There’s this strange reptile in New Zealand that can live for more than a hundred years, saw through prey with jaws that move side to side instead of up and down, and maybe even monitor the Sun’s movement through a third eye buried beneath scales on its head?

This creature is considered a living fossil because it’s eerily similar now to how it was when it first emerged in the Jurassic Era, around the same time much-larger-but-now-extinct reptiles like the Apatosaurus made their appearance. So, what exactly is this reptile?

Well, broadly it’s a tuatara. But because the Brothers Island tuatara, as it’s known, evolved in isolated conditions, whether it’s classified as a unique species of tuatara has been up for debate. It turns out that the diversity of life on Earth isn’t quite as simple as a branching family tree, where each limb and twig are distinct species, originating from a shared common ancestor.

In reality, species are more like a network of streams, branching off a big river. Those streams can diverge, cross with other streams, split away, and link back up again later. And it’s not always clear where one section of water ends, and another begins.

Hi, I’m Dr. Sammy, your friendly neighborhood entomologist, and this is Crash Course Biology. Sorry guys, I think I dropped my theme music.

Oh wait, here it is! [THEME MUSIC] For most of the 1900s, the Brothers Island tuatara was recognized as the same species as another, less isolated population of tuataras. Lumping these two types into a single species gave the impression that the Brothers Island tuataras were doing better than they really were, surviving in a fairly widespread area in New Zealand. That is, until 1990, when DNA evidence suggested that the Brothers Island tuatara was, in fact, a distinct species.

This led to special protections, as it made clear how few of these creatures there really were. But then, in 2009, additional DNA evidence complicated the picture, showing that Brothers Island tuataras are not quite as distinct as previously thought. So, now we’re back to thinking of all living tuataras as one species.

What all this illustrates is just how tricky it is to draw clear lines between species. But we keep trying because where and how we draw those lines can impact how those species are protected, and even whether or not they survive. And understanding differences between species can help us piece a little more of life’s great puzzle together.

But where do new species even come from in the first place? The evolution of species is called speciation. And this process of one species splitting into two falls between the little view and the big view of evolution.

Zoom in and there’s microevolution— changes in specific populations, like, when a bunch of mosquitoes live in the same area, breed with each other, and develop pesticide resistance over time. Zoom way out, and there’s macroevolution – changes in broad groups of living things. The ones that feel like they should be spelled out in all caps. “Now available on planet

Earth: FLOWERING PLANTS. DINOSAURS. MAMMALS. Get ‘em while supplies last.” But in between, we have species: groups of organisms that can potentially interbreed with other members of the same group and produce fertile offspring.

That’s according to the biological species concept, just one of the ways scientists view species and the one that we typically use in this series. Think of the biological species concept like this: a sleepy basset hound and a tireless border collie look and act like they’re from different planets, but they can have a litter of weird-looking puppies that can make more puppies. That’s because they are the same species.

Meanwhile, cross a zebra and a horse, and you’ll get a bundle of joy known as a “zorse.” Personally, I would’ve gone with “hebra” but ya know, ok, to each his own. This is considered a hybrid— a cross between two species. But sadly, that zorse can’t make more zorse babies.

Zebras and horses have different numbers of chromosomes, the structures that carry their DNA. The zorse inherits a number somewhere in between, and can’t reproduce because of those chromosomal differences. But life, ever the wild wonderful thing, sometimes breaks these rules.

There are some hybrids can reproduce. You might know a few of them, without knowing that you know, if you’ve ever bitten into a juicy plumcot, otherwise known as a hybrid plum-apricot. But it’s not only plants.

There are hybrid animals that can reproduce, too. Including some lizard females that don’t even need a male to do so. And that throws a monkey wrench into our biological species concept.

Which is why biologists have proposed other ways of drawing lines between species. Like the phylogenetic species concept, which focuses less on who can breed with whom and more on ancestry and evolutionary history. Or, the ecological species concept, which parses species based on differing environmental conditions, like how polar bears can live in a different environment than grizzly bears.

These are just a few of the ways that scientists think about dividing up species. And we haven't even mentioned DNA testing as a tool to determine species, even though it got us into a pickle with the tuataras. DNA analysis can help us understand how certain groups of organisms are distinct, but like the other methods, none of them can draw clean lines in every case.

There are simply always exceptions. Whatever we think we’ve figured out, life tends to be way ahead of us. That said, in most cases, for any two species to be distinct, there must be reproductive isolation, or an inability to interbreed.

Something more than a Romeo & Juliet-esque feud between families, or a heavily chaperoned school dance. Sometimes, those barriers are prezygotic — meaning, the stuff that stops mating or fertilization of an egg from occurring at all. But could be sheer distance: like, if two coconut crabs live on different islands, making romantic meet-ups impossible.

It can also be behavior— two bees whose calendars aren’t synced up, so they’re looking for love at different times of day. Or there can be a mismatch between physical parts: like, damselfly species with reproductive bits that just don’t fit together. Or, a cup-shaped flower might attract pollinating bees, but those bees will never spread their pollen to a trumpet-shaped flower, with an opening fit only for a hummingbird’s beak.

