- Hello, my name's Lindsay Turnbull,
and I teach biology at the University of Oxford.
In this video, I want to show you
why nearly all species reproduce sexually.
It's actually a huge puzzle for biologists
and it forms the core of chapter three of my book,
"Biology: The Whole Story".
(birds chirping)
(frogs croaking)
In 1928, the legendary songwriter Cole Porter
penned some famous lyrics.
He wrote, "Birds do it, bees do it,
even educated fleas do it.
Let's do it. Let's fall in love."
Well, I hate to be the party pooper,
but I don't think bees or fleas
are capable of a complex emotion like love.
Instead, what Cole Porter was really talking about was sex.
That is what is shared by all multicellular organisms.
Let's think for a minute about how any organism reproduces.
For single-celled organisms, reproducing is very simple.
All it has to do is copy the genome,
double up the ribosomes, make a bunch of proteins,
and then it can simply split itself into two cells,
making sure that each one has a copy of the genome
and all the bits it needs.
But as a multicellular being,
you can't just carve yourself in two.
Instead, most multicellular organisms
set aside a group of cells
and they're going to be responsible
for creating the next generation.
And the rest of the cells in your body, I'm afraid,
are just doomed to die when you do.
When individuals reach adulthood,
those special cells spring into action
and they start to produce sex cells or gametes.
Now, these come in two forms.
There are egg cells, which are large and immobile,
and there are sperm cells,
which are much smaller but can swim actively.
But whichever type they are,
there's something very unusual about sex cells,
and that's because they only contain
half of the normal genome.
Now, the genome in our bodies,
and in those of all multicellular creatures,
is actually a double act.
So you have two copies
of this crucially important instruction manual.
And when gametes are formed, they're only given half of it,
one copy of the instruction manual.
And that's not enough to build a new organism.
So each sex cell has to go out and find another sex cell,
and only then will they have enough genetic material
to start a new organism.
Let's take a look at some fish
who've heard the song of Cole Porter.
They're in a stream somewhere
and they've chosen a nice location
which will be a good place for the young fish to grow up.
And the adults are gathering
and there's a bit of a frenzy going on.
And then at some point,
the females start to release their egg cells into the water
and the males will respond by producing clouds of sperm.
And then the adults will simply disappear
and take no more interest in the small developing fish.
Now, at the point the adults disappear,
the egg cells and sperm cells
still have some unfinished business.
So if we look into the water, we can see what's happening.
The egg cells, which are huge,
are rather few and far between,
and then there are very large numbers of sperm cells
gathering around them.
And the sperm cells are trying to enter the egg
and there's a bit of a race going on
because only one sperm cell can enter each egg cell.
As soon as it has done, the egg cell membrane will harden
and prevent any other sperm from entering.
And as the sperm cell
pushes through the cell membrane of the egg,
it will push its half genome inside
and now the egg cell has two halves of a genome.
That means it's got all the genetic material it needs
so that it can start to divide and become a new fish.
And this moment, called fertilisation,
is a crucial step on the pathway that any organism takes
from starting life as a single cell
to ending up as a large multicellular being.
So let's just recap
what sexual reproduction is actually about.
Multicellular organisms produce sex cells
that only contain half
of the genetic material that they actually need.
Then they have to go out and find another sex cell.
And by joining together in this process of fertilisation,
they can restore the genetic material that they need.
So you really have to ask yourself, what is the point of it?
So the problem that biologists face
is that there is an obvious cost to sex
and we can see that if we consider a fish who is asexual.
So let's imagine
that this fish produces egg cells in a different way.
She just gives each egg cell an entire copy of her genome.
That means that the egg cells
can start dividing straight away and form new fish.
They don't need to be fertilised by another sex cell.
And when those fish mature, they'll all be female
and they can all immediately start laying eggs of their own.
So this population is going to increase really quickly.
Now, if we compare that to the normal sexual fish
and see what she's doing,
she's going to give each of the egg cells half of her genome
and then they're going to be fertilised by sperm cells
which came from males.
And that means half of her offspring are going to be male.
And the males don't have offspring of their own.
They're just used to fertilise the egg cells
from the females.
