Given the right conditions, life in the city might just lead to speciation. Wouter Halfwerk digs into the potential for and evidence of urban speciation in his latest paper in Frontiers in Ecology and Evolution. Halfwerk points out that many studies have revealed both genetic and phenotypic divergence in urban populations, but asks are we seeing urban speciation occur? Urban populations might diverge simply because of non-adaptive processes like genetic drift or founder effects (check out: Urbanization Influences Gene Flow and Genetic Drift). Perhaps one of the most often cited examples of this is the London Underground mosquitos, which have genetically diverged from their aboveground counterparts. Yet such strong barriers to gene flow are relatively rare across urbanization gradients. Arguing that urban speciation does in fact occur despite gene flow across urban ecosystems, Halfwerk discusses the evidence we should collect to empirically examine this evolutionary process.
Ecological speciation
For urban speciation to occur in the face of ongoing gene flow, it would require assortative mating (through opportunity or preference) arising from adaptive processes (e.g., divergent natural or sexual selection). Theory tells us that speciation in the face of extensive gene flow can occur if three conditions are satisfied:
- Adaptive variation exists — Traits that are directly or indirectly involved in reproductive isolation must diverge between urban and non-urban populations. We’ve seen that trait divergence occurs quite commonly in urban populations across diverse taxa, and in some cases we even have some evidence that these shifts may be adaptive. New studies are being published every day documenting phenotypic shifts in urban environments (Check out Johnson & Munshi-South 2017 for a review of this topic). But how do these traits influence reproductive isolation?
- Reproductive isolation is occurring — A second criterion is that urban populations must be reproductively isolated. Reproductive isolation may be more common than we realize, arising directly by selection on reproductive traits as well as indirectly by selection for locally adapted phenotypes. Divergent sexual selection may result in divergence in sexual signals and mating behaviors, or in reproductive timing and output. For example, the urban light environment influences reproductive timing in anoles, and anthropogenic noise leads to shifts in the songs of urban birds. Divergent natural selection may also indirectly result in reproductive isolation. Selection may favor locally adapted individuals, putting immigrants at a disadvantage when it comes to surviving long enough to interbreed (e.g., thermal tolerance), or locally adapted phenotypes may express condition-dependent secondary sexual traits (e.g., diet-dependent coloration). But are these traits genetically based?
- Divergence has a genetic basis — Lastly, trait shifts and reproductive isolation must be determined by genetic mechanisms. A growing number of studies are exploring genomic variation in urban organisms. Studies examining genomic bases of phenotypic shifts have found genetic differentiation associated with, for example, thermal tolerance in anoles and cognition in burrowing owls. Many more studies have found genomic differentiation of uncertain consequence, for example in acorn ants in France and white-footed mice in New York City.
So how common is the phenomenon of urban speciation? It turns out that we don’t really know yet! This uncertainty arises because most studies do not assess all three of these criteria. Making things more complex, tradeoffs might arise between phenotypes favored by natural selection and sexual selection. For example, male tungara frogs must balance tradeoffs between competition for females versus predation when vocalizing in urban habitats.
How to study urban speciation
Halfwerk concludes that despite overwhelming evidence for each of the three conditions necessary for speciation to occur, we still lack conclusive evidence that urban speciation is occurring. This isn’t to say it is not occurring, but rather we are not using the right tools in the right combination! Halfwerk advocates integrative approaches to tackle each of these three components simultaneously in the same system. Specifically, researchers should evaluate non-random mating in urban populations with respect to trait variation, validate these findings with experimental mate-choice trials, and link these phenotypes to genetic divergence through common garden rearing experiments and/or genome-wide association studies. Only through this integrative framework can we really evaluate the evidence for urban speciation.
In summary, although we haven’t yet documented urban speciation, the possibility of it occurring under the right conditions opens the door for fascinating future studies that will help us understand the speciation process more generally. Urban environments provide a fantastic opportunity to study incipient speciation and the underlying mechanisms directly, whereas in most other ecological settings we must infer these early mechanisms from already diverged populations or species.
Read the paper: Halfwerk, W., 2021. How Should We Study Urban Speciation. Front. Ecol. Evol. 8: 573545. doi: 10.3389/fevo.
Featured image: Tungara frog calling. photo by iNaturalist user Brian Gratwicke
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