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[0:00]Hello, welcome to the second part of a multi-part series on the theory of evolution, and in this part, we're going to talk about the mechanisms of evolution. And there's many different mechanisms of evolution, and they often get confused with each other. And so, I'm going to try to break it down, and this is probably the most confusing part of understanding the theory of evolution. The one that people tend to have most trouble with is natural selection, but it's not the only mechanism, and you need to understand that the theory of evolution is actually made up of a bunch of different parts that all work together. So let's dive into that and remember that evolution itself is the change in heritable traits of biological populations over successive generations. And remember, when we're talking about heritable traits, we're talking about things that can be passed down to offspring, but we're not necessarily talking about genetics. We're talking about the genotype, which is the actual code for a living organism, not the phenotype, which is the expression of that code. Although the phenotype can affect the genotype, and that's where we get into some of the more confusing elements of evolution. But for right now, let's just assume that we're talking about the genotype, the code, the DNA of an organism, and how that changes over time. And one of the first and most obvious mechanisms of evolution is mutation. And mutation is simply a change in the nucleotide sequence of the genome of an organism, virus, or extrachromosomal DNA or RNA. It sounds like a lot, but basically, what it's saying is that the DNA code of an organism can change. And this happens fairly regularly. Sometimes when cells divide, they don't divide perfectly, and the DNA isn't copied perfectly, and you can end up with a mutation. Sometimes external factors can affect the DNA and change it. For example, ultraviolet light is known to cause mutations. And so, when you get a tan or a sunburn, what's actually happening is that you're damaging your DNA, and your body is producing melanin in order to protect that DNA. And sometimes those mutations can be helpful, sometimes they can be harmful, and sometimes they can be neutral. But a mutation simply means that the code has changed. And usually when we're talking about mutations, we're talking about a single base pair change. So that adenine that was supposed to be there is now a guanine. And that can cause a variety of things to happen, but it is a change, and it is heritable, meaning that if that change happens in a gamete, in a sperm or an egg, then it can be passed on to the offspring. And if it's passed on to the offspring, then it becomes part of the evolutionary process. Another mechanism of evolution is genetic drift. And genetic drift is the change in the frequency of an existing gene variant in a population due to random sampling of organisms. What does that mean? Well, basically, it means that sometimes things just happen by chance. For example, if you have a population of organisms and some of them have a particular gene, let's say a gene for blue eyes, and others have a gene for brown eyes. And let's say that by pure chance, a bunch of the blue-eyed organisms get wiped out by a natural disaster. That's not because brown eyes are better than blue eyes, it's just because by chance, those blue-eyed organisms happen to be in the wrong place at the wrong time. And so, the frequency of the blue-eyed gene in the population goes down, and the frequency of the brown-eyed gene goes up. And that's genetic drift. It's a random change in the frequency of a gene in a population. And it's not because one gene is better than the other, it's just because of random chance. And this is especially important in small populations. If you have a very large population, it's unlikely that a random event is going to significantly change the frequency of a gene. But if you have a very small population, then a random event can have a very significant impact. So genetic drift is one of those mechanisms that people often forget about, but it is a very important part of evolution. Another mechanism of evolution is gene flow. And gene flow is the transfer of genetic material from one population to another. And this can happen in a couple of ways. For example, if you have a population of organisms and some of them migrate to a new population, they can bring their genes with them. And so, the frequency of those genes in the new population will change. Or if you have two populations that are close to each other, and they interbreed, then genes can flow between those populations. And this is a way that populations can become more similar to each other, or it can be a way that new genes are introduced into a population. For example, if you have a population of fish in one lake, and a population of fish in another lake, and then a flood happens, and some of the fish from one lake get swept into the other lake, then those fish can introduce new genes into that population. And that's gene flow. It's the movement of genes between populations. And this is a very important mechanism of evolution because it can increase genetic diversity within a population, or it can decrease genetic diversity between populations. And so, it's a very important part of how species change over time. And finally, we have natural selection. And natural selection is the differential survival and reproduction of individuals due to differences in phenotype. And this is the one that most people are familiar with. It's the idea that individuals that are better adapted to their environment are more likely to survive and reproduce, and pass on their genes to their offspring. And so, over time, the frequency of those advantageous genes will increase in the population. And this is often summarized as "survival of the fittest," but that's a very misleading phrase. Because it's not necessarily about who's the strongest or the fastest. It's about who's the best adapted to their environment. For example, a very fast gazelle might be able to outrun a lion, but if it's in an environment where there's no food, then it's not going to survive. So fitness in an evolutionary sense means reproductive success. It means how many offspring you produce that then go on to reproduce themselves. And so, natural selection is a very powerful mechanism of evolution because it's non-random. It's not just by chance that certain genes increase in frequency. It's because those genes are actually providing an advantage to the individuals that have them. And so, over time, populations become better and better adapted to their environment. And these four mechanisms, mutation, genetic drift, gene flow, and natural selection, are the main drivers of evolution. And they all interact with each other in very complex ways. For example, a mutation might introduce a new gene into a population. And then natural selection might act on that gene, and either increase its frequency or decrease its frequency. Or genetic drift might cause the frequency of that gene to change just by chance. And gene flow might introduce that gene into a new population. So they're all interconnected, and they all work together to drive the process of evolution. And it's important to understand that evolution is not just about natural selection. It's about all of these mechanisms working together. And that's why the theory of evolution is so robust, because it accounts for all of these different ways that species can change over time. And in the next part of this series, we're going to talk about the evidence for evolution, and how we know that evolution is actually happening. So I hope you'll join me for that, and thank you for watching.