Species and the environment | Mechanisms of evolution | High school biology | Khan Academy

So we tend to view evolution and natural selection and the formation of new species, which is often called speciation, as a slow process that could take tens or hundreds of thousands of years, or in many cases millions of years. And that's why it's always interesting to see examples of when it might actually be happening more on a human time frame.

So this right over here is a picture of a male Central European black cap. Historically, Central European black caps would spend their summers in Central Europe, right over here in what we would now consider Germany. So let me write summer right over here. And then when it gets colder in Central Europe, they would migrate and spend their winters in Spain. So this is where they would spend their winters.

Now whenever we talk about migratory patterns like this, it's always interesting to note that it's not that all of the birds always go on the exact same migration path. As we've talked about in many videos, you tend to have variation in a population. There are birds that, because of genetic differences, they might go in other directions. They might go that way, they might go that way, they might go that way. But those variants probably weren't successful. Some of them might end up in the North Sea, some of them might end up freezing to death, some of them might not be able to have sources of food, while the dominant variant that wintered in Spain was successful. It was warmer there; they had access to food.

Now this is where it gets interesting. In the 1960s and 1970s, it became more and more popular to put backyard bird feeders in England. So some of those Central European black caps just happened to go in that direction. They might have always been happening to go there, some small percentage of the population, but those didn't do so well. But now all of a sudden, you have these backyard bird feeders in England, and so these birds were able to be quite successful. They were able to get food; they were able to thrive.

And so over time, this became a larger and larger proportion that was able to successfully go in that direction and then migrate back in the summer and then reproduce. Now not only did that gene not get selected against, and now the birds that had the genes to go in that direction were thriving, but they also started mating with each other more and more frequently because they would all get back to Germany earlier. Because when the winter ended, they had a shorter distance to travel on the way back than the ones that went to Spain.

And so, as they started reproducing with each other more and more and more, we started to see differences beyond migration direction in these two groups of Central European black caps. The ones that traveled northwest started to see rounder wings versus pointier wings for the ones that were historically going to Spain. Well, pointer wings are better for traveling long distances, and rounder wings are more maneuverable. If you don't have to travel as long of a distance, that is more desirable.

Now it's not like these birds knew that they needed rounder wings, but once again, even in the pool that was going to the northwest, you would have had variations—some pointy wings, some round wings. But the round wings might have been more successful, so that became a more pronounced trait in that group. While similarly, the ones that went to Spain, some of them might have had rounder wings, but they might not have been as successful to get all the way to Spain and back. So the pointier wings, that phenotype, seemed to thrive.

So this is an example of how an environmental change— and here the environmental change is people deciding to put bird feeders in their backyards in England in the 1960s and 1970s—has actually, over just the course of roughly 30 or 40 generations, begun what some biologists view as a speciation event. The ones that go northwest can still interbreed with the ones that go to the southwest, but over time, if the two groups keep breeding amongst themselves because of the timing of their migration, they might not be able to interbreed anymore. And then you would have two different species.

So this is just one example. There are many other examples about how the physical environment can contribute to the expansion of some species, the emergence of new distinct species, or in certain situations cause a decline in a species and, in the extreme, extinction, which we'll talk about in another video.