Speciation: Of Ligers & Men – Crash Course Biology #15
By Micah Moen
You and me? We’ve got some
stuff in common. 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.