Speciation: Of Ligers & Men – Crash Course Biology #15

Speciation: Of Ligers & Men – Crash Course Biology #15

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.

100 thoughts on “Speciation: Of Ligers & Men – Crash Course Biology #15”

  1. I would like to add that there are multiple definitions of what constitutes a species. The sterility vs. fertility idea has issues and there is a lot of debate among biologists over what constitutes a species. Darwin himself demonstrated that fertility and sterility exist on a spectrum and there isn’t a clear-cut line. Other than that, great video!

  2. He has a dog named Abby.

    I have a sister named Abby.

    And I watch crash course.

    And so does she.

    Does anyone think I’m idiotic? (In a good way…it’s not offensive if you mean in a good way.)

  3. all the dislikes are from ken ham-ers lol and yes that is a thing in short god did it science is wrong cause jebus said so

  4. what can a corgi guard a home from? i guess something like squirrels and mice i was going to go with rats but they might be a prob for that thing lol

  5. why are humans the exception to the rule of isolation and mating together successfully Speciation while the Black squirrel and the red squirrel can not

  6. For the most part you could say dogs and wolves have now been reproductively isolated for over 1000s of years – yet they remain the same species as most canids even foxes, etc. So is the reasoning behind them not turning into 2 separate species that it hasn't been enough time or would you predict that over time you would find a split in the canidae all together with medium to big dogs and wolves , coyotes on one end and the smaller ones with foxes due to more sexual selection and non random mating? sorry for not being very accurate with my thoughts.

  7. I actually looked away for a minute when he said "HEY PAY ATTENTION", so it actually made me jump

  8. reproductive isolation: two pops of the same species can no longer mate successfully.
    post-zygotic/pre-zygotic isolation:
    Allopatric speciation: when one species diverges into two diff species bc of geographic isolation
    sympatric speciation: when one species diverges into two diff species bc of reproductive isolation

  9. Exam tomorrow I see people from years ago with the same struggle good luck to everyone present and future

  10. i began scrolling through the comments with the video still playing then he goes "HEY PAY ATTENTION" and it felt like I committed a crime

  11. If hybrids are sterile how us humans contain upto 4 % Nianderhal gene given they were different human species.

  12. this video needs an update, since ligers have been shown to have offspring. What's more, a lot of different bird species mate all the time. The definition of species has now become extremely varied, there's not really an easy way to define it

  13. There is a noticable change in Hank's overall approach to narrating in this video than in the previous videos. I like it. He seems a little less quirky and more confident. I would love to know he got coaching, or had a life change.

  14. There is no evidence that a finch beak has undergone any changes whatsoever, . A finch is born with its beak shape and dies with it. and this is passed down through the generations.. Unless it is shown that the finches with different beaks interbreed and produce offspring that can thrive.

  15. What about the Gellar bear? In another video you said that even though polar bears and grizzly bears can mate and Gellar bears are fertile that polar bears and grizzly bears are desperate species even though grizzly bear- polar bear offspring are fertule

  16. Can anyone specify the random changes in the nucleotide base sequence in a new species that has resulted from a speciation event? A new species should have a unique sequence of nucleotide bases.

  17. I can understand why some people don't believe in macroevolution since people explain it so poorly. Cheetahs are an example of a cat turning into something other than a cat. They are the only living species of cat that don't have a cutaneous sheath around their claws.

    Imagine that you came up with a list of all the characteristics that an animal has to have in order to be 100% cat. Let's say that having a cutaneous sheath is one of those characteristics. Just to make the math easy, let's say that your list contains 20 characteristics and that the cheetah has the other 19 characteristics. Wouldn't that make cheetahs only 95% cat? Now, all cats belong to the family felidae. Since cheetahs arent 100% cat, do they really belong to the family felidae? Personally, I think the cut off point should be at least 50%… anything at least 50% cat belongs to the family felidae. However, it's arbitrary.

    A lot of people don't know this, but the species taxon is the only taxon that is found in nature… everything else, including the family taxon, are man-made. I can't overestimate how important it is for you to understand and accept that last sentence. Think about it in order to understand it's implications. None of the species of cat alive today are 100% cat unless you simply define one of the species as being 100% cat. It's your right to do so, since again, it's totally man-made. But as soon as you define one species as being 100% cat then that automatically means that none of the others are 100% cat.

    To make matters worse, some of those 20 characteristics that cats have may be subjective and not completely black and white. For example, let's say that being agile was one of your 20 characteristics. There are over 40 species of cats still alive today. Surely some of them are more agile than others. What if the cheetah is only 80% agile? Wouldn't that bring down it's cat percentage down to 94% (5 x .80 = 4)? And surely, if you believe in speciation, in a few hundred thousand years a new species can arise from the cheetah that only has 18 characteristics. So it's even less of a cat than the cheetah and even closer to that 50% cut off point that I previously mentioned.

