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- Hello, my name's Lindsay Turnbull,

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and I teach biology at
the University of Oxford.

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In this video, I want to show you

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why nearly all species reproduce sexually.

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It's actually a huge puzzle for biologists

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and it forms the core of
chapter three of my book,

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"Biology: The Whole Story".

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(birds chirping)

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(frogs croaking)

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In 1928, the legendary
songwriter Cole Porter

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penned some famous lyrics.

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He wrote, "Birds do it, bees do it,

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even educated fleas do it.

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Let's do it. Let's fall in love."

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Well, I hate to be the party pooper,

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but I don't think bees or fleas

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are capable of a complex
emotion like love.

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Instead, what Cole Porter was
really talking about was sex.

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That is what is shared by
all multicellular organisms.

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Let's think for a minute about
how any organism reproduces.

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For single-celled organisms,
reproducing is very simple.

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All it has to do is copy the genome,

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double up the ribosomes,
make a bunch of proteins,

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and then it can simply
split itself into two cells,

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making sure that each one
has a copy of the genome

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and all the bits it needs.

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But as a multicellular being,

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you can't just carve yourself in two.

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Instead, most multicellular organisms

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set aside a group of cells

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and they're going to be responsible

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for creating the next generation.

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And the rest of the cells
in your body, I'm afraid,

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are just doomed to die when you do.

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When individuals reach adulthood,

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those special cells spring into action

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and they start to produce
sex cells or gametes.

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Now, these come in two forms.

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There are egg cells, which
are large and immobile,

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and there are sperm cells,

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which are much smaller
but can swim actively.

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But whichever type they are,

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there's something very
unusual about sex cells,

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and that's because they only contain

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half of the normal genome.

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Now, the genome in our bodies,

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and in those of all
multicellular creatures,

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is actually a double act.

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So you have two copies

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of this crucially important
instruction manual.

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And when gametes are formed,
they're only given half of it,

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one copy of the instruction manual.

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And that's not enough
to build a new organism.

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So each sex cell has to go
out and find another sex cell,

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and only then will they
have enough genetic material

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to start a new organism.

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Let's take a look at some fish

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who've heard the song of Cole Porter.

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They're in a stream somewhere

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and they've chosen a nice location

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which will be a good place
for the young fish to grow up.

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And the adults are gathering

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and there's a bit of a frenzy going on.

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And then at some point,

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the females start to release
their egg cells into the water

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and the males will respond
by producing clouds of sperm.

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And then the adults will simply disappear

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and take no more interest in
the small developing fish.

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Now, at the point the adults disappear,

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the egg cells and sperm cells

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still have some unfinished business.

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So if we look into the water,
we can see what's happening.

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The egg cells, which are huge,

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are rather few and far between,

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and then there are very
large numbers of sperm cells

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gathering around them.

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And the sperm cells are
trying to enter the egg

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and there's a bit of a race going on

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because only one sperm cell
can enter each egg cell.

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As soon as it has done, the
egg cell membrane will harden

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and prevent any other sperm from entering.

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And as the sperm cell

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pushes through the cell
membrane of the egg,

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it will push its half genome inside

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and now the egg cell has
two halves of a genome.

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That means it's got all the
genetic material it needs

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so that it can start to
divide and become a new fish.

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And this moment, called fertilisation,

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is a crucial step on the
pathway that any organism takes

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from starting life as a single cell

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to ending up as a large
multicellular being.

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So let's just recap

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what sexual reproduction
is actually about.

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Multicellular organisms produce sex cells

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that only contain half

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of the genetic material
that they actually need.

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Then they have to go out
and find another sex cell.

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And by joining together in
this process of fertilisation,

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they can restore the genetic
material that they need.

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So you really have to ask
yourself, what is the point of it?

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So the problem that biologists face

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is that there is an obvious cost to sex

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and we can see that if we
consider a fish who is asexual.

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So let's imagine

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that this fish produces egg
cells in a different way.

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She just gives each egg cell
an entire copy of her genome.

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That means that the egg cells

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can start dividing straight
away and form new fish.

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They don't need to be
fertilised by another sex cell.

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And when those fish mature,
they'll all be female

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and they can all immediately
start laying eggs of their own.

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So this population is going
to increase really quickly.

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Now, if we compare that
to the normal sexual fish

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and see what she's doing,

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she's going to give each of the
egg cells half of her genome

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and then they're going to
be fertilised by sperm cells

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which came from males.

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And that means half of her
offspring are going to be male.

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And the males don't have
offspring of their own.

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They're just used to
fertilise the egg cells

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from the females.

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So her population is not
gonna grow nearly as fast.

