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Current time:0:00Total duration:5:01

Video transcript

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 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 referring to when we think of evolution an 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 ultimately drives evolution but what is natural selection exactly well why don't we jump right in look at an example let's say it's ten thousand years ago and people survived by hunting and gathering but they also have to worry about you know 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 has slightly longer legs and the other guy now these longer legs put him in 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 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 - the ones with the longer legs have a 50% chance of surviving and reproducing while the shorter leg 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 Fourche 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 and when we started but let's take in another generation further so half of our long legged people reproduce whereas only one quarter 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 sixty-seven percent 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 be 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 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 and 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 in nature itself now finally I just want to point out that natural selection does not apply to acquired characteristics you know if a father teaches a son how to hunt and this makes a child more likely to survive you know that isn't a trait that's selected for 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 Jin 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 learn that natural selection only applies to heritable traits ones that are genetic and passed down from generation to generation