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 zoologists 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   episode, we’ll dive into the true frontiers  of the field and even more open questions. If you want to learn more about the western fence  lizard Bizarre Beasts just did a video on them.   In this series, hosts Hank Green and Sarah Suta  introduce you to a new bizarre beast and explore   what makes these animals so weird to us.

From  birds whose babies have claws on their wings,   to lizards with glowing bones, the show examines  the how and why of some of the world's most   amazingly strange critters. And if you want to  take a bizarre beast home, check out the Bizarre   Beasts pin club! The links for the channel  and the pin club are in the description below Thanks for watching this episode  of Crash Course Zoology which was   produced by Complexly in partnership  with PBS and NATURE.

It’s shot on the   Team Sandoval Pierce stage and made with the  help of all these nice people. If you’d like   to help keep Crash Course free for everyone,  forever, you can join our community on Patreon.