More in common than, say, you and my dogs Lemon and Abby here.

For starters, you and I are probably the same species.

And Lemon and Abby are dogs, which is a different species.

As you may have guessed by now, this video is going to

be about species! But at the very end,

we're going to talk about dogs. So hang in there,

because the puppies are coming.

Before we bust out the puppies, let's talk about people.

Our species, Homo Sapiens, is the single remaining member of

the genus Homo.

Our buddies Homo Erectus and Homo Habilis

and Homo Neanderthalensis

bought the farm a long time ago. So these days, all us Homo sapiens

are pretty different from even our closest living relatives in the

animal Kingdom, the chimps and bonobos.

Humans are a species, a specific type of organism that's different

from all the other types of organisms out there.

But what is it that makes us human? Well, we're a specific type of

animal called a primate.

Monkeys, apes, lemurs, and tarsiers are also primates.

Unless you're Sacha Baron Cohen or something, most of us are

lacking significant body hair.

We're bipedal, meaning we stand on two feet, and we've got these

huge-normous brains, that allow us to do all kinds of stuff like

talk real good, solve complicated problems,

write bad poetry during adolescence, and figure out how

little we can get away with tipping a mediocre waiter at a

restaurant without seeming like a total prick.

THAT, my friends, is something that giraffes rarely have

to deal with.

But being a species is more than having a bunch of stuff in common.

Instead we describe a species as a group of organisms that can

interbreed and produce fertile offspring.

Seems pretty simple, right? Two of the same species that

can produce blah blah blah...

HEY! Pay attention! That last part is important! The two organisms

need to be able to produce fertile offspring.

It seems like it would be enough for organisms of the same species

to be able to make babies, but those babies need to

be able to make babies, too.

Now it turns out, two animals of a different species can sometimes

technically have a baby. Take, for instance,

the noble liger, Napoleon Dynamite's favorite animal,

which I know because I had the very best Napoleon Dynamite costume in

the United States for Halloween in 2005.

But, I didn't just bring ligers up to brag.

A liger is what happens when a male lion and a female tiger

have a little cub.

Only, it's not very little because a liger is generally larger than

both of its parents. And ligers are sterile.

Which leads us to our understanding of what makes a species: lions and

tigers are different species because they don't produce

fertile offspring together.

We call animals like ligers hybrids, the offspring resulting

from the cross-breeding of two distinct species.

And even though hybridization between two animals is a dead end

when it comes to creating a new species, we know that

through evolution, or the change in the heritable characteristics

of a species across generations, new species have formed

in the past, and they continue to develop all the time.

It's tough to nail down every single way this process we call

speciation can happen, but we know of at least a couple ways that

species evolve into other species. And they both involve

one requirement: reproductive isolation, meaning two populations

of the same species can no longer mate together successfully.

Note that I said successfully. One way populations can become

isolated from each other is that they can mate, but their offspring

aren't fertile or viable. Ligers are a good example of this.

So are mules, they're the product of a male donkey and

a female horse. Unlike lions and tigers,

donkeys and horses don't even have the same number of chromosomes,

so even though the donkey sperm can fertilize the horse egg,

the mule won't have the genetic instructions it needs to produce

its own sex cells.

This kind of isolation is call post-zygotic, because the parents

can form a zygote together, but after that it's all

over for their lineage.

Other examples of post-zygotic isolation include pairings of

species that always lead to miscarriage or no development

of the embryo at all, or things like big fetuses

that kill the mother at birth.

The other type of isolation is pre-zygotic, meaning the isolation

happened between groups of the same species before an egg even

thought about getting fertilized. This can include stuff like

behavioral changes within a species, like when birds of the

same species start singing two different songs to attract mates.

Or when one group of a species that does all its business in the

daytime gradually becomes nocturnal, so the two groups

never end up hanging out at the same time.

Pre-zygotic isolation can also be geographic,

meaning simply that the populations are separated

by great distances or physical barriers,

so that they can no longer get together to bump uglies.

When one species diverges into two new species because

of geographic isolation, it's called allopatric speciation,

allopatric coming from the Greek for "different countries."

The two populations of a species end up evolving differently

because conditions are different on each side of this river here.

It might be colder on one side of the river,

so the animals on this side grow thicker,

more luxurious coats because those guys just do better over there.

They probably also put on thicker layers of fat,

and change their behavior, and accumulate a bunch

of other possibly random changes. Meanwhile, on the warm side

of the river, these animals also accumulate changes, and lose some

fur and add a bunch of sweat glands. Given enough time,

and given a complete lack of gene flow between the two populations,

thick-coated animals will eventually only be able to breed

with other thick-coated animals,

and sweaty animals with sweaty animals.

