[MUSIC] [BIRD CHIRP] During WWII, Londoners took nightly refuge
in the Underground as German buzz bombs exploded above. But they were greeted with a different
buzz: Mosquitoes. In the 19th century, when London’s Tube
was being dug, a population of mosquitoes colonized the tunnels. They originally preferred
biting birds, but since pigeons don’t ride the subway, they found a new target in rats, mice and people. Now, just more than a century later, when
the tube-dwellers try to mate with their above-ground cousins, it almost never results in viable
offspring. The subway skeeters are becoming a new species right before our eyes, with
different behaviors, and different genes. Species are always more likely to emerge when
a population gets isolated. Maybe one goes underground. Maybe a river
runs through them. Or maybe one drifts out to sea. Once separated, a different drift occurs:
genetic drift. As time goes on, mutations accumulate, traits are selected by each environment,
until the two groups are sufficiently different that they can’t mate. Or, as scientists call it, reproductive isolation. London’s mosquitoes didn’t invent this.
It happened with the dozen or more tortoise species Darwin found in the Galápagos. It
created us and our extinct Neanderthal cousins. Small, random changes, challenged by nature,
have spawned each of the tens of millions of species alive today, and the hundreds of
millions that are stuck underground. Stay Curious.