Cities are evolutionary hotspots, home to moths that changed color to camouflage themselves amongst pollution-stained trees, “superworms” that can munch on heavy metals, and birds that changed their tune to battle with noise pollution.
A new paper indicates that cities may be causing these kinds of evolutionary changes to occur much faster than scientists thought was possible.
If current evidence about the pace of change is correct there will be “significant implications for ecological and human wellbeing on a relatively short time scale,” the paper concludes.
Nature isn’t quite so natural anymore. Our interaction with the world around us has changed it drastically, and nowhere more so than in cities, vast artificial landscapes lacking in biodiversity. However, cities are also evolutionary hotspots, home to moths that changed color to camouflage themselves amongst pollution-stained trees, “superworms” that can munch on heavy metals, and birds that changed their tune to battle with noise pollution. Species are changing their bodies and behavior to fit into the human environment.
According to a recent review paper published in the journal Trends in Ecology & Evolution, these changes may be occurring much quicker than scientists imagined to be possible.
“What the evidence is saying is that ‘rapid evolution’ is occurring,” Marina Alberti, the author of the study and a professor of urban and environmental planning at the University of Washington in Seattle, told mongabay.com
As we build our cities we tinker with ecosystems, creating conditions that force species to adapt. In doing so we unleash change in wildlife communities. Previously scientists thought such change took a long time to occur. However, Alberti writes that “human-driven trait changes occur roughly twice as fast as those driven by [natural] forces,” according to research cited in her paper.
According to Alberti, if current evidence about the pace of change is correct there will be “significant implications for ecological and human wellbeing on a relatively short time scale.”
In urban areas we change the number of species and their diversity, but we also create habitats — or a lack of them — that cast us as “selective agents” with the power to decide who and what can live there. Arthropods (insects, spiders, and crustaceans that lack a backbone), birds, fish, mammals, and even plants, are being forced to adapt to our urban spaces or perish.
A classic case is the peppered moth (Biston betularia), which is usually just that: peppered, with little black dots on its mainly pale body. But during the nineteenth century, when smog and pollution loomed over English cities, researchers found that peppered moths living in polluted cities were almost completely black. This allowed them to blend in with soot-stained trees, where they typically rest. In less-polluted areas, where trees were not blackened by soot, lighter moths were more abundant. After pollution levels dropped in the 1960s and factories began to clean up their act, the black pigmentation receded. City-dwelling white moths’ vulnerability to predators drove the proliferation of black moths.
As another example, studies in England and Wales have shown that urban great tits (Parus major) sing their songs at a higher pitch than those in the countryside, probably so they can be heard over the din of traffic and their new two-legged neighbors.
Evolutionary change is not only confined to our urban cores, however. Demand for power, food, and water stretches the reach of urbanization far beyond the outer city limits.
Take the earthworms (Lumbricus rubellus) of Wales and northern England, for instance. These worms, considered an entirely new species upon their discovery and dubbed “superworms,” feast on heavy metals such as copper, zinc, arsenic, and lead that occur in soil around old mining areas. This adaption has allowed the superworms to thrive where others would not. They developed their peculiar ability to cope with exposure to toxic heavy metals after prolonged pollution of the soil.
“These worms seem to be able to tolerate incredibly high concentrations of heavy metals, and the metals seem to be driving their evolution,” Mark Hodson, a professor of environmental science at the University of York who discovered the superworms’ peculiar eating habits, told National Geographic News. “If you took an earthworm from the back of your garden and put it in these soils, it would die.”
Likewise a study cited in Alberti’s paper found that fish species have adapted to human-caused changes in their habitat. Reservoir-dwelling blacktail shiners (Cyprinella venusta) were found to have changed almost beyond recognition. They have smaller heads, deeper bodies, a shorter dorsal fin — even their eyes are in a different place — than their river-dwelling brethren. The researchers who undertook the study believe these changes were driven by the lack of water flow in reservoirs, and may have affected the blacktail shiner’s fitness and maneuverability there.
There may well be other C. venustas out there. “The study of human-driven eco-evolutionary dynamics is still at its infancy,” said Alberti. Even though the idea that people could cause species to evolve was hypothesized over half a century ago, scientists are only now gaining firm evidence to support it. “Such studies require long term observations and research,” Alberti said.
Alberti’s study takes a cross-field look at the role of urbanization in driving evolution. She believes the challenge of understanding our influence on the evolutionary process is one that is too big for one field of science to take on. To fully understand how urbanization will affect evolutionary change, experts from fields such as urban studies, ecology, and evolutionary biology will need to work together, she said.
Alberti’s paper compares current trends to the Great Oxygenation Event, one of the largest extinctions in Earth’s history, which wiped out many oxygen-intolerant species beginning roughly 2.5 billion years ago. But despite the paper’s cataclysmic overtones, Alberti sees hope for the future.
She suggested that rather than continuing down the Great Oxygenation route, humankind can learn to co-exist with species and ecosystems, evolving together and changing our ways so the result will not be so catastrophic.
Nature may no longer be quite so natural, but by adapting our urban spaces to accommodate nature and leaving remaining natural places be, we may secure species’ continued survival, according to Alberti.
“Change is inevitable,” she said, “but we can learn how to let nature to take some control.”
- Alberti, M. (2015). Eco-evolutionary dynamics in an urbanizing planet. Trends in Ecology & Evolution 30(2):114–126
- Haas, T.C., Blum, J., Heins, D.C. (2010). Morphological responses of a stream fish to water impoundment. Biology Letters 6(6): 803–806.