This propagation of specific traits based on how kick-ass

those traits make the animal that has them is called

natural selection. And a guy named Charles Darwin or Chuck Darwin,

or Chucky D to his friends...

was the one who let us know what was up with natural selection

and how it can lead to allopatric speciation.

Stop me if you've heard this one before, but Darwin visited the

Galapagos Islands in the 1830s. So Darwin was obsessed

with barnacles, but that didn't keep him from noticing the finches,

which were actually misidentified by him as grosbeaks,

on each island were all pretty similar to the finches on the

other islands AND very similar to the ones on the mainland of

South America BUT they were also obviously their own species.

Darwin believed that the process that led to these finches becoming

separate species was incredibly slow, so slow that we couldn't

actually witness the process, we just had to take his word for it.

Now, for a long time after Darwin made these observations,

allopatric speciation was the main explanation for how species diverge.

But today we know that's not the whole truth. Now, we've got lots

of new-fangled DNA testing and other special gadgetry that tells

us that one species can diverge into two without being

geographically separated, but instead, when they're

reproductively isolated in some other way. This is called

sympatric speciation, meaning "same country," and it also means

that it's time for a trip to the chair!

So, here's a little biological love story for all you

romantics out there. Peter and Rosemary Grant,

two British evolutionary biologists (they are, in fact, a married couple)

have, since the early 1970s, been spending 6 months of each

year together on a secluded island in the Galapagos

studying Darwin's finches, trying to catch them in the

act of evolving. These are, mind you, the same animals

that Darwin studied, and the ones that he

said were evolving at an imperceptibly slow pace.

The island in the Galapagos that the Grants hang out on

is called Daphne Major, and when they started their research in 1973,

it was occupied by two different finch species:

the medium ground finch and the cactus finch.

But in 1981, another finch arrived on Daphne Major

from a nearby island. It was a ground finch-cactus finch hybrid,

and it was a whole lot bigger than either of the local finches.

Its beak was also extra wide, and its song was like a mashup

of the jams ground finches sang on its home island and the ones

sung on Daphne Major. The newcomer set to work crooning to the local

ground finch ladies, and eventually landed one.

The Grants followed the descendants of these two

birds for the next 28 years.

But after about 4 generations, Daphne Major experienced a

severe drought which killed many of the finches.

There were only TWO surviving descendants of that one

immigrant finch, sort of like cousins of each other, basically

and they mated with each other, and that seems to have set the

stage for speciation to occur. The descendants of these two

survivors sang a very distinctive song that was learned from their

parents and which was different from the other

Daphne Major finches.

Gradually as the finch population on the island rebounded,

the hybrid finches, the great-great-

great-great-great grandchicks

of that one bird, began mating exclusively with each other.

In December 2009, the Grants announced that, since the drought,

the lineage of that one immigrant ground finch has been genetically

isolated from the other local finches on the island.

So, that, my friends, is both super romantic and also an

example of super-quick sympatric speciation in action.

Okay, so I promised you puppies, so I'm gonna give you puppies.

You've probably noticed that, you know, a corgi looks pretty

different from a greyhound. They were bred to be different.

Corgis were bred to herd animals and guard farm houses,

while greyhounds were bred mostly to run.

Dog breeding kind of takes the "natural" out of natural selection,

in fact, it's what we call artificial selection,

but it's still a kind of selection.

You've probably wondered what it would be like if a corgi and a

greyhound had puppies together? Because they CAN

have puppies together. Even though that's really weird.

What's that, Lemon? You're both girls?

Oh, well- anyways..

My point here is that they're the same species, meaning that

these dogs, even differenter dogs, like an Irish wolfhound

and a chihuahua, could have fertile offspring together.

Like, how? How... How? Would- HOW!?

Various dog breeds are similar enough that post-zygotic

isolation isn't an issue. But in a natural setting,

a chihuahua-wolfhound pairing would be extremely rare

because of the difficulties involved in the

gettin' it on process. Or "pre-zygotic obstacles."

So, think about it like this, if you were to put a bunch of

chihuahuas and a bunch of wolfhounds on an island somewhere

they probably wouldn't breed together and if they did,

the birthing process, at least for the chihuahua mommies would be...

Gah!

Oh god.

But what this means is that the gene flow between the two groups

would stop, and they would become reproductively isolated.

Over time, they could become different enough that they could

no longer successfully breed together at all, and thus become

different species.

Thank you for watching this episode of Crash Course. If you missed

anything don't forget to go back and review. If you have any

questions, please ask them in the comments or of Facebook or Twitter.

We will endeavor to answer them. Thank you to everyone who helped

put this episode together. We'll see you next time.