Adaptive Radiation - Online Tutor, Practice Problems & Exam Prep

Adaptive radiation is an evolutionary process where a single lineage diversifies into many species, often filling ecological niches after mass extinction events. A prime example is the Cambrian explosion, which introduced nearly all animal phyla around 530 million years ago, leading to complex body plans and diverse life forms. Key factors include ecological opportunities and evolutionary innovations, such as the development of homeotic genes like Hox genes, which regulate body structure and promote rapid diversification. This highlights the dynamic nature of evolution and the intricate relationships between organisms and their environments.

Adaptive Radiation

We've been talking about extinction, but almost the inverse of extinction is going to be adaptive radiation. Now, adaptive radiation, we're going to define as evolutionary events where a single lineage gives rise to many diverse species. So where extinction, we said metaphorically, kind of prunes the tree of life, adaptive radiation, we're going to say rapidly expands the tree of life. You can think of it as making the tree of life bushier in certain places. This happens when a lineage diversifies to fill many ecological niches.

Many ecological niches, and these niches are often open to be filled after mass extinction events. All right, so we can look at our image here. We have this phylogenetic tree, and we've looked at this tree before. We see here in red, right? We see all these lineages going extinct at the same time.

Those lineages ending. That is our mass extinction. But then look there in green, right? Look right here. Well, here we have what looks like a single lineage that makes it through that extinction event, and then it immediately starts diversifying.

This lineage starts splitting into all these speciation events, and you get this real bushy part of the tree, all these new species evolving. This is our, meant to write that in green, adaptive radiation. Alright. So why do these happen?

We're going to give two basic reasons here. First is the ecological opportunity. Now, this happens when species fill those open niches like after a mass extinction, but it's not just after mass extinction events. This often happens, for example, on volcanic islands when a new island emerges from the ocean. The species that get there first is often a little bit random, but there are lots of open opportunities, open niches to fill, and those species will diversify.

They'll go through speciation events filling those niches. A classic example of this is Darwin's finches on the Galapagos. But the example we're going to look at are the Hawaiian Drosophila. There are actually all sorts of examples from the Hawaiian Islands because these are large volcanic islands that have had many adaptive radiations. The Hawaiian Drosophila, there are about 1000 species of Drosophila of these fruit flies that live in the Hawaiian Islands.

That's about a quarter of all Drosophila species on the entire planet, right? A quarter of all Drosophila species you find on the Hawaiian Islands. What are they doing there? Well, they're doing all sorts of stuff. Some of them are living that very standard fruit fly lifestyle.

They're laying their eggs on rotting fruit, but others have evolved to fill open niches. Some lay their eggs only on rotting bark, some on specific species of leaves. There are some flies that only lay their eggs in spider egg sacs. Right? That's very different than you find in Drosophila in other places in the world, but that's because when they got to those islands, there were these open niches and they diversified to fill them.

Alright. Now it can also happen because of an evolutionary innovation. This, we are going to say, is when a novel or, you could say, new trait or group of traits, as I'm indicating there with an up arrow, increases fitness. Now, these organisms are able to outcompete other organisms in the niches that they live in. An example of this is the angiosperms, also known just as the flowering plants.

Alright. So what's an angiosperm? It's almost easier to tell you what isn't an angiosperm. Ferns are not angiosperms and conifers, things with cones, like pine trees, those aren't angiosperms. The vast majority of other plants that you are likely familiar with are going to be angiosperms.

That includes things like trees with leaves. Almost all trees with leaves are flowering plants. All cacti are flowering plants. And things with flowers, right? Tulips there, all the grasses, water plants, those are all flowering plants, or almost all of them are. That's a huge group of organisms, a really diverse group of organisms.

Why were they able to outcompete? Well, part of it's in the name, right? Angiosperm. 'Sperm' refers to the seed, and 'angio' well, that means something like a vessel. The seeds of angiosperms have a protective coat, and they are in a fruit.

That allows them to disperse more widely and often last much longer. Now, there are also other adaptations that they have. Well, they have flowers, for example, so that's a different way of reproducing. They transport water through the plant differently than other plants do. Altogether, this group of traits allowed them to outcompete the ferns and the conifers that were all over the world when this adaptive radiation happened.

