We humans like to get
our heads around all of the complexity around
us by classifying things. And you could imagine
there's no more obvious thing to classify than all of the
living things around us, than all of the life
that surrounds us. So what I want to
start talking about is, how do we classify all
of the life around us? And this is more often
generally referred to taxonomy. But the most basic
question you have when you look at
all the life around, you start to see similarities
between some of these living things. You see, obviously this
thing right over here is more similar to the
things that look like it, than it does to the grass
behind it, or to that tree. And so we start saying, well
maybe I should group this thing right over here into a group
with other things like it. And that very most
building block of how we classify all of
the living things around us, is putting them into
buckets called species. So for example, this is
one particular animal, but we see other animals
that seem to look like it, and so we say they're all
part of the species of lions. And this animal,
it's one animal, and there's other animals
that have stripes, but some might be fatter,
or taller, or skinnier, or whatever else,
darker, or lighter, but we say they're
similar enough, that we call them all tigers. We call all the animals,
even though they might be a little bit bigger,
or skinnier, or fatter, or lighter, or darker, we
call all of them-- they look similar to this thing
right over here-- we would call this a donkey. We would call the
things that we think are like this animal
right here, a horse. Now, that might seem like a
pretty straightforward way to think about it. Oh, everything that looks kind
of like this character right here is a lion,
anything that looks kind of like this character
right here is a tiger. But that, by itself, is not
a good enough definition for a species. Things that look like
each other or things that act like each other,
because what we'll see is that there's some
things that could be very different, at least
in and how they look or act, but are actually
closely related. And we'll talk about what it
means to be closely related. And then we can see things that
look very similar, that they have similar structures or
they have similar behavior, like for example bats and
birds, but they are actually all very, very
distantly related. So we need a more exact
definition for species than just things that
look like each other, or just things that
act like each other. And so the most typical
definition for species are animals that can interbreed. And when we say
interbreed, literally they can produce offspring
with each other, and the offspring are fertile. Which means that the offspring
can then further have babies, that they're not
sterile, that they're capable of breeding
with other animals and producing more offspring. And to show an example of this,
this right here is a male lion. You find a male lion
and a female lioness and most of the time they will
be able to have offspring, and those offspring
can go and mate with other lions or lionesses,
depending on their sex, and then they can
have viable offspring. So it seems to work out
pretty well for lions. Same thing is true of tigers. Now, it does turn
out that if you get a male lion and a female
tigress they can breed. They can breed and they
can produce offspring. And their offspring--
which was made famous by Napoleon Dynamite, he
was kind of fascinated by, these are kind of
fascinating animals. Their offspring
is called a liger. You get a male lion
breeding with a female tiger you produce a liger,
which is a hybrid, it's a cross between
a lion and a tiger. And they're fascinating animals. They're actually larger
than either lions or tigers. They are the largest
cats that we know of. But these ligers cannot be
referred as a separate species. Or you can't say
that lions and tigers are the same species,
because even though they are able to interbreed, their
offspring, for the most part, is not fertile, is not
able to produce offspring. There have been one off
stories about ligers being mated with either
a lion or a tiger, but those are one off stories. In general, ligers
can't interbreed. And in general, this
combination isn't going to produce offspring
that can keep interbreeding or that are fertile. So that's why we say
that lions and tigers are different species. And that liger, we wouldn't even
call it as a species at all. We would actually call it a
hybrid between two species. Now the same thing is
true-- and actually you might be asking
yourself, well, this was a male lion and
a female tigress, what if we went the
other way around? What if we had a female
lioness and a male tiger? In that case, you would
produce something else called a tiglon or a tiglon,
I actually don't know how to pronounce that. And that is a
different hybrid that has slightly different
properties than a liger. I encourage you to look
up what a tiglon is. Similarly, you give
me a male donkey. And donkeys are clearly
a species by themselves, because if you give me a male
donkey and a female donkey they can reproduce, produce
another donkey, and then that donkey can
mate with other donkeys to produce more
and more donkeys. So not only can a donkey
interbreed with another donkey, but that product,
that child donkey, can keep interbreeding
with other donkeys. Similarly, horses, they
can interbreed and produce fertile offspring. But if you give me a male
donkey and a female horse they can mate and they
can produce a mule. But once again, like
the ligers, mules are not, at least
as far as I know, mules they're not fertile. Mules cannot produce
further offspring. They cannot interbreed
with each other. And because, even though
donkeys and horses can breed and produce mules, their
offspring aren't fertile. We don't consider
donkeys and horses part of the same species. And we would consider mules,
like a liger or a tiglon, we would consider them a hybrid. So these are all hybrids,
or we would call cross. In general, the
word hybrid is used when you have two things,
two different types that are somehow coming together,
somehow having a combination. And once again, like the
case with the tiglon, you might say,
well, what if I had a female donkey
and a male horse? And then you would
actually produce something called a hinny, which
isn't as common as a mule. And people like to
use mules, they're actually very good work
animals because they have some of the good properties
of both donkeys and horses. Hinnies are less common, but
once again, it is possible. And they have different
properties than mules. And I do want to
emphasize this idea. Because when we started
off we just tried to think about, well, how
can we classify things? And we said, hey, maybe things
that look and act similar, we can call a species. And maybe things that
look and act different, we shouldn't call them species. But I want to show you a
very typical case, one that's really all around us all the
time, where this definition-- animals that can interbreed
and the offspring are fertile-- really does seem to become much,
much more important than just some notion of animals
that look alike or animals that act the same. And the best example
of that is with dogs. As I said, this is a very
typical species here, because dogs-- and I just
took a sample of some of the different types
of breeds of dogs-- they can look very,
very different. It's obvious, look at the
difference between these dogs. For example, this
little chihuahua here and this dog
right over here. Obviously, they're
size-wise, their look, and even how they act are much,
much more different than maybe how this donkey would act
relative to this horse, or how this lion would act
relative to this tigress. And they obviously
look very different, they have completely
different sizes, but these two things
actually can interbreed-- although for these
two in particular, it seems like the mechanics
would get kind of difficult-- but assuming they get over
the mechanical hurdles they could interbreed and
produce fertile offspring. Same for these two characters,
same for these two characters over here. And because of that, even
though all the different breeds of dogs-- and most
of this is really due to humans' doings
of trying to breed for a specific traits-- even
though they look so different, and even though they act so
different, because they can interbreed, and they could
produce fertile offspring, we consider all of
these things to be members of the same species.