City People Are Forcing Rats to Evolve, Making Them Harder to Kill

Make jokes about pizza rats all you want, but the way cities affect animals’ genes could have serious implications for human health, according to a review paper published on Thursday in Science. Head lice, rats and cockroaches, the authors noted, have grown more genetically resistant to the chemicals that are supposed to kill them. And bedbugs in Boston likely have noticeably different DNA than their distant relatives in New York or Los Angeles.

Genetic changes can happen in a variety of ways, the paper notes. The environment determines which characteristics make an organism fit to survive and pass along their genes to the next generation: That's natural selection. Random mutations happen, too. These processes may be affected by urbanization, the paper notes. Humans not only reshape the environment around them but bring pesticides and pollution along. Some of those things may increase the likelihood of new, random mutations or introduce new pressures on the animals that live in the area.

But there are other mechanisms at work as well. The basic architecture of cities can influence how we define populations. New roads or buildings may split animal populations, leading individuals on one side of a barrier to become more genetically similar to each other than those on the other side. Even the walls in an apartment building can be enough of a barrier. One species of cockroach described in the paper, Blattella germanica, is known to mate only with others in the same apartment. Populations from one unit, then, look genetically different from others in the same building. 

NYC Skyline One World Trade Center, in Manhattan. Cities like New York may be influencing the genetic characteristics of their animal inhabitants, according to a paper published in "Science" on Thursday. Drew Angerer/Getty Images

Half of the world's human population live in urban areas, but that doesn't necessarily mean cities are making our genes more similar to those of our neighbors. The paper notes that animals that can get around easily—birds and humans, for example—generally stay genetically diverse. (And anyway, the trend of people moving to cities appears to have slowed, according to a recent analysis from the Brookings Institution.)

That said, changes in our environment can and have affected human genetics in the not-so-distant past. One notable study looked at the genetic effect of a famine in the Netherlands when Nazis occupied the country during World War II. Researchers found slight changes in the genetic code of children who were conceived during the famine. The changes, more properly called epigenetic changes, affected the way one gene related to growth and development would have been transcribed. And on average, the human populations of cities founded thousands of years ago tend to be more genetically resistant to certain diseases, like leprosy or tuberculosis, that they’re more likely to have encountered.  

Still, more research needs to be done so we can better understand how our cities are changing us—and the pests we live with. 

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