With more than 55% of the world’s population now living in cities, the frantic pace of urban life is changing life on a genetic level. Through rapid evolution, we are seeing the formation of genetically distinct populations right in our backyards.

We’ve known for a long time that humans impact other species, usually for the worse. But nowhere is this clearer than in urban centers. These concrete constructions, pollution, and overall chaos introduce brutal new selective pressures on insects, amphibians, birds, and rodents. For them, the rule is simple: adapt to this dramatic change or go extinct.

But life is stubborn and adaptable. A relatively new scientific field, urban evolutionary ecology, is studying how city-dwelling species are undergoing adaptation, mutation, and selection in real time. We don’t have to wait millennia for the fossil record to catch up; urbanization is driving species evolution today.

From heat tolerance to pesticide resistance, nature is fighting to survive in cities. Here are just some examples showcasing that.

1. Manhattan’s White-Footed Mice

Image credits: Animalia.

One of the earliest studies on rapid evolution focused on the white-footed mouse (Peromyscus leucopus) in New York City.

Researcher Jason Munshi-South has spent years analyzing these populations. His team found that mice in parks like Central Park are effectively living in high-density “islands,” isolated from one another by oceans of concrete. They don’t do this in the wild. This is likely due to the lack of natural predators and competitors in the city, an abundance of human-derived food, and barriers (like roads and highways) that prevent dispersal, trapping populations in small “habitat islands”.

Biologically, this results in strong genetic differentiation. The researchers identified specific DNA mutations in the urban dwellers related to metabolism and heavy metal tolerance. Essentially, these mice are genetically rewriting themselves to digest processed, fatty human food (the “cheeseburger diet”) and survive soil pollutants that would likely kill their rural cousins.

2. The Puerto Rican Anole Lizard

Image via Animalia.

In San Juan, Puerto Rico, the tropical lizard Anolis cristatellus is showing us exactly what it takes to survive on concrete.

A study by Kristin Winchell (NYU) sequenced the genomes of these reptiles and found 93 specific genes related to limb and skin development that had diverged from forest populations. The city lizards have evolved larger toe pads and longer limbs to grip smooth walls and fences rather than textured tree bark.

But it’s not just about grip. Winchell’s team also discovered that urban lizards have evolved a “high-plasticity genotype” that grants them superior heat tolerance. They can function at temperatures roughly 1.47°F (0.82°C) higher than forest lizards, which is a critical edge in the sweltering urban heat island.

3. The London Underground Mosquito Debate

Image in public domain.

The London Underground mosquito (Culex pipiens form molestus) is often cited as the textbook example of rapid evolution—a species that supposedly evolved in the Tube tunnels (the London underground) over the last 150 years.

This is still debated, but recent DNA analysis published in Science has turned this story on its head. Researchers Yuki Haba and Lindy McBride discovered that this mosquito likely didn’t evolve in London at all. Instead, it probably diverged from other mosquitoes 1,000 to 10,000 years ago in the Mediterranean (possibly Ancient Egypt), living alongside early agricultural humans.

It turns out these mosquitoes were “pre-adapted” to human environments. When the London Underground was built, they simply moved in. It’s a powerful lesson in how ancient human history continues to shape modern urban ecology, and a reminder of how challenging it is to study urban adaptations.

4. Raccoons Are Becoming Cuter

Image credits: Neil McIntosh.

Humans might be accidentally domesticating raccoons without realizing it. Urban raccoons are losing their reactivity to humans, attracted by our trash and the lack of large predators.

In a study published in Frontiers in Zoology, researcher Dr. Raffaela Lesch found that urban raccoon snouts are 3.56% shorter than those of their rural counterparts. This is significant because a shortening snout is a hallmark of “domestication syndrome,” a pattern of physical changes (like floppy ears and smaller jaws) that occurs when animals are selected for tameness.

The hypothesis is that by selecting for raccoons that are less aggressive and bold enough to raid our bins, we are inadvertently selecting for changes in their neural crest cells, which affect both behavior and facial structure.

5. Cliff Swallows Are Evolving to Dodge Semi-Trucks

Image via Wiki Commons.

If you think navigating rush hour traffic is hard, try doing it as a bird looking for food and dodging predators. In southwestern Nebraska, researchers Charles and Mary Brown have studied Cliff Swallows nesting under highway bridges for over 30 years.

In the 1980s, the roadkill count was high. But over time, the number of dead swallows plummeted, even as the population grew. When the Browns measured the birds, the data was stark: the birds killed by cars had a longer average wing length than the survivors.

Shorter wings allow for a more vertical take-off and quicker pivoting, helping the birds dart away from oncoming vehicles. The long-winged birds couldn’t maneuver fast enough and were removed from the gene pool by the grill of an 18-wheeler.

6. White Clover Is Disarming Itself

It isn’t just animals that are changing; plants are evolving, too. White clover (Trifolium repens) typically produces hydrogen cyanide, a chemical defense mechanism to stop herbivores from eating it.

The GLUE project (Global Urban Evolution), led by Marc Johnson at the University of Toronto, analyzed clover from 160 cities across 26 countries. They found that urban clover has largely stopped producing cyanide.

In the city, there are fewer cows and deer to munch on the leaves, so the metabolic cost of making poison just isn’t worth it. Additionally, in many cold cities, plants without the cyanide mechanism are actually more tolerant of freezing temperatures. The city clover has effectively “learned” that it doesn’t need to carry a weapon to survive the concrete jungle.

The Concrete Jungle is the New Wild

Recognizing that genetic changes are happening right now due to our infrastructure has massive implications for how we view nature. We tend to think of conservation as something that happens “out there”—in the Amazon rainforest or Yellowstone—but we are overlooking the ecosystem right outside our window.

A recent review assessed the growing body of research on urban evolution, noting that it’s important to stop looking at cities as biological wastelands.

Gad Perry, one of the study’s authors, argues that we need to flip the script. He outlines four main takeaways for the future of urban conservation:

1. Cities have value: Contrary to traditional thinking, urban areas are legitimate targets for conservation efforts.
2. Unique refuges: Because cities are so different from the wild, they might actually provide a safe haven for species that are struggling in their natural habitats.
3. A crystal ball for climate change: Cities are “heat islands,” often significantly warmer than their surroundings. Studying how animals adapt to a sweltering city today could teach us how they will survive a warming planet tomorrow.
4. Novel biodiversity: Rapid evolution is creating new variations of life that exist only in cities.

“There is a need for much more research on urban evolution that can teach us about evolution as a process,” Perry notes. “Urban green spaces have great documented value for both humans and other species. Because of that, our current focus only on ‘wild’ areas is unfortunate and short-sighted.”

There’s another important takeaway from all of this. If city life can reshape the genomes of mice, lizards, and birds in just a few decades, what is it doing to us?