As of 2008, more people live in urban than in rural areas, and human populations in cities will only continue to increase. One goal of urban evolution research is to determine how the expansion of cities impacts genetic diversity of plants and animals. Cities may alter genetic diversity by changing how organisms move across the landscape. Organism movement, or lack thereof, changes gene flow within and between populations and therefore the ability of life to adapt to urban environments.
Urban areas may affect movement of organisms across the landscape in one of two ways. First, the urban fragmentation model suggests that cities act as a barrier to species movement, thus reducing genetic diversity and increasing population differentiation between urban and rural sites. In contrast, the urban facilitation model posits organisms move freely through cities. If the facilitation model is true, urban and rural areas will have similar genetic diversity and high gene flow among different sites.
To evaluate which model best explains patterns of genetic variation in cities, Dr. Lindsay Miles (University of Toronto Mississauga) and colleagues analyzed 164 studies that measured gene flow and genetic drift in urban areas. These studies spanned metropolitan areas around the world and a range of species from plants to insects to mammals. Overall, over 90% of studies showed an effect of urbanization on genetic drift. Of those studies, 62% found support for the urban fragmentation model, such that measures of drift were higher in urban areas. Similarly, 85% of studies found that cities impact genetic differentiation. Two-thirds of those studies show increased differentiation between urban and rural sites, which also supports the urban fragmentation model.
Interestingly, many of these effects were species-specific and cannot be generalized to higher groups, e.g. not all birds had the same response to cities. In addition, the age and size of the city that was studied greatly influenced these evolutionary dynamics. These results highlight the need to continue sampling more species and cities worldwide. Further work in this area will help us better understand organisms’ evolutionary trajectories on an increasingly urban planet. To learn more, visit Dr. Miles’s website or follow her on Twitter.
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