When we introduced natural selection, we said that it's an idea that can feel kind of simple and straightforward, but it has a lot of depth and nuance to it. Here, we want to explore some of that depth. Alright. In our previous example, we used white and brown rabbits. Well, it turns out this is a real example.
We're going to be talking about snowshoe hares. Now most snowshoe hares, you probably know, they live in areas like in North America and Canada, where there's snow during the winter, and so the snowshoe hare turns white during the winter to be more camouflaged. But importantly, not all snowshoe hares do. Some snowshoe hares remain brown, and especially in areas sort of near the edge of their range where it's a little warmer, where there's not always as much snow, you have more hares that remain brown. So that's going to be our variation.
Well, we talked about overproduction. It turns out that snowshoe hares, the females can birth 12 offspring or even more than 12. We'll say 12 plus offspring per year, and those rabbits are sexually mature at just a few months. So you have a real chance for a lot of rabbits to be born very quickly. Well, not all of them can survive, but in a year with little snow, you can expect what's going to happen.
The white hares are going to be killed at a higher rate. Right? So we have our new population here from reproduction, but the animals that are catching these, they're going to be picking off those white ones a lot more frequently than they're picking off the brown ones because they're not as camouflaged. Well, then we have evolution. If we come back, right, the population will have fewer white hares in the future.
We expect that the population is going to look different. It's going to have more brown hares because they pass that variation on. Alright. So let's take a closer look at these and some insights from what we just talked about. The first thing that I want to know is that natural selection requires existing variation.
The population changed because there are already brown and white rabbits. If there is not variation in the population, natural selection can't do anything. If all the rabbits turn white every year, well, we got no story here. There's going to be no change. Natural selection does not introduce variation.
Natural selection selects from variation that's already in the population. Our next insight is that members of the same species are competing. Right? We may have been thinking about, well, can these rabbits get away from the fox? Right?
And you think of that as the competition. But what's really important is not, Does this rabbit get away from the fox? But rather, Does this type of the rabbit get away from the fox more often than other types of rabbits? What gets passed on depends on which rabbits in this population are surviving. That leads us to this idea of fitness.
Remember fitness is the likelihood that an organism is able to reproduce. Well, fitness, we're going to say is relative. Now there are ideas of you know, there is something called absolute fitness, but in very general terms, the way you want to think about it is in relative terms. Right? If you're able to survive and reproduce and have one offspring, that might be great, but it's not very good if the next rabbit over has 82 offspring.
Right? That is not a very good fitness. You're not putting your traits in the next generation nearly as much as the next one over. It's also, though, going to depend on environment. Our fitness here, which rabbit was more likely to survive and reproduce, completely depended on this idea that there was not much snow this year.
If it snows a lot, well, these fitness values are going to be completely different. Different rabbits are going to be surviving and reproducing. Our next idea though is that survival and reproduction is based on probability. In our example here, rabbits of both colors were caught by the fox. And, right, if you have a population, rabbits have something like 80% mortality.
80% of the offspring die. Well, if your coat color means that you only have 70% mortality, that's pretty good. However, it still means that that one rabbit is probably going to die. It just means that as this gets played out thousands of times over, hundreds of thousands of times over across big populations, the results are going to be very predictable. Alright.
Our next idea that we want to talk about is that the populations evolve, individuals don't. Right. In this example, the individuals either survived or they didn't. They reproduced or they didn't. And at the beginning, at the end, we have white and brown rabbits.
Right? We have that same variation. What changed in the population was the frequency of the trait. Right? That idea, the frequency of the trait, that's a population level idea.
The population changes, individuals survive and reproduce, or they don't. Our next idea is that natural selection increases the number of adaptations to the current environment. Right? Over time, natural selection is going to make populations more adapted to the environments that they live in. As more and more traits become more and more adapted, natural selection sort of homes these organisms to survive in these environments.
But let me go to the current environment. Right? If this environment changes, which environments often do, then the population, well, it's going to evolve as different organisms, and different variations now have higher fitness. Alright. We're going to play with these sort of, like, nuances around natural selection some more.
Give it a try, and I'll see you there.