This example is kind of big, so let's work through it. It says the phylogenetic tree below shows the relationships of most modern mammal orders. Placental mammals are highlighted in the blue box, and diversification within orders is shown using triangles. An example organism from each order is illustrated. Alright, so let's take a look at this phylogenetic tree here.
Alright, so as we look at it, each one of these is going to be a different placental mammal order because the placental mammals are in this blue box here. And an order, that's a level of organization, it goes order, family, genus, species. So, that's actually kind of a big group of mammals. It's often dozens of species, more or less, just, you know, depending on the order. And then on this tree, just for reference, we also see the marsupials.
Those are mammals with pouches, and we see the monotremes. Monotremes are mammals that lay eggs, like the duck-billed platypus there. Alright. So, 'a' says the KT extinction event occurred about 66,000,000 years ago. Draw a line across the phylogenetic tree indicating when the extinction event occurred.
Alright. Well, we have our timeline here, and it goes 50, 75, 100,000,000 years. So, 66,000,000 years, that's going to be about here. Alright. So, I'm just going to draw a line across this tree just like that.
Alright. That is my KT extinction event, the extinction event that wiped out the dinosaurs. Alright. Now 'b' here says, at the order level, does the placental mammal adaptive radiation seem to begin before or after the KT extinction event? I'll take a look.
Well, if each one of these is an order, do these branches start before or after the KT extinction event? Well, all the placental mammal orders that we have drawn here, those branches start on the before side of that extinction event. Right? So, I'm going to say here before. Alright.
'c' says, did diversification within orders of mammals occur before or after the KT extinction event? What do you think? Well, our within order diversification, that's these triangles. So, the question that we have is basically on which side of the line do you find the beginning of these triangles? Well, they're all on the after side of the line.
So, within order diversification starts after that extinction event. Alright. Now finally, I'm going to remove myself from the screen here so we can read this one. It says, based on these data, did the KT extinction lead to an adaptive radiation that included new orders of mammals or new species within existing orders? All right.
Now, based on our answers before, what do you think? I'm going to say within orders. New species within orders. Right? It looks from this phylogenetic tree that, you know, starting at about 1,000,000 years ago, we were already having an adaptive radiation of mammals.
That adaptive radiation was well underway by the time this extinction event occurs. Now what happens is after that extinction event, it sort of gets supercharged because there's no more dinosaurs, and we get all this within order diversification. Now, to be clear, there certainly probably was some within order diversification before this line. What happened? All those species went extinct.
Right? It was only probably one species from these different orders that actually survived. Alright. Now our final part of this here, let me remove myself one more time so that we can read it. It says circle three orders on the tree where the KT extinction event seems most closely linked with the adaptive radiation of placental mammals.
Alright? So with that, what we're looking for are really just places where these triangles touch that line, touch that extinction event. So we can go up from the bottom and see whether that triangle starting is sort of really close to that line that I drew. Well, up from the bottom, first, we have the primates here and this triangle, hey, that touches the line. That diversification seems to start right at that extinction event.
So, I'm going to circle the primates here. Right? That's a chimpanzee that's drawn, so I'll circle that one. Next up, we have the lagomorphs. These are rabbits.
Well, there's a little break there. Right? This diversification comes a little bit after that line, but then we have the rodents there, and this triangle seems to touch the line, so I'm going to circle the rodents. Alright.
Going up, we have the perissodactyls. These are odd-toed ungulates, things like horses and rhinoceroses. And then we have the carnivores here, all carnivores. Both of these, well, they don't diversify for a while after. Those triangles don't quite touch the line, but then we get to the artiodactyls here.
Artiodactyls, things like cows, these are the even-toed ungulates. Cows, deer, giraffes are in this group. Hippos and even whales and dolphins. We can see that this one definitely touches that line. So I'm going to circle that one.
Right. I've circled 3, but I can keep going. We have Chiroptera, the bats. That one touches the line. This is a group of, it's actually sort of a small order of moles in Africa, but it touches the line.
We see here, the elephants. It does not come anywhere near the line. The manatees here and our armadillos here, none of those, touch the line. So I circled 5, but we only had to do 3, but we did it. Right?
Those 5 that I circled, it seems that that diversification, that adaptive radiation seems really, really closely tied to that extinction event. Alright. So again, a takeaway here, mammals were already adaptively radiating before the KT extinction event, but it seems to have gotten supercharged once those dinosaurs go away and all those niches open up. Alright. More after this.
I'll see you there.