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There are lots of ways that evolution happens, and natural selection is just one of them. In this episode of Crash Course Biology, we’ll find out how this process works and shapes traits in all living things —from ginkgo trees to howler monkeys. We’ll also learn how extra-grippy toes help some lizards survive hurricanes.

How Spiders Stay Alive 00:00
What's Natural Selection? 1:50
Requirements for Natural Selection 3:41
Environmental Effects 5:40
Types of Natural Selection 7:39
Evolutionary Trade-Offs 9:23
Artificial Selection 10:34
Review & Credits 11:35

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CC Kids:
Whenever I see a spider web, I always think about the song ‘Stayin’ Alive,’ by the Bee Gees.

You know the one. [Singing] Ah, ah, ah, ah, stayin' alive, stayin' alive. Ah, ah, ah...

A little weird, maybe, but hear me out. That spider web represents the undeniable adaptability of life. Out of ever-changing environments, some shared DNA from a common ancestor, and a whole lot of time, life has accumulated more traits than a Swiss army knife.

Thick fur for beating the cold. A deep root system for surviving drought. Opposable thumbs for gripping, grabbing, and doing this stuff! [drumroll] [cymbals crash] And that includes spider webs.

They’re made of silk stronger than steel, gelled together from ooze rocketed out of their backside. But this weird booty goo can be spooled into designs that form a dinner-catching snare —deadly for unlucky bugs, but life-giving for the spider. All of these traits that help organisms go about the business of not dying long enough to pass on their genes are called adaptations, and these adaptations are shaped by both natural and artificial selection.

They represent thousands, perhaps millions, of different answers to that question every organism faces: “How am I gonna stay alive?” So, when I see a cobweb, I don’t think, “Here’s a mess to clean up.” I think, “Here’s one of  those millions of answers.” I can’t help but hear a rhythm: [to the tune of "Stayin' Alive"] “Well you can tell by the way I spin my web, I’m an adapted thing, I have evolved.” Hi, I’m Dr. Sammy, your friendly neighborhood entomologist, and this is Crash Course Biology. Now are you ready for the second funkiest song you’ll hear today? [THEME MUSIC] Adaptations can be found across life’s spectrum.

You’ll see them in the feathers of a bird’s wing, the spiky thorns of a flower’s stem, and even in your own ability to digest the lactose sugar that’s found in milk – assuming that you’re not lactose intolerant. All of these adaptations are responses to that same question: “how do we keep ourselves alive long enough to reproduce?” Enter natural selection, the process by which populations adapt and change based on inherited traits. It is one of the forces behind evolution’s ongoing four-billion year-long craft project.

It has no goal, no final conclusion. It’s just about what works in a certain environment at a certain time—a problem that’s always changing, and has many possible solutions. Like how these awesome opposable thumbs were an adaptation by our common primate ancestors.

Those that evolved the opposable thumbs didn’t do so for a specific purpose, but because the thumbs gave them an advantage over those without them, they survived longer and were able to pass on their genes more often, selecting for that adaptation in the future. The specific alleles, or versions of genes, that organisms inherit are called genotypes. The word genotype can describe just one gene, several, or all of them.

A genotype is genetic information that helps determine phenotype, which is the actual expression of an organism’s anatomy, internal functions, and behavior in the real world. Phenotype includes not just the outside stuff, but also the inside stuff —like, the structure of cells. Natural Selection acts on traits in an organism’s phenotype.

But the connection here isn’t as simple as “inherit this gene, get this quality.” Instead, phenotypes arise from a mash-up of genotype and environment. Like, identical twins share identical genes, but sometimes you can still kind of tell them apart —like these twins have different freckle patterns. For natural selection to happen, you need a few important things: First is variation.

Populations need to have a variety of traits. That can mean features some individuals have and others don’t. Or it can mean traits that come in a range —like the length of your legs or the size of your ears.

No diversity, no evolution. Second, those traits have to be genetically inherited, passed from parents to offspring. If an individual with an adaptation doesn’t reproduce, that adaptation won’t get passed along.

And third, for selection to happen, traits need to be tied to different outcomes. So they need to lend either an advantage or a disadvantage in the business of living long enough to pass on genes. Imagine, for example, a deer has a genetic mutation for white fur.

That trait would probably make them a bright beacon in the woods —they would be more likely to become a coyote’s afternoon snack or a hunter’s trophy than to live a long life and have lots of offspring. So that trait is likely to be selected against in the population. But maybe another deer inherits a mutation for brown and green stripes, allowing them to blend in really well in the forest, like deer camouflage.

That trait might be helpful in avoiding predators— giving that deer a better chance at stayin’ alive. So that trait would likely be selected in the population. Helpful traits tend to stick around, passing on to future generations and becoming more common in a population.

Like an insect body that looks like a leaf. Eyes that work like night-vision goggles in owls. Or snake jaws that unhinge to swallow food bigger than their head.

Adaptations like this can arise over long timescales. And if they work, selection can keep them around. Like, today’s gingko trees still look very similar to fossil gingko trees from 200 million years ago.

