This episode is sponsored by Skillshare. [♪ INTRO].

We generally think of species as discrete groups of organisms that simply can't interbreed. But hybrids, the offspring of two species, do happen.

And they can be very useful. Take mules, for example, the offspring of a male donkey and a female horse. They're bigger than donkeys and hardier than horses, making them helpful pack animals.

But they're also sterile, and that's because, well, mixing species usually doesn't work. And that makes it all the more surprising that there are cases where three or more species come together. These multi-species hybrids are really rare, but they can teach us a lot about the inner workings of genomes.

Most of the time, cross-species mating isn't fruitful because for an embryo to survive and develop, there has to be enough consistency between its parents. The sperm has to be able to recognize and fuse with an egg, for example. And even if an embryo starts developing, you still have the issue of incompatible genes.

To really simplify it, you can think of different genes in a genome as a sports team. Over time, players start to work really well together and the team plays better overall. But if you take those players and stick them on other teams, they don't always perform as well, and in some cases, they can completely clash with their new teammates, making the whole team struggle.

It's basically the same idea with cellular machinery, except that inter-gene clashes can't be solved with a mid-season trade. All of these barriers make it unlikely for hybrids to occur, and even more unlikely that such hybrids will produce their own offspring. But sometimes, they do just that.

In the summer of 2018, for example, a keen-eyed birder discovered a 3 species hybrid warbler in Pennsylvania. Warblers are small, often colorful birds that sing pretty songs, hence the name. This bird looked kind of like a golden-winged warbler, but was singing like a chestnut-sided warbler.

Ornithologists sequenced some of the bird's mitochondrial DNA, the DNA that lives in the cell's energy factories, and almost always comes solely from the egg, as well as some parts of its regular genome. And it turned out to be the offspring of a hybrid female, herself the result of a golden-winged warbler and a blue-winged warbler mating, and a male chestnut-sided warbler, which is in a different genus from either of the hybrid parent species. This odd pairing probably occurred because the female couldn't find a more closely related mate.

Ornithologists already knew that warbler populations weren't doing so hot, so this is seen as more evidence that population declines are significant. But it also adds support to the idea that warblers are a rapidly evolving group where species split off behaviorally long before their genomes become incompatible. Studying such groups can help scientists understand what factors cause species to diverge and what exactly makes genomes incompatible.

Now, this hybrid warbler isn't the only multi-species hybrid you might see if you live in the eastern US. It used to be that wolves were the big canine predators in the northeast and coyotes were the top dogs in the southwest. But almost a century ago, people started to see what appeared to be coyotes moving eastward.

These weren't coyotes with wanderlust, though, they were coywolves: hybrids between coyotes and wolves. And they're one of the most clear examples of a phenomenon called hybrid vigor. Like I said, usually, mixing the genomes of different species fails.

But sometimes, it can more than work, creating an animal that's better in some ways than one or both of its parents. Wolf attributes like large size and wider skulls made coywolves better at hunting large prey than their coyote parents, which allowed them to basically take the ecological place of wolves in areas where humans had hunted them to near-extinction. And it isn't just wolf genes helping coywolves succeed.

Researchers in 2014 looked at lots of individual mutations in their genomes, what geneticists call single-nucleotide polymorphisms or SNPs, and found that coywolves are actually a hybrid consisting of two wolf subspecies, coyotes, and dogs. So, maybe you should call them coy-dog-olfs or something like that. Since dogs are so well-adapted to living with people, blending them into the mix might have helped these hybrids adapt to human-dominated environments, like the towns and cities that have sprung up on what long ago used to be wolf territory.

And by studying how the genes from all these species interact, these hybrid canines are helping geneticists understand how mixing genes from multiple species can help organisms survive, something that happens all the time in plants. Unless you're avoiding gluten, you probably have a 3-species hybrid in your pantry. Common wheat, which supplies roughly 20% of the world's calories, is actually a hybrid between 3 species of grass.

And its multi-species origin is probably what made it so useful to us. Sometime roughly 300,000 to 500,000 years ago, two wild species of grass hybridized, but in a remarkable way. The hybrid retained both complete sets of chromosomes from its parents, literally, twice the amount of DNA.

Humans soon domesticated that hybrid, known as Emmer wheat. Then, around 10,000 years ago, it crossed with another wild wheat species, resulting in modern wheat which has complete chromosome sets from all three of the species that went into it. Now, in most animals, having extra sets of chromosomes, what geneticists call being polyploid, is a big problem.

That whole incompatible players thing applies even more so with extra genomes. Yet, for reasons that remain elusive, plants seem to be able to handle way more genetic weirdness, so polyploidy is much more common in them. And having all those redundant genes turns out to be pretty useful when you want to get a plant to evolve a particular trait through selective breeding.

There's just that much more genetic material that can mutate without killing the plant. Even if a particular mutation isn't super helpful, there are backup copies of that gene which ensure all the necessary things still happen, allowing individual genes to be more flexible. So being a polyploid, 3-species hybrid is probably what allowed us to breed wheat to have traits we really like, like seed hulls that fall off the kernel more easily so they're easier to process.

By studying wheat and other multi-species hybrids, scientists can gain a clearer understanding of what works genetically and what doesn't. Whether they're grasses, mammals, or birds, these species mash-ups are helping unlock the mysteries of the genome, and that could lead to everything from better crops to improved gene therapies. Multi-species hybrids show us how bringing together different things can create something special.

And that's also true in photography. I love the way double exposure photos bring together two or more completely different images to create a stunning work of art. This effect can be created with photo editing software, but these neat dual images have been around much longer than that!

And photographer Tabitha Park has a class on Skillshare which shows you how to make them the old-school way: with your camera. In this class, she shows you three different methods for creating photo overlays. What I like most about it is that her instructions are so clear and easy to follow.

She's one of Skillshare's top teachers, which means her classes are high quality and she's an engaged teacher. Students receive thoughtful feedback on their class projects and she's available to answer questions you may have. If you want to take your photo game to the next level, you can check out some of her other classes, which cover all sorts of photography tricks from creating amazing backdrops to how to edit using Lightroom.

And there are tons of other great teachers on Skillshare, too. Right now, Skillshare is offering SciShow viewers 2 months of unlimited access to their over 20,000 classes for free! So you can take some great photos, cook a new kind of cuisine, or learn something else new, all while supporting SciShow.

You can follow the link in the description to check it out for yourself! [♪ OUTRO].