Cities create accidental experiments in plant, animal evolution

From clover to lizards, a city’s nonhuman residents evolve in human-scale time

Anolis cristatellus

GOING TO TOWN  After generations of city life, widespread tropical lizards Anolis cristatellus and white clover are not quite the same the same as their country cousins, though not always in the way scientists expect.

Geoff Gallice/Wikimedia Commons (CC BY 2.0); Frank Vincent/Wikimedia Commons (CC BY-SA 3.0 migrated)

AUSTIN, TEXAS — Cities have become great unintentional experiments in evolution. Urban life can alter the basic biological traits of its plant and animal residents, down to the taste of leaves or the stickiness of toes, researchers reported at the 2016 Evolution conference.

For white clover (Trifolium repens), leaf taste matters as a defense against grasshoppers and other predators, Kenneth Thompson of the University of Toronto Mississauga said June 19. Variations in two genes let clover booby-trap its leaves and stem to release a warning burst of cyanide when bitten. A little taste doesn’t kill a nibbler but can send it spitting away to another plant.

Clover’s cyanide-doping genes are more common in warmer locations. So in Toronto, Thompson expected the elevated temperatures typical of cities to mean more bitter clover downtown than toward the outskirts. He was startled to find just the opposite. The results couldn’t be explained by fewer clover-eaters downtown. Grazers, mostly snails, were no more likely to damage clover at the urban edge than toward the city center, the researchers found.

The same unexpected pattern of cyanide defense increasing away from the city center also showed up in Boston and New York, but not Montreal. The explanation, Thompson and his colleagues now propose, lies in what happens during winter.   

Urban snow cover can insulate plants from the worst of winter’s stinging cold, a special threat for cyanide-carrying clover. Cyanide is toxic to plant tissue as well as to its predators. At temperatures above freezing, clover leaves and stems aren’t at risk because cells contain only the molecules that will eventually form cyanide. Stored separately, the ingredients usually don’t come together in their dangerous form until an animal bites, crushing and ripping cells. Such a defense damages some plant tissue but the sacrifice can save the rest of the plant. Freezing, however, ruptures the safety system and sets the cyanide loose on the clover for no apparent benefit.

In Toronto, Boston and New York, the buildup of urban heat thins the protective snow cover, more so downtown than toward city margins. The closer to the city centers a clover plant grows, the less likely it is set up a cyanide defense and risk poisoning itself during hard freezes, Thompson said. Montreal, however, has such prodigious snow cover that even downtown clover can stay pretty well insulated.

The clover study with its multiple cities illustrates one opportunity that analyzing urban adaptation provides. “It’s basically a replicated evolution experiment,” says Jason Munshi-South of Fordham University in Armonk, N.Y., where his lab studies local rodents and pigeons.  Comparing cities “is where we need to go.”

Urban evolution, says Kristin Winchell of the University of Massachusetts Boston, “is a very young field, growing very fast.” She presented her latest data June 20 in her ongoing project to put together the first comprehensive picture of how a lizard is adapting to urbanization. Puerto Rico’s agile, abundant Anolis cristatellus lizards have colonized dense cities despite the perilous open expanses between urban trees. City lizard legs tend to be 2 to 5 percent longer than those of their forest counterparts, and the elaborate toe pads that help the lizards skitter up walls have more folds, she and her colleagues reported in the May Evolution.

Urban lizards may need all the anatomical advantages they can get to race around cities: The smoothest thing a lizard copes with in the wild, a leaf, is much rougher and easier to grip than a smooth metal surface. Videos of lizards dashing up slanting lab racetracks show varying footwork abilities, typically better among urban residents. While the lizards’ origins weren’t indicated, in one video, a lizard darts up the slippery unpainted aluminum surface, pauses, then shoots to the end, beating a lizard that also stopped and then steadily slid backwards.

Differences between city and country lizards persist even in a generation raised in the same environment, Winchell reported. That might mean the two groups of lizards have somewhat different genetic makeups. She still has to test the alternative of some carryover effect in gene regulation that mothers pass to their offspring.

Winchell has a way to go before putting together the whole picture of how cities change lizards. But however these stories turn out, they have a special pull: They take place on human-scale time. And as Munshi-South puts it, the evolutionary history of urban animals “is one we created ourselves.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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