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And this is what biologists
refer to as the cost of sex.

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It appears that an asexual female

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could easily outcompete a sexual female

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because she can just grow
her population much faster.

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So we have to ask ourselves,

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if sex has such a massive
cost, why bother with it?

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Well, first, actually, not all species do.

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There are a few lizard
species, a snake species,

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and some unusual small
microscopic creatures

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called rotifers that live
in ponds, that don't.

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They just consist of
all-female populations

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that reproduce asexually.

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But what's interesting
about all of these creatures

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is that they've only appeared

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in the last million years or so.

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Now, you might think,

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well, that's a very long
time, but not really.

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Animal life has been on the
planet for 540 million years

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and there are no asexual
lineages that date back that far.

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So there must be some
long-term advantage to sex.

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What might it be?

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Well, let's look again

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at the offspring of
those different females.

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So the asexual female we said
hands each of her egg cells

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a complete copy of her own genome.

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That's why they're all female.

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It also means they're going to be

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genetically identical to
her and to each other.

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The sexual female is different though.

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She's given each of her egg cells

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a random half of her own genome,

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and now they're going to go
off and merge with a sperm cell

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that could have come from
all sorts of different males.

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So they're all going to get

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slightly different genetic
material from the male.

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And so each of her offspring

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are going to be quite different.

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And you probably notice
if you have siblings

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that they're not
genetically identical to you

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unless you're an identical twin.

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And we think it's this genetic
variation that's so crucial

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because we know that the
environment can change.

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Think about the peppered moth.

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For the asexual female,

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she's having to bet
that if she's done well,

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then the environment won't change

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and her offspring,
which are just like her,

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will also prosper.

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But for the sexual female,
she's kind of hedging her bets.

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She's producing different
kinds of offspring.

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So if the environment changes,

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hopefully one of them at least
will be well adapted to it.

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And you might think,

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but does the environment
really change that fast?

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Well, the physical environment might not,

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but a crucial part of
any species' environment

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is other species.

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They have to worry about
predators, competitors,

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and crucially, parasites.

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Now, parasites get into
the bodies of organisms

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and they start taking resources.

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And very often they're
small things like bacteria

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with fast generation times,

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which means they can
evolve pretty quickly.

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So multicellular creatures
have got to be nimble

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if they're going to
stay ahead in that race.

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And it seems that sex is
crucially important in doing that.

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Well, irrespective of the
costs and benefits of sex,

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sexual reproduction has
had profound consequences

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for life on our planet.

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Individuals that belong
to the same species

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can exchange genetic information
via sexual reproduction.

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When they produce sex cells,
they are able to combine

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with the sex cells from other individuals.

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And that means that
members of the same species

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kind of take part in a
shared common genome,

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but they don't share genetic information

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with individuals belonging
to other species.

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They're reproductively isolated from them.

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And that's actually

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one of the commonest
definitions of a species.

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We say that individuals
belong to the same species

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if in theory they can
have sex with each other

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and produce fertile offspring.

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Now, this isolation is important

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because it means that each
species can evolve independently

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and follow its own
evolutionary trajectory.

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And it's only by doing that

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that we can build this
incredible diversity of life

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that we have on our planet.

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If we also compare the
genomes of different species,

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we can find out how similar
or different they are,

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and we use a technique
called genome sequencing

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and we can use that technique

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to build family trees
of different species.

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Here's one, for example,
for the great apes.

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Now for the great apes,
there are only four species.

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There's the orangutan, the gorilla,

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there's us, modern humans,
and there's chimpanzees.

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And by comparing the genomes,
we know how closely related

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those different species are to each other.

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So we know, for example,

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that humans and chimps are most similar.

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Their genomes are very similar indeed,

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and that's because they last
shared a common ancestor

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about 5.5 million years ago.

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But humans and chimps collectively

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last shared a common
ancestor with gorillas

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about 8 to 10 million years ago.

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So the genome of gorillas
is more different

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to that of humans or chimps

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than the human/chimp
genomes are to each other.

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And if we go further back in time

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to about 14 million years ago,

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then that's when the
orangutans branched off.

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So prior to about 14 million years ago,

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there was only that one
species of great ape

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that eventually gave rise to all four.

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Well, I hope you enjoyed today's video,

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and if you did and you found it useful,

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then please do share.

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There's a lot more depth and detail

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in chapter three of my book,

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and you can find a link to it below.

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There's also extra information, of course,

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about why males and females
look different in some species

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and not in others,

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and how sex is determined
in different species.

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If you'd like to watch the next video,

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then there's one coming
soon on chapter four,

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which is all about energy.

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(birds chirping)