Darwin Comes to Town

1. Cities are evolving ecosystems, not just human habitats

To my biologist's eye, the inner city, for all its hustle and bustle and thoroughly unnatural appearance, becomes a constellation of miniature ecosystems.

Urban biodiversity is rich and complex. Cities are not biological deserts, but rather dynamic ecosystems teeming with life. They harbor a diverse array of plants, animals, and microorganisms that have adapted to thrive in human-dominated landscapes. This urban biodiversity is often a mix of native species, exotic introductions, and novel combinations that wouldn't occur in natural habitats.

Cities create unique ecological niches. Urban environments offer a variety of habitats not found in nature:

• Concrete structures mimicking cliff faces for birds and lizards
• Artificial light sources attracting insects and their predators
• Heat islands providing warmer microclimates
• Human food waste supporting opportunistic scavengers

These novel conditions drive the evolution of urban-adapted species, reshaping the ecological relationships and food webs within cities.

2. Urban environments drive rapid evolutionary changes in wildlife

Never before in the history of our planet has a single life form been so dominant.

Human-induced rapid evolutionary change (HIREC) is occurring. The intense pressures of urban environments are causing species to evolve at unprecedented rates. This rapid evolution is driven by:

• Strong selection pressures from pollution, noise, and artificial lighting
• Novel food sources and nesting sites
• Altered predator-prey relationships
• Extreme habitat fragmentation

Examples of urban evolution abound:

• Peppered moths evolving darker coloration in polluted industrial areas
• City birds developing higher-pitched songs to be heard above urban noise
• Plants producing heavier seeds in fragmented urban green spaces
• Mosquitoes adapting to life in underground subway systems

These changes can occur within just a few generations, demonstrating the power of urban environments to shape evolution.

3. Pollution and artificial light shape urban species adaptation

Pollution can never be completely avoided in human habitation.

Chemical pollutants drive adaptation. Urban environments are awash in novel chemical compounds that wildlife must contend with:

• Heavy metals from industrial processes and traffic
• Pesticides and herbicides from landscaping
• Pharmaceutical residues in wastewater

Species that can evolve tolerance to these pollutants gain a significant advantage in urban ecosystems. For example:

• Killifish in polluted harbors have evolved resistance to PCBs
• Urban pigeons with darker feathers can better detoxify heavy metals

Artificial light at night (ALAN) is a major evolutionary force. The ubiquity of nighttime lighting in cities is driving rapid changes in nocturnal and diurnal species:

• Moths evolving reduced attraction to artificial light sources
• Birds shifting their dawn chorus earlier to avoid peak traffic noise
• Predators adapting hunting strategies to take advantage of well-lit areas

These adaptations to ALAN can have cascading effects throughout urban food webs.

4. Fragmentation of urban habitats influences genetic diversity

The gene pools of urban wildlife get split into a multitude of tiny slivers.

Urban landscapes create habitat islands. Cities fragment habitats into small, isolated patches:

• Parks surrounded by buildings and roads
• Green roofs disconnected from ground-level vegetation
• Individual street trees separated by concrete

This fragmentation has significant genetic consequences:

• Reduced gene flow between populations
• Increased inbreeding in small, isolated groups
• Potential for local adaptation to specific urban microclimates

Some species thrive in fragmented habitats. While fragmentation can be detrimental to many species, others have adapted to take advantage of urban habitat islands:

• White-footed mice in New York City parks evolving distinct genetic profiles
• Urban plants producing heavier seeds that are less likely to disperse to unsuitable areas

Understanding these genetic patterns is crucial for urban conservation efforts and predicting future evolutionary trajectories in cities.

5. Animal behavior and communication evolve in response to city life

City birds sing higher, and probably also louder, than the same species in a quiet rural setting.

Urban noise drives changes in animal communication. The constant din of city life forces animals to adapt their vocalizations:

• Birds singing at higher frequencies to be heard above low-frequency traffic noise
• Frogs and insects adjusting their call timing to avoid peak noise periods
• Some species developing visual communication strategies to supplement auditory signals

Novel behaviors emerge in urban settings. Animals in cities often display innovative behaviors not seen in their rural counterparts:

• Crows in Japan using cars as nutcrackers
• Urban birds opening milk bottles to access cream
• Raccoons learning to open complex trash can locks

These behavioral adaptations demonstrate the cognitive flexibility required to thrive in rapidly changing urban environments.

