Urban evolution

Urban evolution is the adaptation of organisms to heavily populated urban areas. Anthropogenic environmental and geographical changes brought to habitats when urbanization takes place are known to have a significant evolutionary impact on organisms inhabiting these city areas.^[1][2]

Biologists have observed evolutionary change in numerous species compared to their rural counterparts on a relatively short timescale.^[3] Strong selection pressures due to urbanization play a big role in this process. The changed environmental conditions lead to selection and adaptive changes in city-dwelling plants and animals. Also, there is a significant change in species composition between rural and urban ecosystems.

Shared aspects of cities worldwide also give ample opportunity for scientists to study the specific evolutionary responses in these rapidly changed landscapes independently. How certain organisms (are able to) adapt to urban environments while others cannot, gives a live perspective on rapid evolution.

Urbanization[]

With urban growth, the urban-rural gradient has seen a large shift in distribution of humans, moving from low density to very high in the last millennia. This has brought a large change in environments as well as society.^[4]

Urbanization brings transformation of natural habitats to completely altered living space which sustains large human populations. The increasing congregation of humans accompanies the expansion of infrastructure, industry and housing. Vegetation and soil are mostly replaced or covered by dense grey materials which alter the previous space. The process of urbanization is still rapidly expanding already established cities or building new ones.

Three factors have come to the forefront as the main evolutionary influencers in urban areas: the urban microclimate, pollution, and urban habitat fragmentation.^[5] These all influence; natural and sexual selection, mutation, gene flow or genetic drift. Which are the processes that drive evolution.

Urban microclimate[]

A microclimate is defined as any area where the climate differs from the surrounding area, and modification of the landscape and other abiotic factors contribute to a changed climate in urban areas. The use of impervious dark surfaces which retain and reflect heat, and human generated energy lead to an urban heat island in the center of cities, where the temperature is increased significantly. And a large urban microclimate does not only affect temperature, but also rainfall, snowfall, air pressure and wind, the concentration of polluted air, and how long that air remains in the city.^[6][7]

These climatological transformations bring selection pressure and certain species have shown to be adapting to the urban microclimate.

Urban pollution[]

Species are evolving (or failing) to cope with contaminants in their environment in higher concentrations than naturally would occur. This is especially true in urban environments, since humans produce pollutants en masse when their population density rises.

There are two main forms of pollution which lead to selective pressures: energy or chemical substances. Energy pollution can come in the form of artificial lighting, sounds, thermal changes, radioactive contamination and electromagnetic waves. Chemical pollution leads to the contamination of the atmosphere, the soil, water and food. All these polluting factors can alter species’ behavior and/or physiology. Which in turn can lead to evolutionary changes.^[8]

Urban habitat fragmentation[]

The fragmentation of previously natural habitats into smaller pockets which can still sustain organisms leads to selection and adaptation of species. These new urban habitats come in all shapes and sizes, from parks, gardens, plants on balconies, to the breaks in pavement and ledges on buildings. The diversity in habitats leads to adaptation of local organisms to their own niche.^[9] And contrary to popular belief, there is higher biodiversity in urban areas than previously believed. This is due to the numerous microhabitats. These remnants of wild vegetation or artificially created habitats with often exotic plants and animals all support different kinds of species, which leads to pockets of diversity inside cities.^[10]

With habitat fragmentation also comes genetic fragmentation, within small isolated populations, genetic drift and inbreeding results in low genetic variation of the gene pool. Low genetic variation is generally seen as bad for chances of survival. This is why probably some species aren’t able to sustain themselves in the fragmented environments of urban areas. However, in populations with lower genetic variation in the gene pool and less gene flow between populations this could provide opportunities. For certain smaller populations occupying niches, adaptive radiation may happen quicker.^[11]

Urban evolution examples[]

That urban evolution is happening can be seen in multiple examples of city-dwelling organisms.

