Evolution is supposed to be slow. This is practically the first thing you learn in any biology course: natural selection works across generations, driven by tiny differences in reproductive success that accumulate over thousands of years. The textbook example is the peppered moth, which shifted from pale to dark colouring across decades of industrial pollution in Victorian England. Dramatic by evolutionary standards. Barely perceptible in a human lifetime.
That picture is now considerably more complicated. Over the past two decades, biologists working across dozens of species and dozens of habitats have documented something unexpected: evolution in response to human pressure is happening fast. Measurably fast. Fast enough to observe within a research career, and in some cases within a few years.
Birds That Changed Their Song
The most directly relevant example for my research concerns bird vocalisations. In 2006, Hans Slabbekoorn and Ardie den Boer-Visser published a study in Current Biology comparing the songs of great tits (Parus major) across ten European cities with their rural counterparts. Urban great tits sang at significantly higher frequencies. The effect was consistent across cities and could not be explained by learning alone.
The mechanism is straightforward: low-frequency sounds are masked by urban traffic noise, which peaks around 100 to 300 Hz. Birds that happen to sing slightly higher have their songs heard more clearly, find more mates, and leave more offspring. Within a few generations, the population's average pitch has shifted upward.
Subsequent studies confirmed the effect across species and cities: great tits in London, blackbirds in Sheffield, house finches in the United States. Urban birdsong is evolving to cope with the sonic environment that humans have created.
Beyond Birds
The pattern extends well beyond vocalisations. Urbanisation has driven rapid changes in body size, life history, physiology, and behaviour across hundreds of species. White-footed mice in New York City parks have evolved distinct metabolisms compared to their rural cousins, with differences traceable to the fatty acids in urban food waste. Atlantic cod evolved to mature younger and smaller in response to decades of fishing pressure targeting larger individuals. Guppies in Trinidad shifted their life histories within just a few generations when predators were removed from their streams.
The unifying theme is speed. These changes are occurring on timescales of decades, not millennia. Human pressures, whether noise, urbanisation, fishing, land use change, or introduced predators, are strong enough to drive detectable evolution within a small number of generations.
Primates and Keystone Species
This is not just a story about animals adapting to survive in the niches we have left them. It is also about the cascading consequences when keystone species cannot adapt fast enough. My experience with the Darwin 200 project in Rio de Janeiro, documenting the reintroduction of howler monkeys to Tijuca National Park, brought this into sharp focus.
Brown howler monkeys are seed dispersers. Their absence from the Atlantic Forest fragments that make up Tijuca has allowed certain plant species to dominate at the expense of others, simplifying the forest structure over decades. The reintroduction is an attempt to restore a function, not just a species: to bring back the ecological relationships that make a forest a forest rather than a collection of trees.
What the howler monkey story illustrates is that the pace of anthropogenic change often exceeds the pace of natural adaptation. Habitats are fragmented faster than populations can colonise new corridors. Species compositions shift faster than dependent species can adjust. The system falls out of balance before evolution has time to restore it.
What This Means for Soundscapes
Here is where the two threads of my research converge. If urban birds are evolving to sing at higher frequencies, and if the restorative effect of birdsong on human psychology depends in part on the acoustic qualities of that song, then anthropogenic evolution is not just a biological story. It has direct implications for human wellbeing.
We do not yet know whether higher-pitched urban birdsong is as restorative as the richer, lower-frequency songs of rural birds. We do not know whether humans, who evolved alongside one kind of acoustic environment, respond equivalently to one that has been modified under urban pressure. These are live research questions.
What we do know is that the acoustic ecology of cities is changing, that the birds are changing with it, and that both the change in birds and the change in human wellbeing are, ultimately, responses to the same set of decisions about how to build cities and what to value in them.
A Reason for Cautious Optimism
Anthropogenic evolution is usually framed as a warning, and in many ways it should be. The speed and scale of human-driven change is staggering, and many species will not adapt quickly enough to survive.
But the same evidence that reveals the threat also reveals remarkable resilience. Life responds to pressure. Evolution is not passive. When we give species the space and time to adapt, many of them do. The howler monkey, reintroduced to a city park after a century of absence, will, over generations, begin to change the forest again. Nature, given a chance, tends to fill the silence.
Whether we give it that chance is, as always, up to us.
Joseph Roy is a PhD researcher in environmental psychology at Bangor University and a 2023 Darwin 200 Global Leader. His research on birdsong and human wellbeing connects to broader questions about anthropogenic change and the future of natural soundscapes. Read about the Darwin 200 project. Connect on LinkedIn.