Darwins_Finch:
keeganst94:
The fish thing has come up before on some science Youtube channels I watch, and the discussion is usually around how a term that is so basic about something so important to humans is basically meaningless. “Fish” is so broad and arbitrary that it’s basically impossible to make a proper definition of it, which is somewhat unusual even for paraphyletic (not evolutionary based) groups. Like, we’re more closely related to bony fishes than sharks are to bony fishes, and that’s also true for lobe-finned fish and lung-fish compared to ray-finned fish, but we group all of them together and not us. So it’s not evolutionary, but it’s also not about body-plan because of things like lampreys, starfish, or jellyfish. You can’t say a fish is “all mobile multicellular aquatic organisms” because most people wouldn’t consider lobster or clams fish. You can’t say it’s about swimming because of things like octopus, salps, or krill. It just comes down to the old porn thing: “I know it when I see it”.
Some random ramblings on ‘fish’, because I’m bored:
Agnatha (“jawless fishes”)
Chondrichthyes (“cartilaginous fishes”)
Osteichtyes (“bony fishes”)
The problem there is that two of those three groups are paraphyletic - that is, they do not include all the descendants of a parent group.
Agantha (mostly extinct critters, but with some extant clades such as hagfish and lampreys) is paraphyletic because all veterbrates evolved from aganathan ancestors. So, taxonomically, either Agnatha is paraphyletic (and thus not much use for inferring evolutionary history), or, if one wished to make it monophyletic and include all the descendants, it becomes synonymous with Vertebrata which is not particularly helpful.
Chondrichthyes (sharks, rays, etc.) is a monophyletic clade, so we’re good here.
Osteichthyes contains two main groups of “fish”: Actinopterygia (ray-finned fish) and Sarcopterygia (fleshy-finned fish). Actinopterygia is monophyletic (The vast majority of what people tend to associate with “fish” are probably Actinopterygians); Sarcopterygia…not so much. Since terrestrial vertebrates (aka Tetrapods) originate from this group, excluding them to only include aquatic members makes them paraphyletic.
So, on the one hand, we can say that “fish” is simply taxonomically invalid because it is paraphyletic. On the other hand, we can add some qualifiers, sort of how “non-avian dinosaur” has become vogue when talking about the “traditional” dinosaurs once we started (properly) grouping birds as dinosaurs; thus, we can say that “fish” includes all non-Tetrapod vertebrates. Still not terribly useful scientifically, but it can be useful as “short-hand” taxonomy. And it then excludes invertebrates, such as molluscs, echinoderms, sea jellies, etc., and an assortment of other swimmy things (e.g., sea snakes, amphibians, cetaceans, etc.). Not that that valid taxonomy has ever stopped folks from referring to “shellfish” and “starfish” and “jellyfish”…
Everything above is correct, and I’d like to make explicit a useful concept: grade . A “grade” is a fuzzy collection of features that describe what a species does and how. The stereotypical grade is “fish”, that is basically that any animal that moves about in the sea. Or, “tree”, which is any tall self-supporting plant.
A grade is useful for communicating with other people. “I caught a fish while sitting under a tree” is a useful statement in some contexts even if “fish” could mean Glyphis siamensis or Henicorhynchus siamensis and “tree” could mean Pinus palustris or Quercus palustris . We don’t often need scientific precision when describing our afternoon’s pasttime.
In terms of utility in describing evolutionary history, the clade , a group of organisms descended from a common ancestor, is superior. Scientists use typically use clades because nature doesn’t make sense without taking into account evolution and using clades makes that easier.
A grade is a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit. The concept of evolutionary grades arises in the context of phylogenetics: the study of the evolutionary history and relationships among or within groups of organisms. These relationships are determined by phylogenetic inference methods that focus on observed heritable traits, such as DNA sequences, protei...
In biology, a clade (/kleɪd/) (from Ancient Greek κλάδος (kládos) 'branch'), also known as a monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach to taxonomy adopted by most biological fields. The common ancestor may be an individual, a population, or a species (extinct or extant). Clades are nested, one in another, as each branch in turn splits into smalle...