Basically: it’s not you, it’s me. Or, barriers to reproduction can be postzygotic. Meaning, fertilization happens, but the family tree ends with the hybrid.

Sometimes, something goes wrong while the hybrid is still developing, so it doesn’t survive. Other times, that hybrid can survive but can’t reproduce, like we saw with the zorse. When these barriers happen populations drift apart and new species can emerge.

Like when two populations are geographically isolated from each other, so they’re no longer swapping genetic material. That’s called allopatric speciation. Like many long-distance relationships, organisms grow apart when geography gets in between them.

Except instead of long airport goodbyes and never getting their favorite hoodie back, something else can transpire: one species can become two. Let’s head over to the Thought Bubble… The Grand Canyon is big. Like, bigger than the entire U.

S. state of Rhode Island. But to a tassel-eared squirrel in a forest oasis, surrounded by canyon and desert? Those barriers might as well be an ocean.

At some point in their evolutionary history, some intrepid squirrels ended up on the Kaibab Plateau— a cool, isolated, high-elevation forest just north of the Grand Canyon, surrounded by desert. And they’ve remained there, far away from their cousins who stayed put on the canyon’s south rim. With time and distance, the Kaibab squirrels have lost touch with their southern kin, the Abert’s squirrel.

Way up on that plateau, they’ve drifted apart genetically. They’ve evolved a look all their own, too: smudged-charcoal coats and creamy white tails. The Kaibab squirrels and the Abert squirrels still have some things in common —like, tasseled ears and a huge appetite for Ponderosa pinecones.

But there hasn’t been a family reunion since. So, today, biologists consider the Kaibab squirrels a subspecies of the Abert squirrels. Different enough that they can’t be lumped together, but not so much that they’ve become a new species.

But the longer the Kaibab squirrels stay put on that plateau, the more they’ll drift apart from their cousins. Speciating in paradise, maybe someday they’ll be considered a species all their own. Thanks, Thought Bubble!

So, when geography sets populations apart, allopatric speciation can arise. But new species can also emerge without any isolation at all. Sympatric speciation happens among organisms on the same home turf.

One way this can go down is when traits that attract mates become more common in a population over time. And that’s called sexual selection. With enough momentum, it can send the speciation wheels a-turning.

Take cichlids, for example, little fish that have diverged into over 500 different species in Eastern Africa’s Lake Victoria. No matter the species, the females prefer bright, flashy males. Among the bottom-dwelling species, males are mostly red —a color that stands out better on the bottom of the lake.

In the shallower parts of the lake, blue colors catch more light— and more eyes from the choosy females, so surface-dwelling males tend to be blue. But as the environment changes around the species, the species themselves sometimes change. As Lake Victoria grows murkier from pollution, there’s been more interbreeding between these once-distinct species.

It’s harder for females to be choosy about colors when they can’t see well. So, as red and blue get blurry, so do the boundaries between species. And sometimes two species that split off from each other, find their way back together.

Differences in habitat can also drive sympatric speciation. That’s what’s happening with these blind mole rats, for example, which live in an area split between two types of soil: rendzina in one area, and basalt on the other. Now that might not mean much to you or me, but it’s a big deal between blind mole rats, who spend their lives tunneling through the stuff.

And DNA shows that rendzina-specialized mole rats are sticking together, having rendzina-specialized babies, while the basalt-loving mole rats are pairing off with fellow basalt lovers. And they’ve been slowly splitting apart… for the past 200,000 years. But sympatric speciation can also happen really fast, in a single generation, through polyploidy: a process where offspring inherit more than two sets of chromosomes.

Sometimes that’s due to a mistake when either the sperm or egg being used in fertilization has twice as many chromosomes as it’s supposed to have. So instead of ending up with the same number of chromosomes as their parents, the offspring end up with twice as many, or three or four times as many, and so on. And they’re most likely to reproduce with individuals that have the same number of chromosomes they do.

Polyploidy can also happen when hybrids produce fertile offspring with extra chromosomes. So, if a plant species with ten chromosomes got together with a plant that had 12 chromosomes, their polyploid offspring would end up with 22 chromosomes. These polyploid hybrids can’t interbreed with either parent species, but they can interbreed with each other.

So, in an instant: boom, new species. We have polyploidy to thank for the bread of our PB&J sandwiches. That’s because wheat is a polyploid, with six sets of chromosomes from three different species.

In fact, over 80% of today’s plant species have taken this speciation fast-track at some point in their history. So, the ability of two organisms to interbreed — or not — is a solid starting point for sussing out the boundaries between species. Where there’s a barrier to baby-making, populations can drift apart.

And that can send the speciation ball rolling. In fact, it led to the dazzling display of diversity on Earth today. Next time, we’ll zoom way out and get the really big picture of how life on Earth has evolved over billions of years.

I’ll see you then! Peace! This series was produced in collaboration with HHMI BioInteractive.

If you’re an educator, visit BioInteractive.org/CrashCourse for classroom resources and professional development related to the topics covered in this course. Thanks for watching this episode of Crash Course Biology, which was filmed at our studio in Indianapolis, Indiana, and was made with the help of all these nice people. If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon.