You know what sounds good right now?

A taxonomic sandwich.

Taxonomic sandwiches usually show us how much time has passed since two animals with the same abbreviated Latin name split off from their common ancestor.

But we can also make taxonomic sandwiches between any two animals -- it’s just like using two different types of bread.

Like together, horses, donkeys, and zebras form the genus Equus.

If we make a taxonomic sandwich between a Equus quagga, the Plains zebra, and Equus ferus caballus, the domestic horse, there’s 7.7 million years of filling.

But the donkey-zebra sandwich has only 2.8 million years of filling because donkeys are more closely related to zebras than horses.

So basically zebras are flashy donkeys and not striped horses.

And yet despite being separated by millions of years of evolution, these three species sometimes mix and match their mates, producing hinnies, zorses, zonkeys, and yes mules.

And if they look alike and can reproduce together it kinda seems like they may not actually be different species.

Scientists put animals into several categories, each more specific than the last, describing an animal’s features and who its closest evolutionary relatives are.

But the bedrock of that classification system, the taxonomic rank of “species,” isn’t nearly as solid as a foundation as we’d expect.

I’m Rae Wynn Grant, and this is Crash Course Zoology!

Back in episode 1, we defined a species as a group of all the animals of the same type that can breed together over multiple generations.

Which is a perfectly valid way to define a species.

But there are over two dozen other distinct ways of defining a species.

Now that we’ve seen the amazing diversity of animals, we can return to the species problem, which is a set of questions, like “what happens if two different types of animals have babies?” that arise whenever we try to define what a species actually is.

There’s even a whole subfield, called microtaxonomy, dedicated to organizing the millions of animals out there into human-made boxes, and trying to resolve all the exceptions that come up.

Which is not just important to the zoologists, but the animals themselves too.

What species an animal is classified as matters for things like the US Endangered Species Act and comparable laws worldwide that provide legal protection for certain organisms on the basis of their species.

A common method for drawing lines between species in the 19th century was to rely on observations.

The morphological species concept says that members of the same species look the same as each other, but different from members of another species.

But some species vary so much in how they look that they overlap with other species in size, shape, color, whatever, making it hard to know where to draw the line.

The opposite problem occurs too -- some distinct species look so similar that it’s almost impossible to tell them apart based on physical characteristics.

Fortunately, we have more methods.

Like the biological species concept which says that a species is a group of naturally or potentially interbreeding populations that are reproductively isolated from other groups.

Which means we have a group of animals that can combine their genetic material using sexual reproduction to make more animals and if any animals in the group tried to mate with animals outside the group, they can’t produce fertile offspring.

Either because their offspring, which is called a hybrid because it’s the offspring of two different types of animals, never develops because their DNA just can’t work together or because the hybrid is almost always sterile, like a mule.

But the biological species concept is also too narrow of a definition.

It doesn’t work for the many asexually reproducing organisms, like aphids, who clone themselves instead of mixing their DNA with another individual.

Or different species whose hybrid offspring can reproduce like female ligers.

And the caveat of “potentially interbreeding” has always been confusing.

Potentially as in it could happen in the wild?

Or potentially as in it could happen in a zoo?

Another option is to use the cohesion species concept, which was introduced in the late 1980s and was written to address some of these drawbacks.

The cohesion species concept defines a species as a population, or series of populations, with genetic or demographic cohesion, meaning they all have pretty similar genes and traits.

This allows for asexually reproducing animals and hybrids to exist as long as they aren’t so common that they change the characteristics of the population as a whole.

Or we could use the phylogenetic species concept, which was developed around the same time and defines a species as a group with a shared and unique evolutionary history instead of focusing on living animals swapping and sharing genes.

All members of the species are descended from the same common ancestor, and share a combination of defining traits that they got from that ancestor.

The tricky part is that using the phylogenetic species concept, it’s possible to define a near-infinite number of species, because most populations will have many subpopulations that are very slightly different from others.

So we have to decide where to draw the line.

Like the black bears that live in the Everglades could be different from those on the tundra or in the Rocky Mountains.

