Our sixth line of evidence for evolution comes from direct observations of natural selection. We'll look mainly at one example here. So you may have heard of this example.
This is a really famous one. This is Biston Betularia, the peppered moth. So we have two different moths shown here.
So historically, these moths were almost all white with a little bit of black dotting. And they lived in England, and if you know anything about the English, they're crazy. about birds and moths, so people have been paying attention to these moths for a long time and collecting them, and they're very knowledgeable. So in 1848 was the first recording of a melanic form, so that's actually a biston bachelorette moth.
You can tell it's the same moth because physically it's it's exactly the same, it's just completely black. And so this was called the Carbonaria type, so it was Bistan Betularia Carbonaria, to distinguish it from this, which is Bistan Betularia Typica. 1848 was the first recorded sighting of one of these guys. By 1900, in parts of England, over 90% of the Bistan Betularia moths were this Carbonaria type instead of the Typica type. So in just 50 years, you go from having a single individual to just like like this to most of the individuals looking like this.
And again, that's in the industrial areas of England where there's lots of pollution that was actually changing the color of the trees, and so there's actually an environmental change that's creating the possibility for this to occur. So there's a guy back then called Kettlewell who decided to perform some experiments to see if, in fact, it was this color change of the trees that were causing this to occur. So what he did was he would go to polluted areas and unpolluted areas. and he took a bunch of moths, the Typica type and the Carbonaria type, and he released them. So here he's releasing both Carbonaria and Typica.
The black bars are showing the, basically the ratios of the ones that he released. So he releases more Carbonaria than Typica in the polluted area. And when he recaptures them later, it goes back, he's able to recapture more of the Carbonaria and fewer of the Typica.
So it looks like these guys are surviving better. These guys. are not surviving as well because he can't recapture them very well. Then he went to the unpolluted areas where the trees are still light colored and did the same thing. Here he released about roughly equal numbers of each and is able to recapture a higher proportion of the typical one, the white ones, than the black ones.
So by releasing some and then recapturing them he's showing that their survival is different in these different regions, right, because it's the ones that are eaten by birds that he is failing to recapture. So he demonstrated higher survival for the light morph in the unpolluted area, higher survival for the white ones. for the dark morph in the polluted areas.
So it's not like just the pollution was turning them all black, it was there seemed to be differences in their survival abilities in these different regions that was causing them to evolve over time to become black in those areas where that was advantageous. And then to follow this up, much later in the 20th century England kind of gets its act together and starts reducing pollution. So here is a plot here of pollution levels coming down as measured by sulfur. dioxide and this is a plot showing that the frequency of carbon area is coming down right so most moths were these black moths when the pollution started coming down and the trees are now cleaner now you can see a decline in the percentage of these moths that with this black moth so we see an example of when the trees turned black the moths evolved to become darker when the trees kind of started to turn back to light we see the reversal of that pattern of evolution and And so this is essentially natural selection, right? Although humans have caused the change in the environment, we weren't deliberately picking which moths live, which moths died.
That was a natural selection occurring on these moths. Now, this is a famous example. So in fact, identifying the exact basis of the genetic difference between these two morphs is actually a work in progress. This is a fairly recent paper. This is from 2011. This is trying to find what location in the genome is responsible for this color difference.
and it looks like it's a fairly simple sort of genetic difference. So one of the things about evolution and about science in general is we have a number of things we've learned, but now we continue and we try to fill in the details, right? So this peppered moth example has been known for a long time, but we're still working on the details, and this is part of what makes science so interesting, is that there's still lots to do.