Natural selection is an explanation for how populations change, but it's also something that we can just see in real time. So here we're going to look at some evidence of natural selection. We're just going to start by saying natural selection can be observed in relatively short time intervals, and we're going to look at 2 examples of this. First is methicillin resistant Staphylococcus aureus or really just antibiotic resistance in bacteria, and then we're going to be looking at coat color in the rock pocket mouse. Now we're using these two examples because these are really common examples that you're likely to see either in lecture, your textbook, or maybe even on a test.
Alright. So let's talk about this methicillin resistant Staphylococcus aureus or MRSA. Now again, this is really just the same story for any antibiotic resistant bacteria though. Now in any population for natural selection to occur, we said we need to start with variation. So we're going to have this population of bacteria here that could cause an infection.
And we have sort of 2 types here that we're calling out with different colors. They are either the not resistant or the resistant bacteria based on whether they are resistant to antibiotics. Now, when people get an infection, you take antibiotics to kill the bacteria. So we're going to add a selection pressure to this population. Alright.
Our selection is going to be the presence of antibiotics. All right. Now antibiotics are supposed to kill the bacteria, but we have variation in this population as to whether or not those bacteria are resistant. So when we add the selection pressure, that antibiotics, well, a lot of the bacteria die, but not all of them. We have a difference in fitness.
The resistant bacteria have higher fitness because we see here the resistant bacteria were more likely to survive the presence of that antibiotics. And so when that happens and the population then reproduces, and maybe people use more antibiotics as this infection spreads to someone else, and then they use antibiotics, so on and so on. Well, then eventually that population evolves resistance. So resistant bacteria become more and more common. And this is a story that is played out in all sorts of different bacteria.
And I'll just note that antibiotics were one of the great public health advances of the 20th century, and most public health experts agree that antibiotic resistance is going to be one of the great public health challenges of the 21st century. Alright. Next example here is the rock pocket mouse. Now this is a story that's really developed in the past 20 or 30 years or so that people have been doing research on this. And this is the idea well, there are these mice that live in the desert southwest, and they have different coat colors, brown or dark coats, and most of these mice live well, in the desert southwest, the ground is largely brown, stone-colored, but there's going to be areas where, well, these mice live on this black volcanic rock.
So we're going to talk about these mice on this black volcanic rock. So the population starts out mostly with tan fur, some black fur though. And predators though, as this population moves on this black volcanic rock, they are hunting mice on this black rock. And so that's our selection pressure. You can imagine what's going to happen.
Who's more likely to survive? Well, our fitness difference. Black mice have higher fitness because they're better camouflaged, and then as this population now reproduces, well, what's going to happen? The population evolves. Right?
That black fur becomes more common in that environment with that black volcanic rock. Right? Now this story is so powerful and has become so widespread because people know the genetics of mice really well. So the researchers have been able to look at not just the coat color, but also they've been able to track the underlying genes as the population changes. And they've seen a very similar story play out in all sorts of different populations of mice as they adapt to different substrate colors, different ground colors.
Alright. So again, these two examples, things you're very likely to see. I'd recommend being at least a little bit familiar with them. We have more examples in practice file. I'll see you there.