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Unable-Fox-312 t1_iy54t9g wrote

Reply to comment by Dorocche in Eli5: Why do birds and fish come in such a spectacular variety of colors and shapes compared to other animals? by thetravelman888

"There's no such thing as a fish"

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LowRepresentative291 t1_iy5jjpk wrote

To put it in perspective: there are species of fish that are more closely related to humans than they are to some other species of fish

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knowledge3754 t1_iy5tlqs wrote

Please elaborate

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Unable-Fox-312 t1_iy5wwd6 wrote

We use the big sloppy category "fish" to describe all kinds of creatures under the sea. It's like if we bundled together all the chimps and certain kinds of birds and maybe one mushroom and decided those were all called arbs because they like to live in trees. It's a useful word in the real world, but the category doesn't map cleanly to any evolutionary branch

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LowRepresentative291 t1_iy7e36x wrote

All life started in the water. Imagine at some point two species of fish diverged from a common ancestor, species X and species Y. Both became ancestors to many subsequent species. Some descendant species of X (irl: lobe finned fish) eventually came to land, and that's where we descent from. Now, thousands of modern fish species have evolved from species X, and thousands have from species Y. A far descendant from species X might look morphologically similar to a species that evolved from Y (they are both "fish") but it shares a closer common ancestor with humans.

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commanderquill t1_iy7ehpi wrote

Life is created, presumably starting from one species. Then there's an explosion of life. Now all kinds of life. Some have fins. Some have feathers. Some have feet.

Some develop into something else, as different to the ones with fins as humans are. But that design fails. Better to have fins. Now they have fins.

Humans come along and see it and go hey, that's a fish.

But this 'fish' maybe used to be a lizard and then became a fish. So it went:

Step 1: ancestor

Step 2: something else

Step 3: something else

Step 4: lizard?

Step 5: fish?

Meanwhile, 'fish' #2 went:

Step 1: ancestor

Step 2: fish?

So you have one fish that came from a lizard and one fish that came from something else entirely. As a result, you have one fish that has a shared common ancestor with humans say maybe one billion years ago and another fish that has a shared common ancestor with humans three billion years ago. That means fish #1 and humans are related by one billion years while fish #1 and fish #2 are related by three billion years.

Conclusion: some fish are more closely related to humans than to other fish, and the category of fish is meaningless.

This is also true of crabs and trees. Mother nature proves to us over and over again that crabs, fish, and trees are the most superior earthly life forms.

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Dorocche t1_iy5drqh wrote

Well, be careful with that one, because I usually hear that one in the context of denigrating paraphyly, and paraphyly is a useful and valid method of taxonomic classification as long as there are also equivalent monophyletic words.

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Unable-Fox-312 t1_iy5en0l wrote

I was hoping people would search and find my favorite podcast. Obvs there is such a thing as a fish; for the sake of accuracy it's probably better to say for our taxonomy there is no branch that contains all the creatures we commonly call fish while also omitting every creatures we don't call a fish.

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Dorocche t1_iy5et4w wrote

No monophyletic branch. But there's a paraphyletic branch, and a definition based on that won't be any less objective or consistent.

I have heard good things about the podcast, though

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Unable-Fox-312 t1_iy5j9i5 wrote

I assume paraphyletic is a short way of saying basically the thing I just did: "there is a single fish branch if you're okay with a bunch of non-fish in it"

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MrSquiddy74 t1_iy77s15 wrote

Sort of?

Paraphyletic is saying "everything in this evolutionary branch except these things".

Take reptiles for example. In common usage, it excludes birds, even though birds are a subset of dinosaurs, which are a subset of reptiles.

The exclusion of birds from the reptile "group" makes it paraphyletic.

Also fun fact! The most closely related animal group to birds is actually crocodilians (crocodiles, alligators, etc)

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Redshift2k5 t1_iy9eyud wrote

it's a sloppy shorthand, fine as a joke but not an explanation

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