• The common blackbird (Turdus merula) may be the first example of actual speciation by urban evolution, due to the urban heat island and food abundance the urban blackbird has become non-migratory in urban areas. The birds also sing higher and at different times, and they breed earlier than their rural counterparts which leads to sexual selection and a separated gene pool. Natural behavioral differences have also formed between urban and rural birds.^[12][13]
• The peppered moth is an example of industrial melanism. These moths changed color from light to dark due to anthropogenic air pollution during the industrial revolution. The black melanism phenotype frequency saw a rise during the time of heavy air pollution and a fall after cleaner air became more normal again in cities.^[14]
• Urban Anole lizards (Anolis) have evolved longer limbs and more lamellae compared with anolis lizards from forest habitats. This because the lizards can navigate the artificial building materials used in cities better.^[15]
• The urban Hawksbeard plant (Crepis) has evolved a higher percentage of heavier nondispersing seeds compared to rural hawksbeard plants, because habitat fragmentation leads to a lower chance of dispersing seeds to settle.^[16]
• The London Underground mosquito (Culex pipiens f. molestus) has undergone reproductive isolation in populations at higher latitudes, including the London Underground railway populations, where attempted hybridizations between molestus and the surface-living Culex pipiens pipiens are not viable in contrast to populations of pipiens and molestus in cities at lower latitudes where hybrids are found naturally.^[17]

Genetic traits[]

Some already existing genetic traits can have an influence on the outcome which species can thrive in urban environments. The difference in , which normally would not affect a species’ population, can result in a dissimilarity in ability to adapt to environmental changes. The predisposition of some species to certain aspects of urban habitats also plays a role in how capable these species are to adapt to city life. Aspects, such as the physical similarity of their original niche, trophic requirements or behavior all make some species seem pre-adapted to their new anthropogenic environments.

Urban environments can also lead to an evolutionary trap, this occurs when a sudden anthropogenic change in the environment causes an organism to make a decision that normally would be adaptive, but now results in a maladaptive outcome, although better alternatives are available. This leads to selection (natural when it kills the animal and sexual when is distracts from copulating)

Behavioral Changes[]

Residing in urban environments can alter the behavior of species as they adjust over time to living in highly modified habitats in close proximity to humans. Some traits such as brain size have been shown to affect a species' skills in adapting to urban areas by allowing greater versatility in their ability to adapt.^[18] Even among species with the adaptations to live in urban areas, interactions with other present species such as competitors can limit the adaption of species to urban environments to the more dominant of the interacting species.^[19] This further limits the species' ability to thrive in urban environments, but for the species that do settle into urban environments changes in behavior have been shown both within and between species.

Avian Adaptations[]

Avian perception of humans as a threat alters as birds adjust to living in urban environments, causing them to become less fearful of nearby humans. Birds living in urban areas have a shorter flight initiation distance, the distance a potential threat can approach before the bird retreats, than rural birds of the same species. Additionally, species with a higher number of generations living in urban environments are less cautious around humans than species that arrived more recently in urban areas.^[20]  As urban birds are less cautious around people they also show higher levels of aggression in defending their territory and nests against humans than their rural counterparts.^[21] Caution in urban animals around humans also varies from city to city, affected by both the positive and negative human actions taken against a species within each urban center as the species associate the actions with the threat level that humans pose and respond accordingly.^[22]

Birdsong has also been greatly altered due to urban influences, primarily due to the large amounts of low frequency noises caused by traffic, which is caused by soundwaves with longer wavelengths. While the frequency of an individual's birdsong does not change,^[23] over generations urban birds have developed a higher minimum frequency to be heard over the ambient low-frequency sounds. Urban birds also sing with a higher amplitude, or volume, than their rural counterparts,^[24] causing a greater impact on the distance birdsong travels than frequency does by projecting further.^[25] The effect of amplitude on birdsong in urban cities, in part as a result of the Lombard effect wherein the volume of an individual increases as the surrounding noise increases in order to be heard, was notable during the COVID-19 lockdowns where the amplitude of birdsongs dropped while still continuing to maintain high transmission distances.^[26] The effect of urbanization on birdsong also includes changes over generation in the frequency and distribution in dialect.^[27]

References[]