This happened only about 100,000,000 years ago during the Cretaceous period. So just think about that for a second. That means this oak tree, this oak tree is way more closely related to cacti, to tulips, to wheatgrass than it is to a pine tree. Think of the amount of diversification, the splitting in that tree of life that had to happen to get all of these things coming from a common ancestor in what is a relatively short amount of time in the history of life.

Alright. We're going to look at adaptive radiation more and examples and practice to come. Do check them out.

Adaptive Radiation Example 1

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.

Which of the following statements are true?

I) Adaptive radiations often follow mass extinction events as organisms adapt to fill previously occupied niches.

II) Adaptive radiations are common on new volcanic islands where organisms can occupy many new environments.

III) The evolution of a new trait can trigger adaptive radiation as organisms are able to outcompete resident organisms in many different environments.

• A. I & II only.
• B. I & III.
• C. II & III.
• D. I, II, & III.

Which of the following would be considered an example of an adaptive radiation?

• A. Finches on the Galapagos are thought to have evolved into many different species from an original founder population.
• B. The peppered moth evolved to have dark coloration in response to an environmental change in industrial England.
• C. Native human populations tend to have less genetic diversity the further away they are from Africa.
• D. Mass extinction events are usually caused by worldwide habitat destruction from things such as meteorites or extreme volcanic activity.

Which adaptive radiation is correctly matched to the extinction event that preceded it?

• A. Adaptive radiation of dinosaurs: late-Devonian extinction event
• B. Adaptive radiation of flowering plants (angiosperms): Ordovician extinction event
• C. Adaptive radiation of mammals: end-Cretaceous extinction event
• D. Adaptive radiation of Cambrian animals: end-Permian extinction event

Cambrian Explosion

If there is one adaptive radiation that you're likely to need to know about, it's going to be the Cambrian explosion. Now, the Cambrian explosion is an adaptive radiation that introduced nearly every animal phylum. And a phylum, that's the level of organization just under kingdom. So these are major animal groups that are coming around at this time.

So, some examples of animal phyla that arrived in the Cambrian include things like the chordates. That's anything with a nerve cord running down its back; you're a chordate. Things like the mollusks, which include snails, clams, squids, and octopuses, for example. Things like the arthropods, which include crustaceans and insects. So these are major animal groups that sort of burst onto the scene about 530,000,000 years ago. Now, we think this Cambrian explosion takes something like 10,000,000 years, but it's sort of centered around that 530,000,000-year mark. Alright. Now this starts the Paleozoic Era.

So, the Paleozoic Era, if you remember when we zoomed in on our timeline of life, starts with the Cambrian and goes until the end-Permian extinction event. So, what was life like before the Cambrian? Well, animal life was mostly soft-bodied. There were a few known predators. Most animals were probably just fixed and stuck on the ocean floor. And there were many sponge-like creatures. Now sponges are animals, and sponges were around before the Cambrian. And what you have around a lot are things that look like this is the classic Precambrian animal there.

Now, when you look at that, you probably don't look at it and think it's an animal. Right? But then the Cambrian comes, and all of a sudden, life is in color. All of a sudden, animals have hard bodies and hard parts. All of a sudden, you have many predators on the scene. You have much more complex body types, things that you would recognize as animals because they have limbs, arms, and legs. They have jaws. They have the first nervous systems. And now, when I say that life was in color, I almost mean literally because we have the first eyes at this time. This is all sort of shown in this trilobite here. Trilobites came around in the Cambrian.

They were this group of animals that were really numerous in the oceans for a long time until the end of the Permian. But they have these hard bodies. They have those first eyes that we find in the fossil record. And when you see them, yes, they look a little weird. They're not like what's around today, but they're clearly animals.

Alright. So, what caused the Cambrian? Well, remember, we say adaptive radiations can come from either ecological opportunity or new traits that increase fitness. But we think the Cambrian is probably from both of those, and almost sort of working together in almost like a feedback scenario. So first off, before the Cambrian, in the millions of years before, there was this big rise in algae, as I'll indicate there with an up arrow.

