TO THE naturalist in me, the world is full of sorrows: extinctions, the deaths of ancient forests, fires and floods. But the evolutionary biologist in me is more sanguine. The process of evolution continues unabated. If anything, humans have caused it to speed up.
Look closely enough and you can see a new world evolving around us. Witness it in the London Underground, where, beside the rumbling trains, a new species of mosquito is in the midst of an evolutionary flowering. And it is far from alone.
For centuries, evolutionary processes were thought to happen at a glacial pace compared with the speed of daily experience. However, over the past decades we have come to realise that evolution can in fact occur very quickly, even within days, as the virus that causes covid-19 has demonstrated. As I argue in my book, , this evolution is occurring disproportionately fast in our cities.
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These urban landscapes might seem a far cry from the Galapagos Islands and the other wild places where the rules of evolution were first uncovered, but no amount of environmental tinkering or destruction by humans can rewrite the rules of nature. And by considering the laws of evolution, we can make predictions about the kinds of new species that will emerge via the radical biological change taking place, mostly unnoticed, right under our noses.
We might have anticipated this rapid urban evolution a long time ago thanks to one of the most robust and influential models of the natural world: the theory of island biogeography. This was devised by the mathematical ecologist Robert MacArthur and the big-thinking ant biologist E. O. Wilson to explain the dynamics of life on islands. Their 1967 book on this concept outlined how the closer an island is to a mainland, the more species will colonise it, and the bigger it is, the more species will be able to survive without going extinct.
Deep in this book was another less-noted idea: that the rate of evolution of new species should also be greater on bigger islands. This applies not only to true islands, but also to island-like habitats. A cornfield is an island relative to the sea of surrounding forest. So is a lake. For face mites, your body is like an island. So is a city, and as the world’s population grows, we are creating more of these vast islands of urbanisation. In these habitats, just like on oceanic islands, the rate of origin of new species should be highest in those that are big, or growing in area. We should be able to look at cities and see evolution in action. Recently, scientists have.
One of the ways that species evolve in cities is through isolation. Some members of a species become separated from their counterparts in other habitats and then diverge. This process has been the driver of the evolution of new species since our ancestors first formed large settlements, relying on stored foods in the early cities and proto-cities. Species such as house sparrows, house mice, granary weevils, grain moths and many others are the result of groups of animals that became isolated from their rural counterparts and diverged.
This process continues apace. New York’s brown rats now have , the latter possibly due to a softer, higher-quality diet. In the UK, urban living is changing fox populations (see “Fox and the city”). And populations that become isolated in a particular city can evolve differently from those found in other cities.
When creatures that live alongside us, such as carpet beetles and certain spiders, arrived in a specific spot they were unlikely to move back. Once isolated, they began to evolve independently of the populations elsewhere, due to chance changes between generations, or natural selection thanks to the unique features of a particular place. The house mice, carpet beetles and rats in cities around the world are nearly all now evolving along divergent trajectories.
Uptown rats
This is even occurring within cities. Jason Munshi-South at Fordham University in New York and his colleagues have shown such divergence in brown rats. Their analysis of rats in New York revealed two distinct clusters that differ genetically, but also probably in other ways that have yet to be discerned. Uptown Manhattan rats seem to avoid sex with the rats from downtown, separated by a geographic barrier, midtown, a mainly commercial district that lacks the household trash and backyards that rats thrive on.
“Radical biological change is taking place right under our noses”
Meanwhile, Elizabeth Carlen, now at Washington University in St Louis, Missouri, has found that in the US are also diverging from each other genetically. It is likely that the lice that live on the birds are diverging more so than their hosts, and the louse-dependent bacteria species even more than the lice, because species with shorter generation times evolve at a faster pace.
It isn’t just blind chance that governs which of these species is likely to diversify and eventually give rise to new species. There are rules at play. Charles Darwin noted that populations will tend to diverge if their “tendency to modification” isn’t “checked by intercrossing” with species from other regions. He was right. What’s more, such intercrossing – what we now call gene flow – is more likely for species that readily disperse, whether on the wing, by catching a ride (be it on a bird, inside a louse or with humans on a plane, train, boat or automobile) or even by running, than for those that don’t.
On oceanic islands, the dispersal ability of a species is the key determinant of whether it is likely to diversify, and evolve into new species. On the Galapagos Islands, the finches, which tend to fly only short distances, have remained genetically isolated from mainland kin and so diverged from their ancestors, with each finch species or subspecies now possessing unique attributes, behaviours and, especially, beaks. So too the mockingbirds.
But the blue-footed booby, and other seabirds with a proclivity for long-distance flight, haven’t. Their greater dispersal ability means genetic mixing with other populations is still at work, constraining the diversification of specific populations.
On remote islands, biological features of organisms determine their ability to move. In cities, human behaviour is a big factor in who moves and who doesn’t. Tighter control of ship hygiene (and hence the ability of rodents to catch a ride), for instance, is probably responsible for the modern divergence of rat lineages among and within cities. Conversely, , a lineage of Demodex folliculorum, appears to have sufficient movement on us as we ride cars, planes and boats, to maintain gene flow from Mexico to Australia to New York and beyond, so avoiding localised divergence.
It is in the context of the concepts of island area and gene flow that the story of the London underground mosquito has begun to be pieced together. The presence of these subterranean insects came to light during the second world war, when thousands of people took shelter in underground stations during bombing raids, and they would come to be called Culex pipiens f. molestus, a form of the species Culex pipiens, the common house mosquito.
The above-ground C. pipiens mosquitoes are seasonal, active in the warmer months, require a meal of blood before they can lay eggs and tend to feed on birds. In contrast, the subterranean form is active year round, the females tend to feed on mammals and can lay eggs without first ingesting blood.
