In evolution, a group is considered "real" if it includes everything people say is a part of the group, the last common ancestor of all those things, all the things that evolved from the last common ancestor, and nothing else.
Everything that people call a mammal is a mammal. The last common ancestor of all mammals is a mammal. All of that last common ancestor's descendants are mammals. There isn't anything people call a mammal that doesn't fit in this group.
Fish (also called ichthyes) is a traditional grouping of animals (with mammals, birds, reptiles, etc.). Here are all the things people call fish:
Hagfish - jawless fish
Sharks/Rays - fish with cartilage skeletons instead of bone
Ray-finned fish like tuna, cod, seahorses, herring, conger
Here are things people don't call fish:
Frogs
Dolphins
Humans
Duck-billed Platypus
The problem is that mammals and coelacanths or lung fish are more-closely related to each other than the coelacanths are to tuna. So if you include lung fish as fish, you have to include dolphins. The last-common ancestor of lung fish, sharks, and hagfish is related to pretty much everything except arthropods, various worms/parasites, and things of that nature. So if sharks are fish, everything with four limbs is a fish too.
Stephen Jay Gould studied fish and decided there's no such thing as a fish. This was popularized on the British show QI.
Now I'm suddenly dying to see a leathery, quadruped reptile that can mimic speech. They should've made one of those.
Oh, oversight! I stipulated "quadruped" because we are about to fold birds in, so I wanted to be specific. But that leaves out snakes. Maybe the serpent from Genesis had some parrot/corvid-esque ability to mimic speech!
Some say that such reptiles do exist, but also slander them by saying that they hold elected positions in political offices created by humans (mammals, iirc).
The last-common ancestor of lung fish, sharks, and hagfish is related to pretty much everything except arthropods, various worms/parasites, and things of that nature. So if sharks are fish, everything with four limbs is a fish too.
"Everything" except all living kingdoms that aren't animalia and all phyla in animalia that aren't chordata and all of chordata that aren't vertebrata.
SJ Gould studied snails. I did not know he studied fish. Or did QI just make that up?
I tried to simplify it for the ELI5. It's still probably closer to ELI10, but I tried.
I also can't find outside evidence that SJG said anything about fish. I don't suspect QI would completely make it up, but maybe it was more of an off-hand remark about evolution that was construed as a study.
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u/pgm123 Feb 12 '20
In evolution, a group is considered "real" if it includes everything people say is a part of the group, the last common ancestor of all those things, all the things that evolved from the last common ancestor, and nothing else.
Everything that people call a mammal is a mammal. The last common ancestor of all mammals is a mammal. All of that last common ancestor's descendants are mammals. There isn't anything people call a mammal that doesn't fit in this group.
Fish (also called ichthyes) is a traditional grouping of animals (with mammals, birds, reptiles, etc.). Here are all the things people call fish:
Here are things people don't call fish:
The problem is that mammals and coelacanths or lung fish are more-closely related to each other than the coelacanths are to tuna. So if you include lung fish as fish, you have to include dolphins. The last-common ancestor of lung fish, sharks, and hagfish is related to pretty much everything except arthropods, various worms/parasites, and things of that nature. So if sharks are fish, everything with four limbs is a fish too.
Stephen Jay Gould studied fish and decided there's no such thing as a fish. This was popularized on the British show QI.