Fish Don't Exist
Recently, the internet has learned me another little thing to ruin conversations with: the fish don't exist1 attitude.
If you are anything like me you can already feel your temper prickling. Outrageous, right? Obviously, fish exist! In fact, some of my best friends are fish. And yet, there’s a teasing allure to the absurdity of this claim - an almost irresistible pull of curiosity. So, what the hell is this about?
Now, humans have been calling things "fish" for centuries: One Linnaeus grouped organisms by observable traits - fins, gills, scales - and called them Pisces.2 Convenient. Intuitive. But utter bollocks. At least in terms of modern evolutionary terminology.
The reason is that modern taxonomy is phylogenetic:3 a valid taxonomic group (a clade) must include all descendants of a common ancestor.4 That’s the definition of a so-called monophyletic5 group. The thing is that "fish" fails this criterion: it is a paraphyletic6 group of organisms,7 leaving out all tetrapods - frogs, birds, mammals, and yes, even us - despite the fact that we all share early lobe-finned ancestors. Sharks (cartilaginous fish) split off way earlier.8 Salmon? Closer to us than to sharks. Eels? Their "fishy" form is just convergent evolution.9 Because monophyly is the standard, "fish" isn’t really a valid taxonomic group. To make it so, we’d essentially have to include every living thing on earth including us humans.
So unless we’re ready to start calling ourselves "fish", the taxonomic group of fish simply doesn’t hold. And if that isn’t something to bring up the next time you go on a fishing trip, a sushi night or a zoo visit, I don’t know what is.
Footnotes:
As far as I can see, this claim was popularised in public discourse by Lulu Miller in her book Why Fish Don’t Exist (2020).↩
Pisces — This is just Latin for "fish", used by Linnaeus in Systema Naturae (1758) to group all finned aquatic animals.↩
Phylogenetic — from Greek phylon ("race, tribe") + genetikos ("origin, birth"); relating to evolutionary relationships and common ancestry.↩
Common ancestors — an evolutionary organism from which multiple lineages descended.↩
Monophyletic — from Greek mono- ("one, single") + phylon ("race, tribe"); a single lineage including an ancestor and all of its descendants.↩
Paraphyletic — from Greek para- ("beside, near, alongside, but not included") + phylon ("race, tribe"); a lineage that includes some, but not all, descendants.↩
Such a generalised "group of organisms" is sometimes called a taxon — a neutral term for any named group, whether it is monophyletic or paraphyletic.↩
Man, sharks. Younger than the mountains, but older than the trees. I'm not even kidding; John Denver would have had a field day with that one. And if that isn’t enough, consider this: the oldest known individual Greenland shark might well have been cruising around while Shakespeare was writing Hamlet in the early 1620s. Another day, another conversation killer.↩
Convergent evolution — when unrelated species independently evolve similar traits, often because of similar ecological pressures.↩