Could this be an example of speciation in response to city living? A clue comes from the fact mosquitoes that look very much like the molestus form are found in the subways, sewers and even flooded basements of cities as far flung as Paris, Minsk in Belarus, Mailuu-suu in Kyrgyzstan, .
Recent genetic studies indicate that the molestus form probably began its divergence from C. pipiens in the ancient cities of the Fertile Crescent, a region of the Middle East. It was here that many urban species originally found their footing. Then, as they dispersed northwards, molestus mosquitoes persisted even in regions with cold winters by taking advantage of the heat trapped below cities.
Other studies by Dana Price and Dina Fonseca at Rutgers University in New Jersey suggest that as the molestus mosquito colonised the underworld it , digestion and immunity, all of which seem to have enabled it to thrive in subterranean sewers rich with organic wastes.
Meanwhile, this mosquito continues to evolve and genetic studies indicate that various molestus populations have begun to diverge genetically from each other. For instance, when the molestus populations in the London Underground were studied in detail, .
We could be heading for a future in which each line of the Underground has its own mosquito species. However, this divergence isn’t seen in the above-ground C. pipiens mosquitoes, which more readily disperse.
Future paths
What can fast-evolving city rats and mosquitoes tell us about future urban evolution and the kind of new species we can expect to see? Although the precise consequences of evolution can be idiosyncratic – it would be hard to have predicted the existence of Komodo dragons or fungus-farming ants, for instance – the broad strokes of evolutionary change are predictable, reflecting nature’s general rules.
Mammals tend to evolve bigger bodies in cold climates; we should expect the same for urban species, with fat-bodied rats in the far north and mouse-like rats around the equator. This is likely to already be occurring, unnoticed. On islands, species often lose the ability to fly and disperse (for example, flightless cormorants in the Galapagos); it is better to stay near home than to fly off in search of land that might not exist. The same has already begun to occur in cities. The seeds of hawksbeard plants growing in beds around trees in the city of Montpellier, France, . Granary weevils have entirely lost their wings and blackbirds in some cities have ceased migrating.
On islands, species also often evolve the ability to reproduce without mating. This is because species often arrive alone, and under these conditions, it is beneficial to be able to produce offspring without a mate. So the subset of arrivals with the genes to reproduce without a mate are more likely to thrive. In cities, this has also happened for some Surinam roaches, American cockroaches and crazy ants – so called because of the jerky, unpredictable way they move.
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Species also evolve traits that reflect available foods. Populations of urban house sparrows in Arizona have larger, stronger bills to deal with the tougher seeds found in the city, which in turn affects their songs. But more often, the change relates to the food provided by humans. House mice and dogs have the ability to produce extra amylase in their mouths to break down starch, as carbohydrates are a significant part of urban diets.
Another way to cope with the demands of city living is to use brainpower. Rooks, for example, have learned to use bin liners as a tool, working in pairs to pull a liner up from rubbish bins to get hold of the food within. Already, a that thrive in cities are those with inventive intelligence, such as rooks, crows, , ravens, jays and, in the tropics, parrots – as demonstrated by Ferran Sayol, now at University College London, who investigated the brain size (a marker of intelligence) in 629 bird species in 29 cities.
“Cities favour species with ever bigger, more inventive brains”
Looking to the future, we should expect cities to favour species with ever bigger, more inventive brains, but this isn’t the only way to thrive in the urban world. Sayol’s research also shows that species with small brains, such as pigeons and swifts, can be very successful in urban environments by having large numbers of chicks, many of which don’t survive. Conditions are variable in cities. One way to deal with that variability is to be smart and make different choices under different conditions. The other way is to produce lots of babies and hope a few, by chance, survive.
But the most predictable features of urban evolution (at least in the near future) aren’t terribly glamorous. They relate not to the island-like attributes of cities, but instead to the dominant selective pressures – the factors that kill. In cities, humans are the key factor, most often through the use of chemical biocides. In response, German cockroaches living in buildings in which sugary roach baits are used have evolved a disinterest in the sugar; as a result they walk away unaffected by our baits. Meanwhile, bedbugs, houseflies, German cockroaches, house mosquitoes and many other species have evolved resistance to pesticides and, so, are harder to control.
This kind of evolutionary pressure can mean serious consequences for the spread of disease. For instance, urban populations of two species of malaria-carrying mosquito appear to be , potentially because of selection due to pesticides used on the city insects. It is a kind of evolution that isn’t as visible as, say, changes in the beak of a finch, yet is still a measure of the awesome power of natural selection.
Cities have set the stage for an extraordinary evolutionary experiment that is unfolding all around us. Granted, the species involved tend to be ones that eat our waste or even our bodies, and their blossoming has come at the expense of the loss of thousands of species of birds, butterflies, mammals and bees due to habitat loss. Yet each is a manifestation of natural selection’s ceaseless working, a reminder that, despite our assaults on Earth, the process of living goes on.
Fox and the city
Over the past century, foxes have taken up residence in many UK cities. Thanks to the abundance of food waste from humans, the home ranges of these animals can be tiny, just 0.4 square kilometres compared with 30 sq km for their rural cousins.
But that isn’t all; the bones of urban foxes are evolving relative to those of their bucolic brethren. A 2020 study by Kevin Parsons at the University of Glasgow, UK, and his colleagues found that , wider snouts than ones that live in the countryside, and the part of their skull that encases their brain is smaller.
These anatomical changes match those seen during domestication – in dogs, for example – leading the researchers to conclude that urban foxes may be self-domesticating in our midst.