Both the cohesion species concept and the phylogenetic species concept work by comparing the DNA of one animal to another with different techniques to decide if they’re related enough to belong to the same species.

DNA barcoding focuses on a few specific genes or parts of the DNA that are suspected to vary a lot between species but not too much between individuals (the barcode!

), whereas genetic similarity takes into account the whole genome, or entire DNA sequence.

But there’s no one right way to define a species, and even still, zoologists will often combine multiple definitions and tools in their work.

It’s hard work defining a new species!

Let’s go to the Thought Bubble.

While hiking in the mountains just outside Santa Barbara, California, we come across this thing we’ve never seen before.

We take observations about its behavior and where we found it and photos of its morphology, or how it looks.

We’ll also want to get something with its DNA in it.

And ideally bring a specimen or two back to the lab because it’s easy to miss things in the field, and allows other researchers to check our work.

Once back, the first thing we do is check there are no previous records of an animal matching this description based on our notes.

Now we write up our manuscript with our field observations and using the specimens we brought back with us.

We need to explain how our species differs from other described species, and how it fits in the metazoan family tree.

One part a lot of zoolgists focus on is its sex organs, because they tend to be very specific to a single species whereas other differences could be due to something like diet or age.

And if we think it’s a new species based on our definitions, we also get to name it!

This creature looks like others in the family Ursidae.

It doesn't have a false thumb, like a panda or a spectacled bear, so it’s probably related to black bears and grizzlies in the genus Ursus.

So let’s call it Ursus stuffulus.

Now we submit our manuscript to an academic journal, and if it’s accepted, we’ll have successfully described a new species!

Thanks Thought Bubble!

About 11,000 new animal species are described each year by zoologists.

But knowing when to investigate a possibly new species is an art in itself.

Sometimes animals just live in a place where we haven’t seen similar species before, or sometimes they look like other species, but are separated by a geographic feature that could stop them from interbreeding.

Ultimately, categorizing animals into different species helps us humans wrap our heads around the stunning level of diversity around us.

But even if we can agree on one definition of a species, there are a lot of cases where species aren’t nearly as separate as we might think.

It comes down to gene flow, or the transfer of genetic material from one population to another -- or like from parents to offspring.

But there are also situations where animals we consider to be separate species can share genes.

Like hybrids probably won’t just mate with hybrids.

They usually end up mating with one of their parents' species, creating offspring that has genes from mostly but not entirely one species.

Like paradise spiders in the genus Habronattus.

In some species, males have brightly colored ornaments that they show off in their courtship dances.

But paradise spiders are notorious for hybridizing -- the males will dance for pretty much any species of female.

These offspring continue mating, and before you know it, genes from one species have made it into the gene pool of another, and those genes can stick around, which is a process called introgression.

And introgression can make things confusing.

Like, take the big bushy eyebrows of these three paradise spiders.

Thanks to all the hybridization between these different spiders, these eyebrows might’ve evolved in one species and then spread to others!

But all that introgression also makes it hard to be sure, and even harder to know who had the brows originally.

Only one thing is clear, there is a lot of DNA moving back and forth between these “separate” species.

Though sometimes we can see some organization in the gene flow chaos if we’re dealing with ring species, or a series of neighboring populations that can interbreed with the groups close to them, but not other populations that are further away until we get two ends to our “ring.” Like the species of Larus gulls in the Arctic.

The European herring gull can mate with the American herring gull, and the American herring gull can mate with the East Siberian gull, and so on as we circle -- or ring -- around the North pole.

The last species in the ring is the lesser black-backed gull of northwestern Europe, which can’t mate with the first species in the ring, the European herring gull, because they’re too genetically different.

There are other ring species out there, even if their locations don’t perfectly form rings.

Like western fence lizards might be a ring species.

So, we started this episode by asking “what is a species” and while some definitions of species are more robust or commonly used than others, none of them perfectly fit the true diversity of all animals into human-defined categories.

The “species problem” is one of many enduring mysteries in zoology.

And in our next and final