- Hey, smart people, Joe here.

Unless you're from another planet, you know that a peacock is famous for its dazzling splendiferous, tail feathers.

And you probably even know what those fancy boys use their pretty posteriors for impressing the ladies.

Oh yeah.

But what you might not know is that this bird, along with countless other elaborately ornamented organisms confused Charles Darwin so much that he had to develop a completely different theory of evolution to explain it.

This video is about Darwin's other great idea, an idea that has the power to explain some of nature's strangest behaviors and most beautiful forms, but took a century to be accepted by most scientists.

And to tell that story, we're going straight to the source to figure out the paradox of the peacock.

Because weird things happen when evolution meets "seggs" or sex.

So why did peacocks confuse Darwin so much?

Huh?

Well, Chuck D changed science forever in 1859 when he dropped his theory of natural selection that animals were plants with traits that are better suited to their environment, survive and reproduce more, making those traits more common over time.

But that idea on its own couldn't explain this or this or this.

Many traits don't seem to be well suited for an animal's environment at all.

In fact, they can even be hazardous to survive.

This is the paradox that perplexed Darwin.

Why would nature allow so many traits to evolve beyond what's optimal for survival?

What Darwin realized is that some traits aren't better suited for the environment.

They're better suited for sex.

We call it sexual selection.

A trait is sexually selected if it helps some individuals reproduce more than others, and this happens in two big ways.

The first is that males can directly compete with each other for access to mates.

This is what happens when sheep butt heads, roosters fight, and elephant seals smack each other with their noses.

I'm sorry, you have to admit, these boys look very silly.

Darwin called this the law of battle, and if this was all there was to sexual selection, it would be pretty simple.

But as you can see, there's still a few minutes left in this video.

Because males competing with each other doesn't explain this.

You can't fight with fancy tail feathers.

The other big way sexual selection happens is in choosing mates.

And most of the time in nature, it's females choosing the males.

Now, it's hard to believe a female chooses which male to mate with by blind chance.

She must be looking for something.

But sometimes choosing a particular mate can have direct benefits like protecting territory or males helping out with parental care.

But frankly, many dads in nature don't contribute anything beyond sperm and a set of genes.

And this is the case for peacocks and peahens.

After matings, moms are pretty much on their own in the nest egg department.

- So - What's in it for these females?

What's in it for you?

Well, sometimes having beautiful or ornamental features can serve as an external sign of health or having quote good genes.

But a lot of the time looking fancy isn't a reliable signal of actually being healthy or strong.

When it comes to peacocks, we know the male must have one or more genes that grow their ornament.

And different versions of these genes make a variety of tails.

For instance, let's imagine there's a fancy tail gene, and different versions make different displays of different sizes.

And if we assume there's something programmed in the female to prefer bigger or prettier ornaments, their offspring will carry genes for the bigger ornament from their father.

And genes for preferring bigger ornaments from their mother generation after generation.

This means bigger fancier ornaments and the preference for bigger fancier ornaments are linked and both become more common over time.

Excuse me, ma'am, do you have a genetic preference for certain sizes of tails?

- Yeah, That is how choosy females turn peacocks from this into this.

- Yeah, baby.

- If peacocks were only under the influence of regular natural selections, smaller tailed males can better avoid predators would mate more, and we might expect peacock tail feathers to shrink over time.

But in reality, because females have some innate preference for bigger tails, smaller tailed males essentially never get to mate.

And sexual selection keeps tales big and fancy, and that's success.

Getting to mate more, getting your genes into future generations, that makes up for the fact that your big dumb tail makes you an easy snack for a predator.

Are you worried about predators?

Sir?

You worried about anything?

This positive feedback loop where you get a fancy trait and the other sex has some programmed preference for that trait was first identified by mathematician and biologist Ronald Fisher, and today we call this sort of snowball effect of sexual selection Fisherian runaway.

It can even apply to behaviors.

For instance, once female bower birds evolved a preference for males who can decorate, whichever males had gene versions that drove them to make fancier decorations, they mated more than the others, and decorations got fancier over time.

A later version of this idea became known as the sexy son hypothesis.

Females who choose physically attractive males will tend to have more attractive sons and therefore more grandchildren because other choosy females will prefer their sexy sons.

It's a little weird, but hey, when it comes to evolution, the whole point of life is getting your genes into the next generation.

You can do that by say, being camouflaged and maybe surviving longer, or you can do that by fighting or fancying your way into mating more often.

Peacocks' elaborate feathers show that females aren't always picking traits that will help their offspring survive.

How do you think that went?

You think she liked it?

Sometimes they pick traits that will help their offspring mate more.

In reality, natural selection and sexual selection are usually happening together, but peacocks are proof that sexual selection can be so powerful that it outweighs regular natural selection.

That's why the peacock paradox isn't a paradox at all.

This idea that females are choosy about their mates started out pretty controversial because it gave female animals a level of power and independence that human females didn't exactly have at the time.

For the most part in nature, it's males who compete with other males for access to mating, and females are more often the ones doing the choosing.

But there are exceptions where these roles are reversed.

For instance, female hyenas are larger and more dominant than males and females compete with each other for who gets to mate and when.

And when multiple queens hatch in a beehive, the first to emerge, will kill her sister queens before they can do the same to her.

And there's this one type of fly where the females put on elaborate decorations and dance to woo the males.

So what about us?

Well, these ideas of sexual selection are sometimes applied to humans, but we've gotta be careful when we do that.

On the one hand, we're animals and we've certainly gotten to be the way that we are, thanks to both natural selection and sexual selection.

I mean, dancing, flowers, muscular dudes on Jersey Shore... Darwin would have a field day with that stuff.

But on the other hand, we've also got the ability to make choices based on cultural and other preferences.

So I wouldn't say that everything humans do in the dating and mating game can be distilled down to evolution.

Darwin's confusion ended up showing us that evolution runs on two engines: survival and reproduction.

I love that peacocks are one of the most recognizable birds on the planet, right?

I mean, ask your average 3-year-old.

They know what a peacock is.

But there's this really interesting secret about evolution hiding there reminds me that there's some really deep mysteries hiding in some surprisingly common places.

And beautiful ones too.

Stay curious.

That was right on cue, buddy.

I really appreciate that.

Five star review.

Excuse me, ma'am.

Quick question.

Do you ... ma'am, what do you think of his genes?

Are those good genes?

Show off.