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

  • 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

  • branched off early or if these branched separately. And so what they did is they gathered a huge

  • amount of DNA evidence. And so you can see here that this was a very large study done

  • on a number of different families a number of different groups of animals. And they looked

  • at mitochondrial DNA, proteins, ribosomal RNA. They looked at a number of different

  • things and they figured out, this is very recent, that this branch and this branch are

  • actually sister branches. The branch up here that makes us and the branch that makes the

  • jellyfish have kind of separated a long time ago and they have been evolving since then.

  • And so this is a great way, this would be a phylogenetic tree that we can use molecular

  • evidence to answer a problem. But you have to gather a huge amount of data before you

  • can actually do that. And on here you can see that they actually have an out group which

  • is group of fungi which is not an animal, but it's a way that we can actually make comparisons

  • to that molecularly and then we can figure out the connections. Okay. Last thing I said

  • I would talk about are cladograms. Cladogram uses what is called a clade. And a clade,

  • if I remember right comes from I think Latin. It means a branch. And so a clade is simply

  • a group that has an organism and all, and I would circle the word all, of it's descendants.

  • And so this right here is a clade, because it has this organism and all the descendants

  • that come from that. Where this, the orange, is going to be a clade because it has this

  • organism and it has all of the descendants that come from that. But green, right here,

  • we would not call this a clade and the reason why is that you would have this organism right

  • here and all of these descendants but you're missing a number of them over here. And so

  • that's not a true clade. And so cladograms are going to, it's the definitive answer of

  • who's related to whom. But you use two things. You use molecular and DNA for sure, evidence,

  • but the other thing that you're going to use are something called synapomorphies. And a

  • synapomorphy is going to be a characteristic that's shared by all of those in the clade.

  • So a couple of good ones as we look through the fossil evidence of dinosaurs, we branch

  • all the dinosaurs into two groups. The ornithischia and saurischia. And these are all, saurischia

  • if I remember right means bird hipped and ornithischia means lizard hipped. And so it's

  • a way to branch these groups and so a synapomorphy would be this characteristic. In other words,

  • saurischia is going to be in this hip structure, is going to be shared by everything in this

  • clade. And so the goal is to have a clade that has similar characteristics and it also

  • doesn't leave anything out. A real example that you're probably familiar with would be

  • reptiles. Reptiles is a silly term because reptiles used to be this blue area right here.

  • It contained things like turtles, crocodiles and birds we left out of that. And so if you

  • look at this, this group is what's called paraphyletic and so reptiles as a group was

  • paraphyletic. It had all of these descendants but it lacked the birds. And we now know that

  • birds are apart of this group. And so the goal of a cladogram is to create what are

  • called monophyletic groups. Monophyletic would be this, yikes, would be this yellow group

  • right here because it contains this and all of the descendants of that, that move from

  • here. So polyphyletic means you have groups that come from different areas. So if we were

  • to put mammals and birds together in one group that would be polyphyletic. And paraphyletic

  • is when you have some organisms but not others. And so what's the goal? The goal of a cladogram

  • is to figure out all of life. So we put all of life on branches and we figure out who's

  • related to whom. And then, hopefully we can use a naming system and classify all of life.

  • It seemed like a daunting task at one time but molecular evidence is giving us an in

  • roads to that and it's a really hot topic as far a biology goes today. So that's phylogenetics

  • and I hope that's helpful.

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

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系統学 (Phylogenetics)

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    Bing-Je に公開 2021 年 01 月 14 日
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