So in our last lesson video, we talked about how organisms are really well adapted to their environments because of adaptation and how adaptation improves the fitness of organisms. And so in this video, we're going to talk about how natural selection actually causes adaptation, or in other words, how adaptation is really just the result of the process of natural selection. Now natural selection, once again, is a process that was first described by the scientist Charles Darwin way back in the 1800s. And really the process of natural selection is exactly what it sounds like. It's when the natural environment selects for very specific organisms within a population that are more fit to their environment. And once again, this word fit here is different than the way that we would use fit in our everyday language. Fit does not mean you have a big bicep, instead, fit is referring to the survival and reproduction of an organism, and so if organisms are more fit, they are more capable of surviving and reproducing. And so in other words, we can say that natural selection is pretty much survival of the fittest. So the organisms that are most fit to their environment are the ones that will survive and be able to pass on their genes to the next generation. Now natural selection has two requirements that we are going to talk about down below right here. Now the first requirement of natural selection is that there must be genetic diversity within a population, and by genetic diversity, all this means is that within a population there has to be organisms that have different features. And so, you can think that there's going to always be genetic diversity within a population. I mean, think about your own family for instance. Are you exactly the same as your parents, or do you look a little bit different? Of course, you're going to look a little bit different because you are genetically different. There's genetic diversity, within, between you and your parents. And so, even within a, population, for instance, of giraffes, there's always going to be a little bit of genetic diversity. So this is something that's always going to be the case. Now the second requirement for natural selection is that there must be some kind of selective pressure for heritable traits that affect fitness. And so by heritable, what it means is that these traits can be passed on to the next generation. And again, fitness is referring to the survival and reproduction of organisms. And so down below what we have is a classic example of natural selection. And so we're taking a look here at the population of giraffes and notice that on the far left, we're looking at a time scale that is early early on, some thousands of years ago, and over here what we have is the present and so this represents our time scale. And so notice that early on thousands of years ago, there was genetic diversity within this population of giraffes. And so you can see that we had some short-neck giraffes like this one over here, we had some medium-neck giraffes like this one here, and some long-neck giraffes like this one here. Now again, in the process of natural selection the natural environment is going to select for very specific organisms that have traits that allow them to survive better. And so what you'll realize is that these giraffes here are all eating from this same tree but what you'll notice is the short-neck giraffe can only access the leaves on the tree that are really low. However, the other two giraffes, the medium-neck, and the long-neck giraffe, they can eat from the lower portions and from higher portions as well. And so what this means is that very quickly the leaves on the bottom are going to disappear a lot faster than the leaves that are higher up. And so pretty quickly here this short-necked giraffe is going to run out of food on the bottom, and so notice he's realizing he's like wait guys, I can't reach these leaves that are higher up and there's no more food down below. And so ultimately, this giraffe is not going to be able to survive and reproduce as well as these other ones because of the natural environment selecting for these organisms that have a little bit longer necks. And so this short-neck giraffe unfortunately has to pass away. And then this other medium-neck giraffe starts to realize what's happening and he's like, oh, shoot. And he's realizing that this layer of leaves here is also going to run out of food because this giraffe is capable of eating this layer as well and so there's going to be a lot more food at the top and this layer of leaves is going to go away pretty quickly. And so, very similarly to the previous situation, this medium-neck giraffe, although it lasted longer than the shorter-neck giraffe, it ultimately is also sadly going to pass away. And so the only giraffes that are left are the ones that were capable of reaching the top of the tree, where there was plenty of food. And notice that the leaves and all the other areas are all gone and that is why these shorter-neck giraffes passed away, because they couldn't survive and reproduce as well as the longer-necked giraffes which had more access to more food. And so he's wondering where did everybody go? And so this is the process of natural selection, again how the natural environment, like for instance just the height of the leaves on this tree, is naturally selecting for organisms, for giraffes that have longer necks. And that's why, today when you go to the zoo, giraffes have such long necks because of this process of natural selection. And so this here really concludes our introduction to natural selection and how this causes adaptation. Again, because this giraffe is very well adapted to its environment being able to access food at higher levels. And in our next video, we're going to talk about how natural selection ties into evolution. So I'll see you guys there.