So her population is not gonna grow nearly as fast.
And this is what biologists refer to as the cost of sex.
It appears that an asexual female
could easily outcompete a sexual female
because she can just grow her population much faster.
So we have to ask ourselves,
if sex has such a massive cost, why bother with it?
Well, first, actually, not all species do.
There are a few lizard species, a snake species,
and some unusual small microscopic creatures
called rotifers that live in ponds, that don't.
They just consist of all-female populations
that reproduce asexually.
But what's interesting about all of these creatures
is that they've only appeared
in the last million years or so.
Now, you might think,
well, that's a very long time, but not really.
Animal life has been on the planet for 540 million years
and there are no asexual lineages that date back that far.
So there must be some long-term advantage to sex.
What might it be?
Well, let's look again
at the offspring of those different females.
So the asexual female we said hands each of her egg cells
a complete copy of her own genome.
That's why they're all female.
It also means they're going to be
genetically identical to her and to each other.
The sexual female is different though.
She's given each of her egg cells
a random half of her own genome,
and now they're going to go off and merge with a sperm cell
that could have come from all sorts of different males.
So they're all going to get
slightly different genetic material from the male.
And so each of her offspring
are going to be quite different.
And you probably notice if you have siblings
that they're not genetically identical to you
unless you're an identical twin.
And we think it's this genetic variation that's so crucial
because we know that the environment can change.
Think about the peppered moth.
For the asexual female,
she's having to bet that if she's done well,
then the environment won't change
and her offspring, which are just like her,
will also prosper.
But for the sexual female, she's kind of hedging her bets.
She's producing different kinds of offspring.
So if the environment changes,
hopefully one of them at least will be well adapted to it.
And you might think,
but does the environment really change that fast?
Well, the physical environment might not,
but a crucial part of any species' environment
is other species.
They have to worry about predators, competitors,
and crucially, parasites.
Now, parasites get into the bodies of organisms
and they start taking resources.
And very often they're small things like bacteria
with fast generation times,
which means they can evolve pretty quickly.
So multicellular creatures have got to be nimble
if they're going to stay ahead in that race.
And it seems that sex is crucially important in doing that.
Well, irrespective of the costs and benefits of sex,
sexual reproduction has had profound consequences
for life on our planet.
Individuals that belong to the same species
can exchange genetic information via sexual reproduction.
When they produce sex cells, they are able to combine
with the sex cells from other individuals.
And that means that members of the same species
kind of take part in a shared common genome,
but they don't share genetic information
with individuals belonging to other species.
They're reproductively isolated from them.
And that's actually
one of the commonest definitions of a species.
We say that individuals belong to the same species
if in theory they can have sex with each other
and produce fertile offspring.
Now, this isolation is important
because it means that each species can evolve independently
and follow its own evolutionary trajectory.
And it's only by doing that
that we can build this incredible diversity of life
that we have on our planet.
If we also compare the genomes of different species,
we can find out how similar or different they are,
and we use a technique called genome sequencing
and we can use that technique
to build family trees of different species.
Here's one, for example, for the great apes.
Now for the great apes, there are only four species.
There's the orangutan, the gorilla,
there's us, modern humans, and there's chimpanzees.
And by comparing the genomes, we know how closely related
those different species are to each other.
So we know, for example,
that humans and chimps are most similar.
Their genomes are very similar indeed,
and that's because they last shared a common ancestor
about 5.5 million years ago.
But humans and chimps collectively
last shared a common ancestor with gorillas
about 8 to 10 million years ago.
So the genome of gorillas is more different
to that of humans or chimps
than the human/chimp genomes are to each other.
And if we go further back in time
to about 14 million years ago,
then that's when the orangutans branched off.
So prior to about 14 million years ago,
there was only that one species of great ape
that eventually gave rise to all four.
Well, I hope you enjoyed today's video,
and if you did and you found it useful,
then please do share.
There's a lot more depth and detail
in chapter three of my book,
and you can find a link to it below.
There's also extra information, of course,
about why males and females look different in some species
and not in others,
and how sex is determined in different species.
If you'd like to watch the next video,
then there's one coming soon on chapter four,
which is all about energy.
(birds chirping)