Name: 生命の木はメチャクチャ (The Tree of Life Is Messed Up)
Uploaded: 2021-01-14T10:28:46.000Z
Duration: 11 min 35 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

様態の副詞

In school, you likely learned about the ranks in the tree of life with

some kind of clever mnemonic, like Dear King Philip Came Over For Great Soup.

This probably gave you the impression that those ranks were pretty important.

Fixed, unchanging steps on the path from general to specific.

And of course, one phylum or order must be treated the same as any other.

and sorting them into groups is called taxonomy.

And it's a foundational part of studying life on Earth.

to understand evolutionary relationships and preserve biodiversity.

But it's also more complicated than it seems on the surface.

Trying to shoehorn the messy, complicated web of interrelationships that is biology

into neat boxes has resulted in a pretty messy tree of life.

And straightening it out is going to take time and effort.

Time and effort we're going to need to put in,

to name organisms before they disappear forever.

If you remember that King Phillip thing, you might also remember that taxonomy

got its start in the 1700s due to the work of a Swedish botanist named Linnaeus.

He came up with a scheme to name and categorize life on Earth,

with ranked categories and binomial, two-part names,

That system, now updated to reflect our understanding of evolution,

forms the basis of what's called rank-based taxonomy.

It runs from the very high-level domain down through

kingdom, phylum, class, order, family, genus, and species,

with each describing more narrowly defined groups of related organisms.

The names and naming process are regulated by international codes,

like the ICZN, or International Code of Zoological Nomenclature.

by describing characteristics shared within the group,

which can be anything from having a cell wall to laying eggs with shells.

but there are some weird quirks to doing things this way.

Part of the problem arises from using a system and groups invented before Darwin,

and trying to paste in evolutionary relationships over top of them.

One of the ways scientists have updated the tree of life to reflect evolution

is to try to keep groups like orders or families monophyletic.

That means everything in the group is descended from a single common ancestor,

and the group contains all the descendants of that ancestor.

can easily and quickly tell you about evolutionary relationships.

Like, giant pandas are in the same family as bears, but not red pandas.

So giant pandas are more closely related to a black bear than a red panda. Easy.

Consider the class of vertebrates called Reptilia.

It includes animals like alligators, turtles, and snakes;

pretty much all of what we'd traditionally consider reptiles.

In day-to-day life, it's usually not a problem to lump all of these critters together.

They're all scaly, generally cold-blooded, and generally do similar things.

But when we start to think in terms of evolution, things get a little trickier.

Because reptiles as a group are what's known as paraphyletic.

In a paraphyletic group, all members of that group descend from a common ancestor,

but the group doesn't contain all descendants from that common ancestor.

Someone really important is missing when we talk about reptiles: Birds.

Birds, which are descended from dinosaurs,

are actually an off-shoot of the reptile family tree.

In fact, crocodiles, which we think of as reptiles, are more closely related to birds

than they are to turtles, which we also think of as reptiles.

And yet birds have their own class: Aves.

You can't have a class within a class, so birds can't be included under Reptilia.

This makes Reptilia a paraphyletic group.

So if you were simply comparing one class to another, you might not realize

how close birds and crocodiles are, nor how ancient and unique turtles are.

Taxonomy should ideally help us understand how life evolved,

but in cases like this, it can be misleading.

Incidentally, we've mostly phased out “fish” as a group,

since in order for it to be monophyletic, we'd have to include humans.

Because we're also descended from the same ancestor.

Over time, we've focused more and more on these systematic relationships,

and we've retired some old groups when they stop making sense.

But it's an ongoing process, and we still have wonky groups like Reptilia.

There are also some weird problems with the idea of the ranks themselves.

At the most basic level, there are tricky problems with how we define species.

But there are also questions about how some of those

higher-order groups are organized as well, like orders or phyla.

That's because there isn't an objective test

for whether something should count as a class or phylum.

It's not like 99% DNA match is a genus, 95% is a family, 90% is a class, et cetera.

This can create situations where groups of the same rank

aren't comparable to one another; they're not equivalent.

Take Cnidaria, a group that includes critters like jellyfish and coral.

Cnidaria is a phylum, a high-level rank just below kingdom.

you might be tempted to think of them as roughly “equal”.

Like, maybe the groups are roughly as old as each other,

or contain about the same amount of genetic diversity,

But depending on how you look at it, Cnidaria is actually a much bigger group.

In fact, the genetic difference between true jellyfish and box jellyfish

is about the same as between a human and a sea urchin;

a group that belongs to another phylum altogether.

So, again, for a system that's supposed to help us understand the world, it's got some serious flaws

Now, don't get me wrong: Taxonomy is really useful.

Life is a continuum, and classification is a tool that we use to understand it.

Without it, we'd struggle to describe the organisms around us

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生命の木はメチャクチャ (The Tree of Life Is Messed Up)

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