Hello, and welcome to what we're calling here the evolution of populations. Now in this video, we're just going to give an overview of those concepts that we're going to go into in a lot more detail coming up, but we just want to paint this picture so you understand how all the pieces fit together. Now we've previously said that we define evolution as a change in a population over time, but we want to note here that you can sort of study evolution at different scales. You can see how it happens at sort of large changes over long periods of time, and we call that macroevolution. These large changes that take a long time to happen.
That's sort of things like thinking of birds evolving from their dinosaur ancestors or, you know, maybe looking at tigers and lions and wondering what caused these two species to be different from each other since they evolved from a common ancestor. Well, in contrast, we can look at what we sometimes call microevolution, and that's going to be these small changes that take a very short time. I kind of think of this as sort of the everyday evolution that's happening out there in the wild, sort of as we speak. That's going to be the focus of this section. Alright, so we have this example here where we have the white and brown rabbits, and we see the fox catching that white rabbit.
And we want to understand here though that for this section, the variation that we really want to focus in on is not the color, but those genes and the alleles that underlie that color variation. So here I've laid over the different chromosomes, and we can see that there's a big A allele or a little A allele that causes that different brown or white color variation. So in this section, what's going to be interesting is not that the fox caught the white rabbit, but that the fox removed specific alleles from the population. Alright. To see what I'm talking about, we're actually going to sort of refine our definition for evolution here.
It's still a change in a population over time, but how we're going to measure that change, we're going to say evolution is a change in allele frequency. Remember, an allele is just those different variants. We'll say here variants of a gene. So if one allele becomes more common over time or less common over time, that is going to be our measure that the population has evolved. So what we're talking about here, obviously, we said populations require variation, evolution requires variation, but specifically we want to focus in on that genetic variation.
Genetic variation. And by that, we mean the population needs multiple alleles. Alright. So where do multiple alleles come from? Well, new alleles are introduced through mutation, and mutation is definitely something that we will be focusing on in this section.
But once a new allele enters a population, it might become more common, or it might become less common and just be lost from the population. That's going to depend on different evolutionary mechanisms. So we're going to talk about 3 of them: natural selection, genetic drift, and gene flow. Alright. I've talked about natural selection before, but we're going to say here that, you know, evolution is caused by natural selection when certain alleles make it more likely that individuals reproduce.
Now you can say survive and reproduce, but remember, survival is important because if you survive, you get to reproduce and pass on those alleles. And those organisms that are reproducing more pass on their alleles more. That will cause a change in allele frequency. Alright. But not everything's due to natural selection. Some change is just random change, and that we call genetic drift. So genetic drift, we're going to say here, these are random changes to allele frequency. Right? Nature is a wild and crazy place. Sometimes things die for random reasons, sort of probability doesn't always work out the way you think it should.
When that happens, we call it genetic drift. Finally, we have gene flow, and gene flow is just the movement of alleles between populations. So if one population, sort of, individuals from one population move into another and they bring different alleles with them, that'll change the allele frequency in that sort of second population, and that we will call evolution. Alright, the stuff that we're talking about here really came together in what we call the modern synthesis. And we call it the modern synthesis, but this actually happened at the beginning of the 1900s, and it's when people combined Darwinian evolution and Mendelian genetics.
Remember, Mendel and Darwin did their work actually about the same time, but people didn't rediscover Mendel's work until the 1900s, the early 1900s. And that's when people sort of understood that these ideas of genes and alleles, that's what was causing what Darwin was talking about. And so people put those together, that they used a lot of math to figure out how all these different things work, and that sort of spawned the field of what we're going to call here population genetics. Population genetics is the study of processes that affect genes at the population level. Right?
So where Mendelian genetics worries about sort of single crosses. Here, we want to really take that whole population view and think about how these alleles and genes are changing throughout whole populations. That's what we've been doing in this section. We're going to practice it more coming up. It'll be fun, and I'll see you there.
