Above: Fig. 2 from Rivkin et al: many species have been shown to adapt to cities. Thompson et al. ask whether or not this could lead to speciation.
Organisms that persist in urban environments are subject to novel selective pressures as they exploit this novel niche space. We now know that this ecological shift can lead to evolutionary change in urban environments in terms of rapid adaptive responses in behavior, physiology, and morphology, with shifts in migration, mutation rate, and genetic drift further driving phenotypic divergence. This raises the intriguing possibility that in time we might observe speciation. Understanding the circumstances that lead to new species is one of the main longstanding issues of evolutionary biology, and urban environments provide a fantastic natural experiment to observe incipient speciation. In their recent article in TREE, Ken Thompson, along with co-authors Loren Reiseberg and Dolph Schluter, explore this intriguing possibility.
The evidence for the potential of urban speciation
Thompson and colleagues dig into this topic by first discussing the reasons why speciation in the city is possible. Urban areas are drastically modified in similar ways globally and we have evidence of phenotypic and genetic divergence between urban and nearby non-urban populations across a wide range of taxa. The authors present evidence that speciation has occurred or is occurring in response to other aspects of anthropogenic change.
Habitat fragmentation can have the immediate effect of isolating populations, for example the rainforest damselfly Megaloprepus caerulatus has diverged into distinct genetic lineages following deforestation. But divergence and reproductive isolation can arise even without physical separation of populations. For example, some plants that have adaptively diverged in response to soil toxins in mine tailings have also diverged in flowering times from nearby populations on unpolluted soils, leading to reduced gene flow – a first step in reproductive isolation. Or it can arise following the breakdown of physical barriers, leading to hybridization and fusion of species. These examples are presented to demonstrate that observing speciation is certainly possible, particularly in anthropogenically disturbed systems.
“Urbanisation alters environments in much the same way – by polluting, fragmenting, and restructuring natural landscapes and thus has similar potential to cause speciation.”
How might urban speciation occur?
Given this background, it may be surprising that similar studies of speciation in cities do not abound. The authors present some supporting evidence from studies on mosquitos, including the “London Underground mosquito“, but admit that there is a dearth of data to rigorously test this idea of urban speciation. This lack of evidence they argue is attributable to lack of attention and not because speciation is not occurring.
Even though there are no robust examples of urban speciation (yet), Thompson and colleagues give us an idea of what urban speciation might look like by detailing the mechanisms that might produce new species in cities. They propose several mechanisms that can lead to divergence and reproductive isolation of urban populations, which ultimately may result in speciation.
Urban populations may diverge because they are subject to different natural selection regimes in their new environment leading to local adaptation to novel urban conditions. In other words, certain traits are favored in urban environments while others are favored in non-urban environments. If selection pressures are similar across cities, then we would expect similar phenotypic shifts to arise independently in different cities. This ecological specialization can then be followed by reproductive isolation that is incidental to this phenotypic divergence. There are several ways in which this reproductive isolation may occur, for example because of phenotypic shifts that limit dispersal distance, shifts in courtship traits (e.g., coloration or song), or local physiological adaptation.
However, even if selection pressures among cities are similar, divergence may arise between cities because of mutation and drift. Lineage specific mutations favored by natural selection and the chance fixation of alternate alleles can lead to reproductive isolation not only between urban and non-urban populations but between cities as well. Other mechanisms for reproductive isolation discussed by the authors include autopolyploidy (resulting in the instantaneous reproductive isolation of offspring from their parents), novel hybridization (e.g., between species that were allopatric outside of urban environments), and reduced dispersal.
Of course, urban speciation is not a foregone conclusion of phenotypic or genetic divergence. Many different factors may impede urban evolution such as inconsistent or variable selection pressures, human-facilitated gene flow, clinal variation, and plasticity. Thus it is important to consider not only the mechanisms of urban speciation, but also the factors that mediate the strength and directionality of phenotypic shifts.
Urban speciation gives us insight into evolutionary processes
Future research on urban evolutionary trends should consider the possibility of speciation and investigate the criteria necessary for speciation to occur. Specifically, Thompson et al. recommend (Table 1) that studies of urban speciation should establish: (1) phenotypic divergence associated with differences in environments (urban populations should be more similar to each other than to non-urban populations), (2) divergence time of populations (urban and non-urban populations may have diverged recently or long before urbanization), and (3) the specific mechanism through which urbanization produced reproductive isolation.
If Thompson and co-authors are correct about the prevalence of urban speciation, then studying this phenomenon could provide tremendous insight into the process of speciation in general. In particular, by studying adaptation and speciation mechanisms in urban environments we can improve our understanding of the evolution of reproductive isolation, the role of plasticity in adaptive evolution, and the repeatability of evolution.
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