Is a shark a fish?

Factual Questions
21

In terms of years since divergence, there is exactly the same evolutionary distance between goldfish and sharks as between humans and sharks. We belong to the same branch as goldfish and split with the lineage that led to sharks at the same time, maybe 420 million years ago. (In terms of generations, since mammals and particularly the ape lineage have longer generation times than most bony fish, there are actually fewer generations between humans and the divergence with sharks than the number of generations for goldfish).

The lobe finned fishes, the lineage we belong to, split from other bony fish very soon after the divergence of bony fish themselves, the oldest fossils being about 418 million years old.

22

No where. They’re actually called fish STICKS

:D:D

23

Erm… Doesn’t it? I’d have thought that this was exactly the implication.

24

Think of it this way. Waaay back hundreds of millions of years ago there was a little fishy creature. It swam around in the water and had little finny things and gills and so on. Then half these creatures split off and developed into sharks and rays. And half these creatures split off and developed into bony fish. Except, some of those bony fish learned to flop around on land, and then evolved legs, and became mammals.

You can see that even though some bony fish stayed in the water and some moved on land, they should belong to the same group. All members of the bony fish group are related by the same amount to the shark group, even though some bony fish have changed a lot by growing hair and legs.

This is the same concept that people use when they say that humans are apes. We’re more closely related to a chimpanzee than we are to a gorilla. But a chimpanzee is also more closely related to a human than it is to a gorilla. Even though a chimp looks a lot more like a gorilla than it does to a human, in fact a chimp’s closest relative is a human, not a gorilla.

This is a hard concept to understand. Most people can easily grasp that a human is more like a chimp than a human is like a gorilla, but they think that there’s a continuum from gorilla to chimp to human which would make a chimp more closely related to a gorilla than a human is related to a gorilla. But that’s not the case. It’s not a continuum or a ladder, it’s a branching family tree. There’s a branch that splits into two twigs, the gorilla twig and the human/chimp twig. The human/chimp twig then splits into the human twig and the chimp twig. But neither twig on the human/chimp twig is closer to the gorilla twig, they are equally distant from the gorilla twig.

25

And along the same primate line, we’re more closely related to Old World monkeys than Old World monkeys are to New World monkeys.

26

OK, the main point I was refuting was the idea that there is a ladder of evolution starting with bacteria, leading up through worms, insects, vertibrates and of course, with humans sitting smugly on the top. I don’t think this is what was being implied, but I think terms like ‘further up the evolutionary scale’ should be avoided as they are misleading and uninformative. The other thing I’d say is that surely more generations does not neccessarily mean more evolution. Small rodents have hugely quicker generations than us, but many of them have deviated very little from there anscestoral archetypes in tens of millions of years, at least morphologically. I have no idea about genetically, I’d guess that the unread DNA of, say a shrew has evolved much more rapidly than that in slower-maturing species, whereas the sections expressed phenotypically may have changed less. Is this true? Even if it’s not though, if a shrew has completely different genes to ancestral forms, coding for pretty much the same bodies, would you really want to call them ‘more evolved’? I’m sure I’m betraying my ignorance here, but since I’ve already admitted to it, I don’t mind.

27

Can you double-check whether you’re misreading what Hari Seldon wrote? Otherwise, what you wrote doesn’t make sense to me.

28

Yes, I did misread it. Oops.

29

If Hari Seldon was replying to me (it is not altogether clear, but it looks like it) he misread what I wrote too. I quite explicitly acknowledged that we are closer to goldfish than we are to sharks.

Are you saying that there is no other metric of evolutionary distance other than time (or number of generations) since there was a common ancestor? That seems implausible to me.

Quite apart from the sort of morphological similarities and differences that biologists used to rely upon in the dark ages of a few decades ago (do they really count for nothing at all, now?), what about differences in DNA sequences? That, surely, is thoroughly quantifiable. Is it not the case that, as measured by rate of DNA change, evolution sometimes goes quickly in a particular line, and sometimes much slower? Does it not even make sense to say (regardless of whether it is true or not) that sharks might not have evolved all that much since the time they split from bony fishes, but that the line leading to humans has evolved much faster over the somewhat shorter time since it split from the bony fishes?

30

But can you relate to a sea slug?

31

You’re forgetting how evolution happened

The common ancestor of all fish is also the common ancestor of all amphibians, reptiles, birds and mammals.

Are you more closely related to a fish than a shark?

Depends on the fish.

32

Okay, start with the common ancestor of all vertebrates.

The first group to split off gave rise to the modern hagfish, or “slime eel”, a noxious ectoparasite which had a cranium but no vertebrae. Everything else, from lampreys to llamas, is more closely related to each other than any of them are to the hagfish.

The next few groups to split off were the “jawless fish,” including 2. the modern lamprey; 3. the conodont animals (eel-haped filter feeders whose only hard part was the bony framework for the filter apparatus, the so-called conodonts; 4. the Osteostracans, 5. the Heterostracans, and 6. the Anaspids, all three of which were bony-armored groups of jawelss fish on three distinct body plans, each of which radiated into three or four major subgroups. There were also a couple of 7-8. minor groups of jawless fish. Paleontologists will argue at inordinate length about which of these diverged before which others; the sequence I listed them gives one of several common orders of divergence.

This now leaves us with the Gnathostomes, the jawed vertebrates. First group to split off here is 9. the Acanthodians, the so-called “spiny sharks” of the Paleozoic. (Though definitely spiny, they were not sharks.) Next probably was 10. the Placoderms, a large array of armored Paleozoic fish including the Arthrodires and the Antiarchs. Some exxperts don’t believe the Placoderms are a natural group, but rather a lumping together of several lineages of jawed fish that didn’t survive.

  1. Next group to split off were the Chondrichthyes, the cartiliginous fish. This includes two main groups (which may be closely related separate groups), the Holocephalians which are made up of the chimeras or ratfish and two extinct related groups, and the Selachians, which include primitive extinct sharks, modern sharks, skates, rays, the dogfish, the sawfish, etc.

What we’re left with at that point is the Osteichthyes, including on the one hand the Actinopterygians, the ray-finned fish that constitute most modern bony fish, and on the other the Sarcopterygians, including a wide range of extinct forms, the three lungfish, the coelacanth, and all tetrapods (land vertebrates).

We can break this stuff down into more detail, but the fact of the matter is that the goldfish, the sturtgeon, the marlin, the lungfish, the snake, frog, the dinosaur, the sparrow, and you and me, are all more closely related to each other than any of them is to the shark – and more closly related to the shark than to the lamprey or any of the extinct forms noted above.

33

The common ancestor of all fish is certainly a common ancestor of tetrapodes, but it’s not the most recent common ancestor of tetrapodes. The most recent common ancestor of the tetrapodes would have been an amphibian-like thing that was not the ancestor of anything that we would call a fish.

It’s like saying that my great-grandfather is the common ancestor of my sister and I. It’s true, but it’s misleading about our degree of relatedness.

34

I read this and thought it sounded pretty disgusting. Then I googled it wished I hadn’t. I’m never going swimming again.

35

Same place buffalo wings come from :wink:

36

I don’t know - it sounds like a great superpower.

37

Last Saturday, about 2AM, yes. The sea slug had an intellectual advantage on my, though, so it was hardly fair!

38

He said they lack limbs, not digits.

39

This was no boating accident!

40

Furthermore, It was no propeller, it wasn’t any coral reef and it wasn’t Jack the Ripper.