In 2009 the United Nations reported that half of the world’s human population lived in cities and was expected to grow to 66% by 2050. The movement of people from dispersed living to concentration in urban environments is a large change both for human civilization and for the environment. Urbanization is the process of changing from natural habitats to dense grey space made up primarily of buildings, roads, and accessory infrastructure (e.g. street lights, underground sewage pipes, power lines, etc) accompanied by dense human populations. While many cities are well established, humans continue to build new cities or expand cities outward in a network of suburban environments. And urbanization is not simply about a transition from green to grey space, other abiotic changes such as changes in light regimens due to artificial lighting, increased pollution, and increased impervious surfaces leading to runoff are found in urban areas. With all of these landscape changes a central question is: how do organisms respond to urbanization? Some species cannot live in urban environments while others thrive. Thus, one of the earliest and most prominent changes is in species composition between urban and nearby rural environments.
Urban ecology has taught us much, but a new subdiscipline of urban evolution is emerging. Evolution is the study of allele frequency changes over time, although biologists may study changes in phenotypes as a proxy if the causative genes and alleles are unknown. Biologists increasingly observe evolutionary change on relatively short time-scales particularly when there are strong selection pressures. The selection pressures due to urbanization have been around for 10s to 1,000s of years, thus it is not inconceivable that species have (and are) adapting to urban environments. When you consider that many of the urban adaptor species studied have multiple generations per year (i.e. rats can produce 3 generations per year, mosquitos 10 generations per year; for comparison generation time in humans is 25 years), a little city time can result in a lot of evolutionary change between generations.
Urbanization creates novel environments which species may adapt to, so I will describe a few of the selection pressures below and how different species are responding.
Air pollution from steel mills has been shown to increase the mutation rate in herring gulls that live close to the plant. Yet this study also showed that gulls in urban areas also have higher mutation rates than their rural counterparts. Many mutations in the genome will occur in genomic regions that are not functional thus will not affect the fitness of the individual. However, some mutations will occur within genes or regulatory regions (DNA sequences which turn genes on and off) and may change the protein sequence or gene expression pattern in a way that decreases the fitness of the individual (i.e. the mutation is “deleterious”) and potentially affects the viability of the population.
Urban environments have novel substrates that wild habitats do not such as concrete and asphalt. There are two big questions here: 1) can species use novel substrates, and 2) if so, how are they adapting? While I think of concrete and asphalt as being very rough surfaces, they are much smoother than natural surfaces like tree bark, especially if the concrete is painted like you would see for houses.
Several studies of lizards show that they do use novel surfaces as perches for sun-bathing, hunting insects, or to escape predators. A study of the Puerto Rican crested anole observed more toe lamellae and longer legs on individuals in urban environments compared to those that lived outside the city. Toe lamellae are scales that help lizards grip surfaces and species with lots of lamellae grip smooth surfaces better than species with few; this study of anoles showed that the same holds for populations (not just different species) with more toe lamellae. Thus, morphological changes are also occurring due to changes in the urban environment.
High densities of humans are a defining feature of urban environments. Many species consider humans as predators and run or fly away as they approach. However, in urban environments the density of humans can be so great that constant encounters would always mean running away instead of foraging, resting, or looking for a mate. Thus populations of many urban species have decreased flight initiation distance. This is a measure of the distance (in meters) between a human and an individual before it tries to escape. Flight initiation is a behavioral response with an underlying genetic basis. One study measured flight initiation distance of blackbirds in 12 cities in Germany and compared the distance to near-by rural populations. The urban populations had a lower distance, thus allowing humans to get closer to them. Additionally, this study observed allele frequencies in the SERT gene, which is related to harm avoidance behaviors, and observed significant differences between urban and rural populations. Thus, this gene is under selection in blackbird populations to decrease harm avoidance behaviors to allow individuals to be more successful living in urban environments.
Urban environments make new foods, particularly human food waste, available to species, and also change the distribution of food resources on the landscape (like when it is clumped at a garbage dump). Processed human foods are calorie dense and may be softer than natural foods. Researchers studying beak shape of house finches in and around Tucson, Arizona found that urban populations had larger bills which could exert more bite force than their desert counterparts. The researchers hypothesized this change was due to selection to eat anthropogenic foods including lots of sunflower seeds found at bird feeders in the city. More interestingly, because bill shape affects how birds sing (which is how they attract mates), urban birds had longer and deeper trill rates and sang fewer notes. This implies that with enough time (and staying on the same evolutionary trajectory) these geographically close birds may not even find birds in the other population attractive enough to mate with, since songs are learned and different song characteristics indicate higher or lower quality mates.
Another way food has induced evolutionary change is an example from cockroaches. Poisons deposited in sweet baits have been used to exterminate roaches because they can be squirted into small crevices where roaches will eat it and die. Over time entomologists and exterminators began to notice that cockroaches avoided glucose-based baits. Scientists identified that the brains of “glucose-averse” roaches would stimulate neurons that perceived glucose as bitter (similar to caffeine) instead of sweet like fructose. This was in contrast to the wild type (i.e. “normal”) roaches who perceived glucose as sweet. This natural selection has occurred independently in populations around the world but also means that poison may have to be put in different baits, like salty peanut butter, to remain effective; thus bringing new meaning to “peanut butter-jelly time.”
Where is urban evolution going?
In terms of big picture questions, we want to understand if evolution to urbanization is repeatable and if we can predict the direction of evolutionary change. While dozens of species have been studied to date, there are many more species that live in urban environments that could be studied. Thus one goal is to increase the taxonomic diversity in urban evolution studies. While observing similar directions of evolutionary change in diverse taxonomic groups is important, we also want to understand if there is repeatability across cities. Cities have common features such as roads, access to water, small and large green spaces, and increased human densities; but they also have their own independent histories including founding age, population density, levels of industrialization, and socio-economics. Thus for different selection pressures, we want to understand which aspects of the city influence, and to what degree, evolutionary change in mutation, gene flow, and adaptation across diverse organisms.