This weed's relatives won't travel far from home. Credit: Gilles Przetak

Urbanization is forcing plants to evolve quickly, but their form of evolution could ultimately put them in danger of dying out.

The daisy-like annual weed Crepis sancta has two types of seed — big, heavy ones that tend to drop straight down, and light, floaty ones that can be caught on the wind and spread farther afield. Weeds growing in patches of soil around trees in the French city of Montpellier have evolved over a period of just 12 years to produce more of the big seeds than the flighty far-reaching ones, says Pierre-Olivier Cheptou from the CNRS in Montpellier who publishes his research team’s findings in the Proceedings of the National Academy of Sciences1.

Cheptou first noticed the trend in the 'field' of a real urban environment. To find out whether it was due to a genetic change, rather than environmental cues, Cheptou took seeds from both rural and urban environments and grew them in a greenhouse. The two plant types were allowed to be pollinated only by similar plants, and once they flowered, the different types of seeds were collected and counted.

The researchers found that the plants grown from urban seeds produced about 14% heavy seeds, compared with 10% heavy seeds produced by the plants from the countryside. Using a mathematical model, Cheptou worked out that the change had happened over about 10 generations of the plant. This is not the fastest evolution seen in plants, but is a remarkably quick period in which to change, Cheptou says: “it’s very short.”

Concrete jungle

Some seeds are built to drop; others to fly. Credit: Eric Imbert

In cities, plants become fragmented — they grow only in certain isolated areas. Cheptou measured the likelihood of seeds landing in an inhospitable site when dispersed in this kind of urban environment. He found that small, light seeds had a 55% lower chance of settling and growing than the heavy seeds that fall straight down into the same patch of ground as the parent. Light seeds were liable to hit a Montpellier parking-lot rather than a patch of soil: habitable ground not covered over with concrete accounts for just 1% of the city.

So having heavy seeds sounds like a good adaptation for a city plant. But the strategy comes with a higher risk of genetic isolation. “It’s clear that after colonization in a fragmented habitat, that fragmentation will prevent gene flow," says Cheptou. And that can put plants at risk of not keeping up with the times.

“This evolutionary scenario has a potential danger,” says Karl Niklas, a plant biologist at Cornell University in New York. “If the environment changes rapidly, locally well-adapted populations may go to local extinction and disappear.”

“The downside is that the population fragment is vulnerable if the environment changes there; it will not be able, ever, to reach new and fertile ground if it gives up the potential for long-distance dispersal,” agrees Martin Cody, a plant biologist from the University of California, Los Angeles. Cody has seen a similar effect on isolated ocean islands, where seeds usually carried by the wind have evolved to become bigger and so less likely to be blown out to sea.