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So today we're going to talk about a topic that's very central to the idea of evolution, and that's natural selection. But before we get into that, I want to talk about what evolution isn't. So evolution isn't when some organism like this monkey magically transforms into a human. And it's also not when an organism changes in some way when it's in trouble, like a person growing wings after jumping off a building. And I want to clarify that this is not what we're referring to when we think of evolution. And evolution is a process that occurs to populations of an organism, not individual members. And it occurs over huge amounts of time, and we're talking millions and millions of years for even small changes. So natural selection is one of the forces that ultimately drives evolution, but what is natural selection exactly? Well, why don't we jump right in and look at an example? Let's say it's 10,000 years ago and people survived by hunting and gathering, but they also have to worry about being chased around by wild animals. So in order to survive, these people need to be able to find food. But they also need to be able to escape from predators. Well, let's say that one of the people of these two has a special genetic trait and has slightly longer legs than the other guy. Now, these longer legs put him at an advantage, because his legs are longer and he can run, let's say two times as fast as everyone else. And because of this, he's more likely to survive when a predator like this bear chases him down. So what this also means is that the guy with the long legs is more likely to reach an age where he's old enough to find a mate, reproduce, and have children who would also have this special trait of longer legs because it's genetic. And because he's more likely to have kids than everyone else, over long periods of time soon more and more of the population will have this special trait. Now, let's look at this idea again but a little more deeply. And let's say there are six people in the world and two of them have longer legs than everyone else, And let's say that the ones with the longer legs have a 50% chance of surviving and reproducing while the shorter-legged people have only a 25% chance of surviving and reproducing. So that means one of our two long-legged people and one of our four short-legged people here will reach an age where they can reproduce, so now these people who survived will each have four children. And naturally, these children will resemble their parents. And the children of long-legged people will also have long legs, and the children of short-legged people will have short legs. So now in our next generation, we have four people with long legs and four with short legs. And you can already see that more of the population has long legs than when we started. But let's take it another generation further. So half of our long-legged people will reproduce, whereas only 1/4 of our short-legged people will reproduce. And this means that by our third generation, we'll have eight long-legged kids and only four short-legged ones. Now, if we number our generations, generations one, two, and three, we can see that in generation one, 33% of the population was long legged. In generation two, 50% of the population was long legged. And by generation three, 67% of our population was long legged. And this is all because that special trait of having longer legs made those people more likely to survive and reproduce than those with short legs, and this is the crux of how natural selection works. So why is it called natural selection in the first place? Well, let's say with our example of the short- and long-legged people. Now, we use the word selection because one trait is advantageous over another and is selected to be passed on to future generations more than other traits. On the other hand, selection can also apply to a disadvantageous trait. If we have people who have really short legs and run really slowly, then those people will be selected against and won't pass on traits to offspring as frequently. Now, we use the word natural because there isn't an individual who's physically selecting which traits are good and which ones are bad. It all has to do with whoever has the greatest probability of surviving. There's no one actually doing the selecting except nature itself. Now finally, I just want to point out that natural selection does not apply to acquired characteristics. If a father teaches his son how to hunt and this makes a child more likely to survive, that isn't a trait that's selected for us since it's not genetic and it's not absolutely passed on to children. So that's why we say that natural selection only applies to heritable traits, with heritable traits being any genetic trait. So what did we learn? Well, first we learned about the concept of natural selection and how traits that help an organism survive are more likely to get passed on to offspring. Next we learned that evolution, which is driven by natural selection, occurs to populations, not individuals, and occurs over a huge period of time. And finally, we learned that natural selection only applies to heritable traits, ones that are genetic and passed down from generation to generation.