    Now, some people will object by saying that a cutaneous sheath shouldn't be necessary in order for an animal to be considered a cat. But again, it's a man-made list, and I'm a man, so I can make that decision if I want. If you don't like my list, then you can certainly make your own list, but surely, if your list is a good list, any future species can lose one or more characteristics until they are less than 50% of whatever animal you are talking about. In fact, depending on what characteristics are on your list, it's even possible that some species of cats are already less than 50% and don't actually belong in the family felidae. I mean, it's not as though taxonomists are gods and can't make mistakes. Perhaps the cheetah currently isn't a cat. I'm not saying that it isn't at least 50% cat; I'm saying that I don't know and neither do you. Think about it.

  18. Keep trying, it's comical relief to watch evolutionist bang their heads against the wall trying to prove what is demonstrably false, namely that adaptation can cross the species boundary, reproductive isolation causes inbreeding and not speciation. Your So pathetic

  19. A doubt – if the 2 extremes of a specie, say early Girrafe are 3 meter and 3.5 meters tall, the descendants should never be taller than 3.5 meters, right? No matter how much isolation you bring. So how do they eventually turn into 6 meter tall giants???

  20. Hello There,
    One of the things that I observed from the past days is that your videos are been cheated . An Indian educational institute named byjus have cheated the content in your videos . All the examples you gave resembles he one given by them .

  21. Natural selection is powerless to drive macro evolution It is a slave to a random process. Random damaged DNA is going to generate highly specified complexity? Not going to happen. It's an antiquated theory that is kept on life support by atheists…. who desperately need it to be true.



  23. Hank, you said that the horse-donkey cross and the lion-tiger cross produced hybrids that are infertile. Does this occur to all "hybrids"? Because, later in the video, you read about the ground finch-cactus finch cross that produces a hybrid yet they are capable of reproduction. Is there a separate word used for crosses of two species that produce fertile offspring or no – they are also just referred to as hybrids?

  24. I find that this is a very superficial chat, it doesn't go 1 mm beyond inside the surface of the problem that tries to solve. We need a more solid information to dive more deeply in the explanation of the real nature as living creatures. The complication of life needs a more analytic approach if we intend to get a good base for understanding a comprenhensive meaning of life, where the only perspective of biology is incomplete and not sufficient robust, even least the vision of Mr Charles Darwin.
    We need a more powerful view to grab the whole content of the problem; meanwhile the ideas of this video are only vane illusions.

  25. Question… would this drought with the finches that caused only two species left to be to be an example of a bottleneck effect?

  26. Ever heard of a phonological loop? Well, mine REALLY liked it when you called Charles Darwin Chuckie D and it kept echoing in my mind.

  27. When he said HEY PAY ATTENTION! I was looking at my phone in that EXACT moment. This means that Hank wants me to study for my exam

  28. I know this is three years ago, but if I say something offensive, will youtube take the comment sections down, and take all the money put into it? ok, u suck!

  29. Could you please take out the word “prick”?
    I play your videos for my students and that word is completely inappropriate.

  30. Adaptation is a fabricated concept. Fluctuations in the genomes, within and confined to, of a species can cause minor structural variations. No living thing has ever been observed or had to "adapt" to survive in it's environment. The genetic code that allows it to exist in it's surroundings is written in it's DNA and constant. If it's genomes are altered, in any way, it would result in the termination of that species. The light and dark colored moths, from the I.R., were distinct species already existing together. The population of the two was proportionally inverted due to industrial pollution darkening tree bark, conversely allowing the light ones to be vulnerable to predators. The "greatest example of observed natural selection and adaptation" never even happened.

  31. My textbook “Exploring Biological Anthropology 4th Edition” says that Ligers and Tiglons are fully fertile…

  32. The theory is wrong. You can't generate precision coding necessary for new body plans via random damage to the DNA

  33. It’s just the crazy birder in me but the images that pop up for cactus finch and medium ground finch are not Galapagos finches but redpolls. Redpolls too have an interesting micoevoltionary history. Right now, their are two species of redpolls recognized in North America , the common and the horay but it was once believed that common and horay redpolls where the same species. However, the question now is if the two redpolls have diverged enough to be true distinct species, or if one is simply a subspecies of the other.

  34. What about species that do not mate with each other in nature, but when placed together in an experimental setting produce viable and fertile offspring. Would they still be considered separate species, because they don't mate in nature?

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