January 30, 2017 — Cities are hubs of commerce, politics, population — and evolution? Yep. Due to human-wrought environmental change, biological evolution moves more quickly in cities than in other areas, according to a new paper published by an international team of researchers in Proceedings of the National Academy of Sciences.
The study, based on analysis of 89 previously published studies, found that for animals and plants, models based on whether urbanization was in play best explained speedy evolution of observable traits such as body size, anatomy, life cycle, behavior and physiology.
“What is the very important implication of this study is that the urban habitat is not simply reducing the number and diversity of species,” says Marina Alberti, lead author of the study and director of the University of Washington’s Urban Ecology Research Laboratory. “We are selectively determining which species can live in cities.”
Alberti gives the example of electrical transmission towers. One study from the United Kingdom found that in soils polluted with zinc due to the towers’ presence, different grasses, such as sheep’s fescue and tufted hairgrass, have evolved different levels of zinc tolerance. However, genes needed for tolerance don’t appear in every individual of a species; if tolerant plants are absent in some local habitats, this could affect the ecosystem by changing the local community composition. Urban environments are the backdrop for many such interactions, which can happen at a speed that hasn’t always been associated with evolution.
“[E]volutionary biologists have been aware of the interactions between ecology and evolution for a long time,” Alberti noted in a follow-up email, “but the potential ecosystem feedback on a contemporary scale has been overlooked until very recently because we thought that evolution was occurring on a very long timescale.”
To take a stab at determining which aspects of urban development hasten evolution’s pace, the researchers coded different kinds of urban disturbance into their models. They found that introduction of new species — predators, prey, hosts and competitors — had a large association with the changes they looked at.
Another potentially major driver of rapid urban evolution is the number and speed of interactions between people and people, people and animals, and people and plants. Counterintuitively, changes to habitat correlated with relatively low evolutionary change, a finding the researchers said might be due to limitations to the study’s methods.
A few words of caution, though: This research is the first to systematically and explicitly examine, using a large global data set, how urbanization affects evolution, so other researchers have not yet replicated it, a process that could help confirm the findings. Meta-analyses like this study rely on already published research, and it’s impossible to get a truly random sample of phenotypic (observable) changes from across the globe, so over- or under-representation of studies from particular areas, ecosystems, or organisms could affect the results. In particular, the authors acknowledge that their conclusions about the relative importance of different urban processes to evolution should be viewed with caution because those results might be influenced by factors including both which species the academic literature considers and the way in which the authors classified interrelated variables that affect evolutionary change.
Still, they recommend that conservation biologists consider this apparent evolutionary change of pace. As plants and animals respond to human reshaping of their environments by evolving in ways that further change the ecosystem, these changes, in turn, can circle back and affect human life.