Here's one of those cool examples of evolution occurring in a short enough time that it can actually be observed. A paper that came out very recently in Current Biology shows that human activity has caused the first steps of speciation in a European bird.
The Central European blackcap normally spends the summer in Germany, then flies off to Spain for the winter. (Not a bad way of life.) But a genetic quirk caused a few of these birds (shown at left above) to head north in the winter instead, to the UK. That should be a very harmful genetic quirk, because there's not a whole lot for birds to eat in the wilds of the UK in wintertime.
However, in recent decades, bird feeders have become quite popular in the UK, and so the blackcaps that mistakenly migrated there were able to survive anyway.
Because the migration distance is shorter for the UK birds, they get back to Germany sooner in the springtime, and that's their mating season, so they tend to mate almost exclusively with other UK-migrators. Then the Spanish birds get back and separately do their thing.
What that has already led to is some minor evolution: the UK-migrators have longer beaks, all the better to eat from a bird feeder with, and shorter, rounder wings, all the better to fly shorter distances with better maneuverability. Plus their plumage has undergone subtle changes.
Presumably the two types of blackcap CAN still mate with each other, but they largely DON'T, and if this continues for much longer, they won't be ABLE to anymore. And then you will have a new species, Dingbattius flynorthicus.
This isn't the first example of human-caused speciation; the domesticated sheep can no longer mate with the wild sheep from which it ultimately descended. But we didn't get to see that happen. Hopefully this story will continue to be followed, and one day we will observe two distinct species. So, Britons, it's February -- go fill your bloody feeders!