Urban Evolutionary Biology .

As a graduate student in the late 1980s and early 1990s I was struck by an undercurrent of discrimination against behavioral biologists who studied domesticated, zoo, or urban animals, and ecologists who studied species around people. Cities?! They are not “natural.” Field biologists, after all, should go to “natural” places and study “natural” processes. But what is natural? Naturalness is, of course, not absolute. An older definition I like is that it reflects the amount of human energy that has gone to modify the system (Anderson 1991); a site with paved roads and buildings is less natural than a site with dirt roads and cabins. But this definition reflects intentional modifications and doesn’t recognize our planetary chemical footprint that includes Persistent Organic Pollutants (Roscales et al. 2016) and plastics (Geyer et al. 2017) to name a few. Restricting our studies to “relatively” natural areas is limiting. How are we to understand and manage biodiversity around ecotourism or our expanding urban footprint? And, the irony was that studying Drosophila and Tribolium in jars and Daphnia in mesocosms was somehow OK because they were “model systems.” I will be the first to admit that we have learned a lot from model systems and I am delighted to see that we have realized that we have a lot to learn from cities! Now, we not only recognize that studying the behavior, ecology, and evolution of urban species provides unique opportunities to study foundational processes, but it also is essential if we are to wisely manage biodiversity and increase urban sustainability on an increasingly urban Earth. Humans have a long history of habitat modification and there have been two major human-driven transitions, as Milot and Stearns nicely remind us in the final chapter of Urban Evolutionary Biology. First, there was a transition to agriculture, which created a series of novel anthropogenic environments. Only later was there a transition to larger population settlements. Now, over 50% of humans live in cities (United Nations 2018) and cities occupy 3% of the Earth’s habitable land (Center for International Earth Science Information Network – Columbia University et al. 2011). The proportion of people in cities, and cities themselves, are expected to grow substantially in the coming decades (Seto et al. 2012). These transitions have clearly influenced the distribution and abundance of organisms on Earth and because of their impact, urban areas may provide replicated experiments, much like islands, to study ecological and evolutionary processes that occur in cities and indeed provide a novel model system for studying adaptation.