Another example can be found in their seeds, which contain a toxin that deters predators from eating them – a trait that has stuck around for thousands, maybe even millions of years. But natural selection isn’t just some long-ago process. It’s happening right now, as we speak.

And changes in the environment can spur changes in populations faster than you might think. Let’s head over to the Thought Bubble… When Hurricanes Irma and Maria blasted through the Caribbean in 2017, their brutal high-speed winds destroyed homes and uprooted trees. Given the degree of devastation, you might not think anoles— small lizards often found climbing trees in this region —would stand a chance at survival.

But some anoles did survive. And it wasn’t just sheer luck that kept them alive, the ones who rode out the storm had traits that helped them cling to life – quite literally! Measurements from before and after the hurricanes showed that the average anole survivor had bigger toes, longer front legs, and shorter back legs.

Those traits gave the lizards a stronger grip, helping them hold tight even in hurricane-force winds. To figure out how these traits kept them safe, biologists used a leaf blower on anoles clinging to a perch to simulate windy conditions — catching any wayward lizards safely in a net. Anoles with longer hind legs had more dangling surface area, so they blew around like a living windsock.

But anoles with stubby hind legs could keep all four feet glued safely. So, some anoles held out because they could literally hold on. That shows that even sudden changes in the environment — like a strong hurricane — can be fuel for natural selection.

Now, biologists are studying the surviving lizards’ descendants to find out if their super- grippy trait sticks around —not just on branches, but across generations. Thanks, Thought Bubble! Now, not everything that lives or dies during a hurricane, flood, or other natural disaster is a product of natural selection.

The traits of these anoles helped them hold on during a hurricane, but probably wouldn’t have helped them much if there had been a fire. This is just one example of how certain traits can be selected for due to elemental factors. Of course, natural selection can push traits in a few different directions.

Imagine, for example, a forest of pine trees with a Goldilocks-style range of heights. There are some short ones, a few really tall ones, but mostly a whole bunch of middle-of-the-road medium-sized ones. That’s a sign that this population is facing stabilizing selection, a version of natural selection that tends to reduce variation at the extreme ends of the spectrum.

In this scenario, little trees struggle to get enough light, and growing super-tall takes a lot of energy. So the middle-of-the-road phenotype has the easiest time surviving and passing on genes, so those extreme phenotypes, for really little or really tall trees, are more rare. But imagine resources, like water for instance, become scarce, so now it’s actually better to be tiny in this environment because the little trees don’t need as many resources as the medium or super-tall ones.

In that case, this population of trees might become shorter on average. That means that directional selection is at play. And that’s when an extreme version of the phenotype is selected for and passed down genetically, so traits are pushed towards one end of the spectrum.

But selection can also push a population towards opposite extremes at the same time, leaving the middle ground entirely, in what’s called disruptive selection. Like, there are some beetles that normally fight other beetles to get the chance to mate. The biggest beetle wins, so one extreme is selected for.

But in certain populations, small beetles can sneak by unnoticed while the larger beetles are fighting and can mate with the females, while the medium-sized beetles can’t win in either of these categories. And the population changes as a result. But just because a trait helps an organism survive in one environment doesn’t mean it helps in other environments.

Like, polar bears have adapted to eat soft, fat-rich marine mammals like seals, but that makes them inefficient at eating bonier prey. This is called an evolutionary trade-off, and howler monkeys face an unusual one: they can either have a loud voice or…big testicles. On one hand, male monkeys with bigger hyoids — a u-shaped bone in their throats – have louder voices, really good for, well, howling.

On the other hand, bigger testicles make more sperm, upping the odds of passing on their genes. But here’s the catch: male monkeys can only have one or the other. It just takes too much energy for their bodies to have both a loud voice and big balls.

So, in howler monkey species where groups of females mate with just one male at a time, male howlers tend to have louder voices, which is great for warding off sexual competitors. But in species where females mate with lots of males… well, it pays to place bets on the sperm. There’s no one right answer to this staying alive thing.

Just millions of possibilities. Now, we know that natural selection doesn’t have any sort of goal or endgame when it comes to evolution. But, unlike nature, we humans do.

So we’ve changed some organisms through artificial selection – selectively breeding plants and animals for traits that we prefer. Like dogs, who all share a common wolf-like ancestor and have been put through artificial selection’s funhouse mirror —resulting in two-kilogram Chihuahuas, 70-kilogram Great Danes, and all kinds of good doggos in between. So, since the beginning of life itself, natural selection has molded and shaped a huge array of adaptations that are still in flux as we speak.

Traits that work well enough to help organisms survive and reproduce tend to hold fast and pass on over generations. And on a much shorter timescale, humans have bred certain species to artificially select traits that we find favorable. Shoutout to Jimmy, the blue heeler/shih tzu mix.

Both types of selection are possible because of variations in traits within populations that are inherited across generations. And there’s no right or wrong answer, no perfect solution. Just an array of ever-changing possibilities— adaptations ranging from spider webs to super-sticky toes.

Each of them offering a different response to that shared, thrumming, four-billion-year-long beat: staying alive. In our next episode, we’re going to take a look at the similarities and differences within populations, and find out why you just might just have a doppelganger out there. See you then!

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