Name: 系統学 (Phylogenetics)
Uploaded: 2021-01-14T04:47:31.000Z
Duration: 12 min 45 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：

Hi. It's Mr. Andersen and welcome to biology essentials video number six. This

様態の副詞

is on phylogenetics. And phylogenetics is essentially the evolutionary history or the

evolutionary family tree of organisms. If you look on this page we've got a number of

pictures, we've got a number of pictures of cetaceans, so a bunch of whales. So the goal

of phylogenetics is to create a phylogenetic tree. In other words a tree that shows who

is related to who. In other words, is the humpback whale most related to the gray whale

or to the minke whale or to the fin whale. And so phylogenetics is actually a really

fascinating area right now because with all of the DNA evidence that we have we're able

to put together a wonderful picture. And our goal is that the phylogeny will match taxonomy.

In other words we can give names to organisms based on who they're related to. But the more

you learn about this the more you realize that all life is very very similar. We have

similarities between all of it. So let's get going on phylogenetics. And so basically speciation

is when one organism or one population or one group eventually diverges and so they

can't interbreed any more. It's the simplest way to think about speciation. And so phylogenetics

and phylogenetic trees require speciation to have occurred. There are a number of different

phylogenetics trees. The one that we'll talk a lot about are called cladograms and they

use what are called clades. And so it's just a specific type. But a phylogenetic tree shows

the evolutionary history of an organism. Now how do we figure that out? Well we could use

all the tools at our disposal. I am going to talk about two specifically today. One

is morphological. Morphological is the structure that you have. And the other is molecular.

And so morphologically I am going to talk about hearts and how hearts have changed over

time as they've required more and more things. And then the last is molecular. In other words

how do we use DNA to figure out who is related to whom. So those are phylogenetic trees.

This is the evidence that we use to figure it out. And then at the end I am going to

talk more about cladograms and how you create a cladogram. And so let's start with the biggest

phylogenetic tree of all, phylogenetic tree of life. So if this is life down here, so

if life began you know 3.6 billion years ago, it's diverged into all these different lineages.

And so this right here would be a phylogenetic tree. A phylogenetic tree of life. Now the

one thing I want to point out and Darwin was the first person to do this, is that whenever

you have a tree that suggests that there's common descent. And what does that mean? Well

bacteria, archaea and eukarya, since they're all on the same phylogenetic tree, it means

that they all came from that common ancestor. And so every time we have a branch point on

here, so what does this suggest, that branch point right there suggests where that tree

diverged into the eukarya and then the archaea that we have. And so the idea of descent is

a long one, but the more evidence that we gather the more we realize that Darwin was

right on. Now we have to figure out who is related to who. And so let's start at evidence

that we have. So the evidence that we can use to make a phylogenetic tree, let's start

with the first one and is morphological. Morphological are the structures that you

have. And so this is a phylogenetic tree of vertebrates. And so we've got early vertebrates,

we've got time periods over on the side, but we're most interested in the mammals. We've

got birds, reptiles, amphibians, fishes. And so how did scientists figure out who's related

to who? Well, we can choose one characteristic and then we can trace that through time. We

can look at one thing and see how it's changed over time. So a perfect example would be the

heart. The heart began in fishes as a two chambered heart. A two chambered heart really

just has one valve. In other words, the blood is going to flow in this direction and then

there's a valve that opens in this direction, so once the blood moves through it, it can't

come back in. And so it's just a muscle that has a valve on the inside of it. And so what's

the function of the heart? Well in a closed circulatory system, in other words, insects

don't use a closed circulatory system, they use, their blood just goes everywhere in through

the tissues. But in a closed circulatory system, in a fish, the blood, which in blue in this

case just means that it is deoxygenated, is going to go through the gills, and then it's

going to be oxygenated, so it's red, and then its going to go through the body and the tissues

in the body, and then it's going to drop off that oxygen and then it comes back to the heart

again. And so a two chambered heart, a better way to think about what a two chambered heart

is, is it's simply a single loop. So we just have this one loop through the gills and back

to the body. And for fishes that works great. The problem is that as we move on to land

there are quite a few more constraints as you move, especially as you move towards being

like a warm blooded organism. And so the constraints get heavier and heavier and so it's okay to

have a two chambered heart, works great if you're a fish, but as we move on to land then

it has to modify itself. And so again we just have this one loop. The major problem is that

once it goes through the gills you loose a lot of the pressure. And so you loose the

pressure and so it's hard to move that through the rest of your body. Works great if you're

floating in water, but as you move on to land, we don't have that pressure. Okay, so let's

go to a three chambered heart. Now three chambered heart arrives in the amphibians. And so things

like a frog have a three chambered heart. So they've got three delineations. We've still

got that loop that goes through the lungs and mostly in amphibians it actually goes

through the skin where they pick up oxygen. But you see that we now have a problem here.

We're not losing that pressure, in other words we're able to pump the blood to the skin and

the, excuse me, we are able to pump it to the skin and the lungs and then we have a

separate loop that goes through the body, so we still don't have to deal with that pressure.

But the problem comes in right here, and that is that we have a mixing of the oxygenated

and deoxygenated blood. And so it's purple. Now is that a problem? Well it's a problem

if you are anything above, or spend more time on land then amphibians do. And so it works

great for amphibians, but you have a mixing of oxygenated and deoxygenated blood. And

so if you look at this loop, it's a double loop, but if you look at it and say okay,

now let's move up to the reptiles and now we have to move more blood and we don't want

as much of this mixing here because we're going to lose a lot of the oxygen. Well think

about it as an engineer, how could you solve this? Well a three chambered heart works like

this and what it does is it has a septa that's built right down here in the middle of the

heart and that septa separates the deoxygenated from the oxygenated. It still has a little

bit of mixing of the blood but that works great because they're cold blooded critters

and so as they move that body, that blood around their body they can actually keep themselves

a little bit warmer. But that's a three chambered heart and as we move on to endothermy, as

we move on to birds and mammals, that just doesn't cut it. And so we think that birds

and mammals both evolved this independently and you can see on here that birds branched

off from reptiles and mammals branched off earlier from a common ancestor. And so we

eventually have the arrival of the four chambered heart. What's the four chambered heart do?

Well you can see that that septa that went right down the middle has completely closed

off. So we don't have any mixing of the oxygenated and deoxygenated blood. And so birds and mammals

have this morphological change and they did it because they're endothermic. In other words

they require a constant body temperature. And so we can trace this morphological evidence

through the organisms and we can say who's related to who. In other words is we have

a three chambered heart that's shared by everything above here, that means that on our phylogenetic

tree we want to at least put those on the same branch. Next I want to talk about molecular

data. So molecular data is looking at the DNA. So looking at the genetic code. And so

this is a study that was done in 2009. And what they were trying to figure out is where

metazoans fit and who is related to whom. And every time I have a new biology book I

find that this is actually organized a little bit differently. But we have this group down

here of the, so the jellyfish and the sponges down here on the bottom. And then we have

this group up here which contains things like us. And so scientists weren't sure if this