Now algae are photosynthesizers, and so they make a lot of oxygen. So that increase in algae caused a rise of oxygen in the atmosphere. Now more oxygen means that animals could potentially get bigger. More oxygen means that animals could potentially be more mobile. And now, this algae also could serve as a food source. So now you potentially have a bigger base to your food chain. Also, we've already noted the rise of predators and predators—animals eating other animals— drives diversification because animals now have to protect themselves. Classic Cambrian animals tend to be hard-bodied and really spiky because they have these first predators that they're evolving defenses against.

Now, we also are just going to say that diversification itself could create new ecological niches. As things are mobile, now they could swim up into the water column instead of just being stuck on the ocean floor. That's a whole new ecological niche. And well, predators could chase them up in the water column. That's a new ecological niche for those predators. You could have predators of predators, which couldn't have existed before. It's just this idea that the more organisms are around, well, that creates new niches for other organisms to evolve.

And then finally, we want to note that there was this new set of animal developmental genes that evolved either early in the Cambrian or slightly before the Cambrian. And these genes are present in many of these animal phyla that burst onto the scene at this time. Now, we're going to look at this in more detail in a future video, but the idea here is that these new genes enabled complex body plans to evolve, or at least enabled these complex body plans to evolve more rapidly. Alright, so to take the takeaway here: right before the Cambrian, animal life is pretty boring. And then we get the Cambrian, and animal life gets crazy, right?

We got animals eating animals. Life is mobile. It's a party. Alright. With that, we've got an example and practice to follow. I'll see you there.

Adaptive Radiation Example 2

Our example says, for each of the possible contributing reasons for the Cambrian explosion below, write why you think they could facilitate adaptive radiation. Alright. So let's go through them one by one. The rise of the predators. How could the rise of the predators cause adaptive radiation of other organisms?

Well, when we said there are more predators, well now other animals have to evolve defenses. Let's say evolve defenses. Right? When there's predators, well now things gain those hard bodies. They get spiky. They can get more mobile. Right? The more predators there are, the more ways there are to try and get away from the predators, and that could lead to some adaptive radiation. Alright, well, what about an increase in oxygen? What do you think there?

Well, an increase in oxygen, we said could allow for animals to get bigger and more active. Bigger and more active animals, I'm going to say. And as animals are bigger and more active, that allows them to exploit new ecological niches. Alright, our final one here, new animal developmental genes. How could that add to an adaptive radiation?

Well, we said that those new animal developmental genes allowed for more complex body types. Alright. With that, we have more practice after this. Give it a try.

Which of the following statements about the Cambrian Explosion are true?

I) Before the Cambrian Explosion, most organisms had soft bodies.

II) The Permian extinction created new ecological niches that were exploited during the Cambrian Explosion.

III) After the Cambrian Explosion, new morphological features like limbs and jaws are observed.

• A. I & II.
• B. I & III.
• C. II & III.
• D. I, II, & III.

Developmental Genes & Regulation

Now I want to take a little bit of time to look at developmental genes and gene regulation. Because when we're thinking about adaptive radiation, we just want to know, well, how do things evolve so quickly? How do they differentiate from each other so quickly? Well, what we want to start off by saying is that many evolutionary changes are changes to gene regulation. Now I know when I think about evolution, I'm often thinking about a mutation to a coding region in a protein.

But what we're saying here is that a lot of mutations are to the regulatory regions or to a regulatory gene itself. Now why that's important is because that means that what we're doing in evolution is not always, you know, evolving new proteins. We're just changing how much proteins are used or when they're used in development. And this is important because small changes, we're going to say early in development, well, those can have really large impacts going forward. Right?

If cells are using different proteins early in development, that can have big implications for how that organism develops going forward. Alright. So now to think about this, I want to bring back this idea of homology. Remember, a while ago we looked at vertebrate limbs, and we have here a bat wing, a human arm, and a whale fin. And we can see that they have the same underlying structure.

Right? They have the same bones. So what's different about them? What's different is how the growth is regulated. Right?

In this bat wing, these bones grow long and thin. In the whale fin, they grow thick and relatively short. And, you know, we see another difference, right, in the bat wing. Right? We're just going to grow more skin between them.

And the whale fin, you know, the tissue is going to grow thicker and and more tough. But again, we're not inventing whole new body parts, whole new structures. We're just growing things at a different rate. Now that idea we call heterochrony. Hetero means different and crony refers to time.

