Extinction | Common ancestry and phylogeny | High school biology | Khan Academy

When we think of the term extinction, we tend to think of events like what happened, what we believe happened, 63 million years ago when a large meteor hit the Earth and killed most of the dinosaurs. I say most of them because a lot of animals that we know today, especially birds—birds like chickens—we believe are actually descended from some of the dinosaurs that survived.

Other species, not just dinosaurs, would have gone extinct, but clearly, some other species did survive. Our ancestors might have been some type of burrowing squirrel, rodent-like thing that maybe was under the ground and was able to survive the immediate shocks, and then somehow find food even when the Earth was covered by dust and there wasn't a lot of photosynthesis going on. But they were able to survive somehow.

What we're going to do in this video, though, is think about a more subtle notion of extinction. It's not always caused by these massive changes into the environment, but one thread, one theme that we will be talking about in extinction is that the environment is, to some degree, always changing. When there is variation in a population, I'm going to draw some variation as I do with the different colored circles. If the environment changes gradually or in certain slow ways, there will oftentimes be variants—members of the population—that will be more likely to succeed than others.

So those variants that are more suitable for those changes in the environment, they will reproduce more, they will survive more, they will become the dominant variant. Over time, they can even become a new species. Over time, you have species that won't go extinct, but they would be adapting, or maybe even evolving into other species that are more suited to the environment.

But our meteorite dinosaur example shows us that change is not always gradual. Clearly, in a situation where the sky is burning, as you have in the dinosaur example, very few organisms are going to be able to adapt to those types of circumstances. But you can also have fast change that might not seem so fast to us as human beings.

For example, this chart right over here shows extinctions since 1500, and I encourage you to pause this video and make sure you're processing what is going on here because that's a skill in and of itself. It's really interesting; this tells us the cumulative percentage of species driven extinct over the last 500 years or so.

You can see, relative to the species that were there in 1500, if we follow, let's say, the line for amphibians, it looks like somewhere between two and two and a half percent of all species of amphibians have gone extinct. This is not saying two to two and a half percent of the amphibians have died; this is saying the species of amphibians, they don't exist anymore. That variant of amphibian genotypes don't exist in our world anymore.

It's pretty bad for amphibians, but we can see it's pretty bad for other types of animals as well. You might say, well, this isn't so bad—two to two to two and a half percent—but once again, these are entire species that we will never see again.

What's also troubling is how these curves are just rocketing upwards. Some of you all might be thinking, isn't there always some baseline level of extinction? Maybe this isn't so much worse than that baseline. What's interesting about this graphic here is it shows us the baseline. This little gray square—they're saying that's the cumulative percentage of species based on a background rate of 0.1 to 2 extinctions per million species per year.

So you see the baseline in this little gray right down here. If we didn't have unusual amounts of extinction, we would just be charting roughly close to this line. But you can see since 1500 we have gone well above it. We know why this is happening: their environments are changing very, very quickly.

Who is the main culprit here of changing the environments very, very quickly? Well, what group's population has really expanded over the last 500 years? You would, of course, say human beings.

Human beings—it’s not just where they live, and they're taking habitats away. You can imagine this environment is very different than the environment of what modern-day Los Angeles would have been, say 500 years ago. And it's not just where we live; it's also the food we need.

We do need food, but that takes up ecosystems—environments where other animals might have lived—and it could be a dramatic change where they can't live anymore. Human land and agricultural land use is a major source of extinctions. But on top of that, we go into the sea, and we sometimes will overfish; that can be a source of extinction.

Lack of diversity, as more and more species learn to live with humans, and maybe our lack of diversity—we only have a few types of crops, a few types of cattle that we grow. There might be certain species that live well with them that are able to live off of them, but they might not have as much variation because the ecosystem is so not diverse.

So if there's some change, the species won't have the variation necessarily to survive. On top of that, you throw in things like pollution, global warming, invasive species, and you can see why these curves are going up in a very, very troubling way.

I will add this isn't just for us to care about animals, although that is a good motivation for us to care about the species, because once the species is gone, you're never going to get it back again. But in general, the less diversity that Earth as a whole has, it means that our ecosystems are going to become less and less robust to change.