Hey, smart people.

Joe here.

You know, a lot of us probably remember when we had...the talk.

[electronic bleeps and bloops] Are you winning, son?

Not anymore.

Obviously.

We need to talk.

Oh, my God, this is not happening.

About...sex.

[in slow motion] No... [light twinkling music] ...ooooo.

This isn't going to be like that, hopefully.

But it is going to surprise you.

First, a seemingly simple question.

What is sex for?

If you were to ask a bunch of people "What's the purpose of sex, biologically speaking?"

They'd probably say, "Reproduction."

[baby groans] I mean, that's how you and I got here, right?

But that is not what sex is for.

Biologically speaking, "sex" isn't what you're probably picturing in your head right now.

We all know that passing genes on to the next generation is essentially the whole point of life.

Survive to reproduce and get your precious genetic information into some offspring so that they can go on and do the same thing.

And there are two primary ways to accomplish that.

Method 1: Divide and copy all your information over to a genetically identical offspring.

Or Method 2: First, you do some fancy cell division.

Then go off and spend a bunch of time and energy looking for a potential partner.

Then you got to attract that partner.

And while you're mating, try not to get eaten by a predator or eaten by your potential mate.

If you survive and get lucky, you mix a random half of your genes with a random half of their genes to make some genetically patchwork offspring.

One of these is obviously much simpler than the other.

And sex refers to just this one bit of the process.

Taking part of the genetic instructions from two organisms and combining them into one complete set of genetic instructions.

Now, plenty of living things reproduce without sex.

Microbes that bud off new copies of themselves.

Or like flatworms, pieces of their body can grow into whole new individuals.

Fungi that release spores.

Even some bitey fish, scaly pals, and danger noodles are capable of virgin birth.

So sex isn't required for reproduction.

Then what is it for?

The truth is, why sex exists is one of the big unanswered questions in evolutionary biology.

I mean, if you think sex is hard to figure out in your life, take comfort in the fact that science hasn't totally figured it out, either, okay?

What makes sex so puzzling is that it's so expensive.

I'm not talking about dinners and dates, presents, and those kinds of things.

Those are pretty much just human problems.

I'm talking about the biological costs of sex.

For starters, sex is slow.

In the time that it takes, say, a bacterium to copy themselves and then divide, we've barely gotten ready for a date.

And finding a mate can be really hard.

I mean, for a tiny bug or some itty-bitty ocean creature, searching for sex can feel a lot like searching for a needle in a haystack the size of the solar system!

And they don't have Tinder and stuff.

In fact, that's why so many organisms have both male and female parts, you know?

Less searching.

But slowness and loneliness aren't actually the biggest costs of sex.

Surprisingly, it's...me.

Okay, not literally me.

But the cost of having males in general.

Consider a model in which every female has two offspring.

In a species that reproduces asexually, we can consider every individual female.

And all their offspring are also female.

In a sexually-reproducing species, she has one male and one female offspring on average.

In the next round, each of the asexual females has two more females.

In a sexual species, that female still just has one male and one female offspring.

The asexual population grows twice as fast as the sexual population.

In other words, sex is twice as costly as no sex.

This is because in a sexual species, females spend half their resources having sons who can't make offspring themselves.

I mean, yes, males contribute their genes in sex.

But in most species throughout nature, males don't do that much else.

Males are costly.

One of the strangest costs of sex is that it can ruin all of natural selection's hard work because sex breaks apart favorable combinations of genes.

Imagine you walk into a poker tournament, and you offer all the winners at each table this opportunity: For the next round, you can keep your winning hand, or you can shuffle your cards with another player.

Surely, the winners would choose to hold on to the cards they have because they're winning hands.

They have combinations of cards that work well together.

Shuffling winning hands together is more likely to make each of them worse, not better.

What determines if you win a poker game is a combination of cards working together, not the individual cards.

And that's how it works with genes too.

It's the combination of genes working together in a particular game or environment that decides whether an individual will survive and reproduce or fail and fold.

And sex forces you to shuffle your hand.

In real life, this shuffling happens during a process called meiosis.

Early in meiosis, each chromosome swaps chunks with its partner buddy to make new mixed-up chromosomes.

Then the newly crossed-over partners split up.

So each sex cell randomly gets just one mixed-up chromosome from each pair.

That way, when male and female sex cells come together to form the next generation, the offspring have one complete set of chromosomes instead of two.

In a species like ours with 23 sets of chromosome pairs, there are 2 to the 23rd or 8,388,608 different ways your mixed-up chromosomes pairs can split up during segregation.

And you add in the mixing of recombination, and that's exponentially even more genetic diversity, which is why you're a special and unique snowflake.

Unless you're a monozygotic twin, maybe.

So sex is a really bananas way to get your genes to the next generation.

It's slow, it's costly, and it can break up winning genetic hands.

But then why is sex so common?

Only about one in a thousand animal species and only 1% of flowering plants are exclusively asexual.

From fleas to trees, pretty much every eukaryote does "the birds and the bees."

