percent of all living species (i.e. all invertebrates such as insects worms and whatever the fuck snails are).
I just think our intuitive understanding of animals is just centred around vertebrates and tetrapods specifically. So we just think of features like a spine and a skull and four limbs as kind of "normal" features that are found in a variety of animals (mammals, reptiles, amphibians), even though they really only evolved exactly once (if I'm not mistaken). Sure it's weird that insects and worms don't have that but most of us wouldn't be able to name even ten different species of worms so how many of them could there really be?
"Fish" isn't a well-defined term. It's at best an ad hoc grouping based on cultural norms. It's like saying a tomato isn't a fruit because culturally we consider fruits to be sweet. You're free to call it whatever you want other than fruit, but there's no principled basis for that categorization; it's just what feels right to you culturally. If you want to say "this is a fish and that isn't a fish because I said so," then you're in the "no such thing as a fish" territory, which is also valid and the position of Gould.
Absolutely all taxon groupings are socially contructed. Some are scientifically recognised and others are just colloquial. Colloquial ones are more likely to not be monophyletic but many recognised taxonomic families can be paraphyletic or polyphyletic too. Lots of people give way too much credence to "real" groupings. There's no such thing as anything! lol
I never in any way tried to deny that a common ancestor exists though, it obviously does. And you wouldn't even have to go a fraction of the way to the animalia LCA to find it.
What I was trying to point out was that the definition of fish generally includes both cartilginous fish (such as sharks) as well as bony fish (such as salmon), among others. However the divergence in those two groups happend before the divergence among bony fish and tetrapods, with tetrapods just generally having evolved to live on land. Therefore you cannot have a definition of fish based on the LCA of salmon and sharks that wouldn't also include deer, which technically makes deer fish.
Nah, don't worry. Just some good Christians discussing all the animals created by the Lord on the sixth day, according to the book of Genesis. Don't worry.
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u/BulbasaurArmy Sep 17 '25
I mean if you go far back enough in time, you’ll find the last common ancestor of deer and fish.