Darwin Comes to Town

Charles Darwin is well-known for his contributions to the science of evolution and has been described as one of the most influential figures in human history. But he certainly wasn’t infallible. Darwin claimed the effects of natural selection took hundreds of years. As he wrote in On the Origin of Species,

“We see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages. And then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were.”

In the 1950s Oxford University’s Bernard Kettlewell conducted a landmark experiment which explained why England’s peppered moths changed from mostly white to almost all black. Over a three-year period, Kettlewell tracked hundreds of moths near the polluted city of Birmingham and in a cleaner coastal forest. Near Birmingham, black moths avoided being eaten by sparrows because they were camouflaged by soot-stained trees; in contrast, white moths were easy to spot and more readily eaten. The exact opposite occurred in the coastal woods. As Britain became more urban, moths demonstrated natural evolution by turning mostly black in less than 50 years.

In his book Darwin Comes to Town, evolutionary biologist Menno Schilthuizen catalogues multiple examples of human-induced rapid evolutionary change (HIREC). Urban environments are very different than rural ones; they have artificial lights, are noisy, and have lots of concrete. These concentrated environments encourage genetic variations – those which increase the likelihood a species will survive – leading to a more rapid evolution of the species.

For example, there are significant genetic differences in bobcats living in Southern California, as separated by the 405 and 101 freeways. In the early 2000’s, a mange epidemic in the bobcat population north of 101 caused a high mortality rate. This produced a natural selection of bobcats more resistant to mange, but only for those living north of 101. The other nearby bobcats were less mange-resistant as they were isolated by the freeways and weren’t forced to adapt.

One of my favorite examples involves the crows of Sendai, Japan. For many years, the crows had been feeding on walnuts by dropping them from great heights. At some point during the 1980’s, some crows discovered it was easier to place a walnut under the wheel of a slowly-moving or recently-stopped car, such as at a traffic light. Over a few years, this habit spread to other crows in the city and is now commonplace.

The urban evolution of animals provides a cautionary tale as well. City lights suppress birds’ estrogen and testosterone, changing their singing, mating, and feeding behaviors. Controlled experiments have shown male blackbirds did not develop reproductive organs when they were exposed to light at night for two years. Behavioral scientist Vincent Cassone warns us about these changes: “If you see these deleterious effects in the birds, you’re likely to see them in humans in short order. The smart thing to do is to pay attention to avian life.”

Given these radical changes in animals, it’s fair to wonder how humans will adapt to cities over time.