So what this says is that changes in the rate or the timing of development, that's going to be responsible for a lot of the evolutionary changes that we say that we see rather. Different structures, we're going to say they are not evolved new every time. Different structures generally just come from different growth rates. All right. So as we're thinking about gene regulation, we need to call a very specific type of regulatory gene, and these are called homeotic genes.

Homeotic genes are genes that determine the specific organization of body parts in organisms. And these are going to be transcription factors, genes that control the expression of other genes, and lots of different organism patterns. So for example, in plants, there are homeotic genes that control you know, what parts of the flower develop into different things. So, you can imagine though, you know, there's a homeotic gene that says, hey, develop into a petal. Well, that means that changes in these homeotic genes, they can cause large changes in a short time.

Right? You can imagine a change that homeotic gene could cause that flower to develop very differently. Now in animals, we have a very specific group of homeotic genes that we need to call out and talk about. They are called the Hox genes. Hox genes control animal body plans.

Now not all animals have Hox genes, but a lot of those animals that we see bursting out of the scene in the Cambrian have them. So we're going to say here, they are first seen in these Cambrian phyla. And one idea is that, well, these Hox genes had to evolve probably just before the Cambrian. And then this set of genes allowed for the diversification of animals as part of that Cambrian explosion because these genes determine the body plan in a head to tail orientation. And the idea here is well, we can see these Hox genes drawn out in a couple phyla here.

We have the mollusks, that snail there, an arthropod, that fly, and we have chordates with that mouse there. And what we see, well each colored box here represents a Hox gene. And you can see in that in all these organisms, well, there's the same order of genes in the chromosomes. We basically have the same genes in the same order on the chromosome. Now it's not exactly the same.

There are some duplications. You can see, you know, one's missing there. You can see in vertebrates. This entire set of genes has been duplicated a couple times, so we have a lot more Hox genes. But they're basically there in all these organisms doing the same thing.

So what are they doing? This first one, well, it sends out a signal. When it's transcribed, it sends out a signal and say, hey, cells. You're in the front of the head. And then going down each one during development sends out that signal that you're in a different body part going down head to tail, and this last one, it sends out the signal.

Hey cells, you're in this segment that's going to become the end of the tail. Now that idea, this tells organisms how to develop. That means that these animals have really detailed information as to where those cells are in the body plan. That allows those cells to develop into more complex bodies. And as we see the vertebrates here, well, we have a lot of Hox genes.

That means that the cells in a vertebrate body plan have extremely detailed information as to exactly where they are in the body that allows them to evolve into a very complex body. So here when we think about the Cambrian, these new genes, well, this allows for complex bodies to evolve because we have detailed information in the body during development about where cells are. But again, because changes in regulatory genes can cause kind of big changes to the organisms, that means that changes to these Hox genes can cause really big changes in animal body plans. And that's exactly what we see during the Cambrian explosion. Alright.

We'll look at this more, an example and practice problem to come. I'll see you there.

The evolution of the Hox gene cluster likely allowed for what key evolutionary innovation?

• A. Genes that control development.
• B. Complex body plans in animals.
• C. Hard-bodied animals in the Cambrian.
• D. Transcription factors that control gene expression.

In arthropods, the Hox genes Ubx and abd-A are largely expressed in the same tissues. The illustrations below show in red which tissues both Ubx and abd-A are expressed in three species of arthropods. Based on this illustration, which statement is consistent with the data?

_{Images adapted from: Jennifer K. Grenier, Theodore L. Garber, Robert Warren, Paul M. Whitington, Sean Carroll}

_{Evolution of the entire arthropod Hox gene set predated the origin and radiation of the onychophoran/arthropod clade,}

_{Current Biology, Volume 7, Issue 8, 1997, Pages 547-553,}

_{ISSN 0960-9822,}

_{https://doi.org/10.1016/S0960-9822(06)00253-3.}

• A. The gene Ubx induces the growth of legs while abd-A inhibits the growth of legs.
• B. Legs can only grow in tissues with Ubx and abd-A expression.
• C. Ubx and abd-A repress leg formation in insects but not in crustaceans or centipedes.
• D. Duplications of Ubx and abd-A allowed organisms such as crustaceans and centipedes to grow more legs.