6. Sexual selection takes on new dimensions in urban settings

In these urban insects, natural selection first makes them evolve into stronger flyers, and this is then even further amplified by sexual selection.

Urban environments alter mate preferences. The unique challenges and opportunities of city life can shift what traits are considered attractive:

• Less aggressive males may be favored in densely populated urban areas
• Bolder individuals might have greater mating success in novel urban habitats
• Tolerance to pollution or artificial light could become a sexually selected trait

Sexual signals adapt to urban conditions. The ways animals attract mates must evolve to remain effective in cities:

• Bird songs adjusting to be heard over urban noise
• Visual displays becoming more important in well-lit urban areas
• Chemical signals potentially disrupted by air pollution

These changes in sexual selection can accelerate urban evolution and potentially lead to reproductive isolation between urban and rural populations.

7. Urban evolution may lead to speciation and new urban-adapted species

Over the past centuries Turdus merula has spawned a new species, Turdus urbanicus, if you will.

Urban populations are diverging from rural ones. The strong selection pressures in cities are causing urban populations to evolve distinct traits:

• Physiological adaptations to pollution and heat
• Behavioral changes in response to human presence
• Shifts in life history strategies (e.g., earlier breeding, reduced migration)

Reproductive isolation is emerging. As urban and rural populations diverge, mechanisms of reproductive isolation are developing:

• Differences in breeding timing between urban and rural birds
• Habitat preferences keeping urban-adapted individuals in cities
• Potential sexual selection for urban-specific traits

While complete speciation has not yet occurred in most cases, many urban populations are on an evolutionary trajectory that could lead to the formation of new, uniquely urban-adapted species in the future.

8. Humans play a crucial role in shaping urban evolutionary processes

We humans are the world's super-accumulators of heavy metals.

Human activities create novel selection pressures. Our behaviors and technologies introduce unprecedented evolutionary forces:

• Architecture providing new habitats (e.g., skyscrapers as artificial cliffs)
• Transportation networks fragmenting populations and introducing roadkill as a selection pressure
• Waste management practices creating new food sources and toxin exposures

Human preferences influence urban evolution. Our aesthetic and cultural choices shape urban ecosystems:

• Selection of ornamental plants influencing urban flora
• Feeding of wildlife altering natural foraging behaviors
• Pest control efforts driving resistance in target species

Understanding how human activities drive urban evolution is crucial for sustainable urban planning and biodiversity conservation.

9. Urban design can harness and guide evolutionary forces

We can engineer our own ecosystem engineering.

Evolutionarily informed urban planning is possible. By understanding urban evolutionary processes, we can design cities that promote biodiversity and ecosystem health:

• Creating corridors to facilitate gene flow between fragmented habitats
• Designing lighting systems that minimize disruption to nocturnal species
• Incorporating diverse native plants to support local food webs

Green infrastructure can evolve with cities. Urban green spaces should be designed with evolution in mind:

• Allowing spontaneous plant colonization of some areas
• Monitoring and adjusting management practices based on observed evolutionary changes
• Creating heterogeneous habitats to promote diverse evolutionary trajectories

By working with, rather than against, urban evolutionary processes, we can create more resilient and biodiverse cities.

10. Citizen science is key to understanding and monitoring urban evolution

Let's build ourselves an Urban EvoScope!

Urban residents can contribute to evolutionary research. The vast number of people living in cities provides an unprecedented opportunity for large-scale data collection:

• Smartphone apps for recording urban wildlife observations
• Community-led monitoring of local plant and animal populations
• Collaborative projects between scientists and urban gardeners

Citizen science enhances public understanding. Engaging urban residents in evolutionary research has multiple benefits:

• Increased awareness of urban biodiversity and its importance
• Greater support for conservation initiatives in cities
• Improved scientific literacy and appreciation for evolutionary processes

By harnessing the power of citizen science, we can create a global network of urban evolutionary observatories, advancing our understanding of how life adapts to human-dominated landscapes.

Last updated: January 25, 2025