 So Darwin had these observations. Based on those observations, he made some inferences. His inferences were like, almost like conclusions that he drew based on the observations that he made. And the first inference that he made, the first thing that he thought of where he was like, you know what, this, if all of these are true, then some critters are going to be more likely to survive than others. Some, okay, I'm going to write it like this, differential survival. Okay, some of them are more likely to survive, and let's take it a step further. Let's say there's going to be differential survival due to inherited traits. Now, I want you to think about this for one second. If, for example, my kids were more likely to survive because I dyed their hair blue. Blue hair is not an inherited trait. That's just something that I was phenomenal and decided to give them blue hair. And then, because I was so phenomenal and they now have blue hair, they're more likely to survive. But that blue hair, they might learn, they might, an inherited trait could be that they're really smart, like their mama, and they might dye their kids' hair blue so that their kids are more likely to survive. I have no idea where this analogy is coming from, but that's not, the hair itself is not an inherited trait. The hair is something that I just added into the mix. This only works, Darwin's little plan here, his little conclusion, only works with inherited traits. So it's not an example of differential survival due to his process unless the variety, the variation, can be inherited. So if we were to go back and think about our population of ladybugs, if for some reason the polka-dotted ladybugs were more likely to survive and the polka-dots were an inherited trait, then this is his conclusion, that this is what happens. And then his second conclusion is, his second inference is that those who's inherited traits give them a survival advantage, those with survival advantage will probably make more babies. I told you it all comes down to babymaking, like everything comes down to babymaking. Not all of them are going to survive. They all have different characteristics, but those inherited characteristics that give them an advantage, a survival advantage means they're more likely to make babies. And this, my friends, is the definition of fitness. I mean, fitness is not how far you can run. Evolutionary fitness is basically how many babies you can make. So I can totally ask you a quiz question where I said, look, Fred is like, he's got awesome muscles, he's really good looking, he lives until he's like 120 years old, and he has one kid. George is, well, we won't describe George, but he lives until he's about 42 and he has 13 kids. Who is more fit from an evolutionary perspective? Oh boy, I forgot what his name was. Was it George? George who had 13 kids. George passed on more genes, so he was a fitter human. He contributed more genetics to his population. The gene pool, more like him. Go, George, go. Sorry for a luck 120-year-old guy who only had one kid. What did you contribute to the gene pool? All right, can you visualize how natural selection can cause changes? This whole thing is natural selection. The fact that not everybody survives, those who do are more likely to make babies. And if the gene pool changes because of this differential survival and reproductive differential reproduction, that's evolution. Now, I was going to tell you something. Oh, that's one way that we can have change in allele frequency through the process of natural selection. And natural selection happens all the time and we can see it happening. We can study it as it happens and we can definitely visualize how, yeah, we can totally change the genetics of a population by selecting certain characteristics that are heritable. It's not the only way that evolutionary change happens. It's not the only way that we can change the allele frequency in our population. I have two more ways to talk about. So let's first talk about gene flow. Feel the flow, dogs?