As of 2018, 55% of the world’s human population lives in villages, towns, or cities, and this figure grows annually as people increasingly adopt the urban lifestyle. The environmental consequences of this urban expansion are vast: grasslands, forests, and wetlands are replaced by roads, sidewalks, and buildings that make up the residential and commercial infrastructure of today’s urban settlements. While cities were historically viewed by biologists as “anti-life”, an alternative view has emerged over the past few decades that places greater focus on the “altered-life” of cities; rather than being devoid of life, urban habitats are increasingly viewed as microcosms of altered species’ assemblages and ecological interactions. More recently, biologists have begun extending this work to explore whether—in addition to affecting the ecology of species—urbanization influences the evolution of species in a way that enables them to adapt to the harsh environmental pressures imposed by the construction of cities.
Because cities are constructed primarily to suit the needs of humans, cities in different parts of the world are predicted to share many features of their environments. This idea may seem obvious to anyone who has visited multiple cities but has surprisingly never been tested on a global scale. Using a suite of nine environmental variables extracted from satellite images of > 6000 white clover populations across 160 cities around the world (Figure 1), the Global Urban Evolution Project (GLUE) tested this prediction, confirming that distant cities around the world converge to similar environmental conditions. Specifically, urban habitats contained more impervious surfaces, had less vegetation, and were warmer than surrounding non-urban habitats. Moreover, populations within urban habitats were less variable in these environmental conditions than populations in non-urban habitats. Together, this means that two distant cities (e.g., Toronto & Tokyo) are more like one another than either is to its nearby non-urban habitat.
Given the convergent environmental conditions in urban habitats around the world, GLUE was interested in examining whether white clover evolves in the same way across all these cities. Indeed, similar environments often—but not always—lead to similar evolutionary responses, and biologists have long been interested in understanding the factors that cause some populations to evolve in parallel while others evolve in unique ways. To explore this, GLUE estimated the frequency of hydrogen cyanide (HCN)—a potent antiherbivore defense that also affects tolerance to environmental stressors such as frost and drought—in each of 40 populations along an urban-rural transect in each city. Overall, significant urban-rural changes in the frequency of HCN (so-called “urban-rural clines”) were detected in 47% of cities, with most of the clines showing less HCN in urban populations (Figure 1). Nonetheless, there is considerable variation not only in the presence of clines but also in their magnitude, with some cities showing very strong changes in HCN frequencies while others show weak to moderate differences across the urban-rural transect. Sequencing of 2,074 whole genomes across a subset of 26 cities (Figure 1) confirmed that urban-rural clines were driven by natural selection: urban and rural populations were more divergent in the dominant alleles and HCN phenotype than expected by neutral evolution.
The variation in the strength of urban-rural clines leads to a natural follow-up question: what predicts this variation? GLUE addressed this question using the satellite-derived environmental data and examining whether the strength of urban-rural clines could be predicted from broad environmental differences between cities, or urban-rural changes in environmental factors within cities. Across all cities, urban-rural variation in water loss and vegetation (a proxy for herbivore damage) were among the most consistent predictors of the strength of clines. Put simply, herbivory seems to select for higher HCN production in rural areas, but in the absence of strong herbivore pressure (i.e., when there is less vegetation across the whole gradient), drought is the primary mechanism structuring clines. These results mean that the factors creating and maintaining urban-rural HCN clines vary across cities, and reflect a balance between both biotic (e.g., herbivory) and abiotic (e.g., drought) selective pressures.
GLUE has brought together over 280 scientists from around the world to answer the question: Are species adapting to urbanization? This study shows unequivocally that the answer is yes, and follows up to show that variation in adaptation across cities can be predicted using knowledge of environmental conditions within and between cities. If adaptation to urban environments is common, as suggested by the study’s results, this could have implications for the ability of species to respond and persist under one of the most environmentally destructive forms of land-use change on the planet—urbanization.
Read more about the history of GLUE here
Check out the paper here: www.science.org/doi/10.1126/science.abk0989
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