This is a paradox.

There must be a reason, a huge advantage to sex that more than makes up for these huge costs.

What is it?

Well, shuffling up your winning hand may actually be the winning strategy because nature doesn't always play by the same rules.

Let's put in a stock clip or something.

Sex creates genetic diversity.

But remember like we showed in that poker example.

For an organism that's already well-adapted to its environment that's carrying a winning hand of genes that are working together, shuffling does seem counterproductive.

Here's the thing, the rules of life's poker game don't stay the same from game to game.

Environments change, and what was a winning hand today might not be tomorrow.

So if we think about our poker tournament again, imagine now that the rules of the game change every hand.

So you don't know if a straight flush will be a good hand or a bad hand until the next round starts.

Now do you want to change your cards?

Remember that asexual reproducers are stuck with the hand that they're dealt, which might seem good at first, but over time, as the rules of the game change, the filter of natural selection changes too.

And they gradually accumulate random mutations too.

Cards in their hand are randomly being changed with no way to fix or get rid of them.

If you're asexual, what was an advantage at first can very quickly turn into an extreme disadvantage.

So once you realize that nature's constantly changing the rules of the game and an organism's environment, sex makes a bit more sense now, especially when that game is parasites.

Whether a parasite can infect a host has a lot to do with your genetics.

Your cells in your body essentially have genetically-programmed locks that parasites are trying to unlock.

In a population where hosts have the same genes, like in asexuals, any parasite that evolves to pick the lock can bring down the entire population.

But by shuffling genes, like in sexual populations, every genetic lock that a parasite sees is a lock it's never seen before.

And even if it opens one, the rest of the population is safe.

The same reasoning applies to any relationship where species are evolving together.

Predators and prey.

Viruses and their hosts.

Biologist Leigh Van Valen called this idea the "Red Queen Hypothesis."

The name "Red Queen" comes from "Through the Looking-Glass," where the Red Queen tells Alice while they're running as fast as they can but not getting anywhere, "Now here, you see, it takes all the running you can do, to keep in the same place."

When the environment and its challenges are constantly changing, the constant shuffling of genes in sex is the key to long-term survival.

Scientists have even seen this play out in real life.

In these small desert pools in Mexico, sexual fish species compete alongside asexual species.

A scientist noticed that the asexual ones carried more parasites, just as the Red Queen predicts.

The sexual fish are a moving target for the parasites, but once a parasite catches one asexual, it catches up to them all.

When a drought dried up these ponds, the sexual fish lost their parasite resistance because they had lost their genetic diversity-- just like the asexuals-- until the scientist seeded the pond with sexual fish from nearby ponds.

Their genetic diversity increased again, and their parasite resistance returned.

Species don't stop evolving just because they've succeeded in the environment.

Have you ever wondered why species like peacocks or antler boys have these huge ornaments that are expensive to grow or make them easier targets for predators?

Well, the Red Queen may be able to explain this too.

What if these ornaments are attractive to mates because they're like big signs that display how healthy and strong an animal is?

In other words, if you've got a healthy mix of parasite-resistant genes, you're more likely to mate.

These visual displays are a way for sex to keep some winning genetic hands together.

Sex is costly.

But it gives the next generation a better chance of surviving a challenging environment.

Some organisms, including many plants, some algae, even bugs, like aphids, they clone themselves asexually some of the time, but reproduce sexually when their environment presents a challenge.

And genetic shuffling might provide a survival advantage.

Switching back and forth between reproductive modes seems super ideal.

So why can't we reproduce asexually?

Like budding off a big toe and growing a baby clone when it suits us?

Sorry, that is horrifying.

Well, in addition to eventually running out of toes, organisms don't get to decide how to reproduce.

All of these mechanisms, all these trade-offs, the costs and advantages, are coded into an organism's biology by their environments, by their ancestors' history.

It can be tempting to say something like, "Sex evolves to create genetic diversity."

But that's not quite right.

Sex evolved randomly, just like everything else.

It's some still-unknown point in the distant history of life.

And the resulting genetic diversity it generated made the organisms that did it a little bit more fit and a little bit more likely to get their genes into the next generation.

And that's what matters in this game.

I mean, it's not fate.

It's evolution.

Stay curious.

I gotta pick all those up now.

Shuffled my genes.

Don't use that last part.

[laughs] Hey, guys, thank you so much for sticking around.

You've probably seen on the channel a special new series that we've premiered recently called In Our Nature.

It's an incredible nature series.

It's co-hosted by my friends Emily Graslie and Trace Dominguez.

We're taking you around the world from literally the African Serengeti to some of the most amazing ecosystems in North America to tell some incredible nature stories.

Check 'em out here on the channel.

I mean, there's baby elephants.

How could you say no?

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Guys, couldn't do it without you.

So thankful for you.

And if you want to join that super cool club, there's a link down on the description where you can learn more about our community, see some behind-the-scenes stuff, get early access to videos.

We'll see you there and in the next video.

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