Thanks to the Monterey Bay Aquarium for
partnering with us on this episode of SciShow.
They hope you find it fin-teresting!
[♪ INTRO]
There’s a funny story about beavers.
In the 1700s, a Canadian bishop asked the Catholic church if it was okay to eat
beaver during Lent, a period of time when Catholics abstain from eating meat.
But eating fish was okay and, look, beavers spend so much time
in the water they should totally count.
And, remarkably, the church agreed.
Beavers, at least as far as it mattered during Lent, were fish.
Well, in most cases, people would probably say that beavers
obviously should not count as fish.
They have fur and live babies and breathe air, for instance.
As for what science says.
Oh boy. Uh, see, the thing is, the answer is surprisingly complicated.
Because beavers are not fish, but that might be because either we’re all fish,
or fish kind of don’t exist.
In the end, this is going to be a story about different ways of
grouping animals together, why we use language the way that we do,
and how evolution has made things kind of funny.
For a long time, people grouped animals together
by some common body plan or lifestyle.
Like “fish”, “reptile”, or “bird”.
This is good because it means we can talk about many
different organisms at once without needing to name each one individually.
This lets us make useful general statements, like “It’s fish tacos for lunch today”.
And this is generally how science worked too.
But over the years, as we learned more about evolution,
it became clear that this old way of talking wouldn’t work as well.
There were too many exceptions.
Too many groups of things that seemed similar
but weren’t when you looked under the hood.
Instead, we developed a new, more scientific way of
grouping animals together called cladistics.
In this approach, we sort living things into groups, or clades,
that reflect their evolutionary history and how closely related they are.
There’s a couple of different ways to do this precisely,
but a good example might be to say that, rather than trying to define primates
by things like opposable thumbs or big brains, instead we would say:
“The group of animals known as primates includes all species that are
descended from the most recent common ancestor of humans, apes, and lemurs.”
That’s a bit of a mouthful, but defining things this way
is actually really useful for biologists.
It gives us a way to make predictions, for instance.
Like if two different species are grouped very closely together,
we can predict that their genomes must be relatively similar.
This runs into funny situations, though.
Like, again, the question of whether beavers should count as fish.
Let’s, take, for example, the evolutionary history
of a very-definitely-a-fish fish, like a goldfish.
Its early ancestors were some of the first vertebrates.
Unlike their close more worm-like cousins lancelets,
they had a backbone, which let them swim more powerfully.
As time went on, this group split.
Some, which would become the ancestors of hagfish and lamprey,
stayed roughly how they were.
But the group that led to goldfish would go on to develop powerful jaws.
Over time, more groups split off as more characteristics evolved.
Sharks and rays split off, while the goldfish’s ancestors got bony skeletons.
In another split, lobe-finned fishes diverged.
These are the ancestors of fish with thick,
fleshy paired fins for limbs, like coelacanths and lungfish.
Meanwhile, our goldfish’s ancestors went on to develop fins
made of bony spines connected by skin.
Eventually, we arrive at today, and the familiar goldfish.
So far so good.
And everything we mentioned so far would be in the
category of what most people would call a fish.
But, it turns out, the beaver's evolutionary story is largely the same.
Beavers share the same vertebrate ancestor as goldfish,
and their ancestors also watched hagfish and sharks split off.
And as lobed-fin fish split off, some of them, a group called tetrapods,
used their fleshy limbs to haul themselves onto land,
eventually giving rise to early mammals, and, eventually, to our beaver friends.
And, of course, we humans are in there too!
In the tetrapod group.
Along with every other tetrapod, like frogs,
and alligators, snakes, birds, and horses.
If you zoom out, the tetrapod branch is just one nestled
among a bunch of different fishy clades.
So, back to the question: are beavers fish?
Remember that in the cladistic approach, for a definition to be useful,
we want to include everything that is descended from a common ancestor so that
we can make predictions about who is related to whom on an evolutionary scale.
So this means that, if we want to be able to say that sharks, hagfish,
and goldfish are all fish, we kind of have three options:
One, animals like humans and beavers are fish.
Two, you have to carve out some ridiculous definition, like:
“Fish would be any organism that is descended from the most recent
common ancestor of both hagfish and goldfish EXCEPT for the subset
of that group that is descended from the most
recent common ancestor of frogs and beavers.”
Which is awkward, but it also kind of defeats the purpose of cladistics.
Or three, fish, as a scientific category, don’t exist.
So in the end, are we fish?
Well, we are all vertebrates. And there are smaller categories like “ray-finned fish”
or “sharks and rays”, but there is no single “fish” category in the cladistic approach!
If that sounds like scientists being unnecessarily pedantic, they do it for a reason.
Despite the foibles, the cladistic approach has been a huge success.
It helps scientists talk about the evolution of
life on Earth in a way that makes sense.
And if there are some weird edge cases, it’s not the fault of the system,
but an opportunity to marvel at the history of life on Earth.
And it also means that we use language in different ways
based on what is useful for that conversation.
That means it’s still okay to call a fish a fish, and,
if it’s Lent, maybe even to include beavers in that category.
Thanks again to the Monterey Bay Aquarium for
collaborating with us on this episode!
They think it would beavery cool if you
checked out their social channels or montereybayaquarium.org.
The people at the Monterey Bay Aquarium are o/fish/ial sea enthusiasts
who love to share the wonders of the underwater world,
so they have curated in-person and online exhibits that
anyone with internet access can enjoy.
They range from live exhibit webcams to crafts,
like puppets and coloring pages, to educational videos in English and Spanish!
And if you want to see what the Aquarium calls fishes,
you can surf their webpage called “Animals from A to Z.”
There, they give you the option to filter your environment and just see fishes,
complete with cool facts and conservation practices for each one.
[♪ OUTRO]