By Mark Kinver
Science and nature reporter, BBC News
A species of plant found in cities has evolved rapidly in order to adapt to the challenges of surviving in the concrete jungle, a study suggests.
Heavier seeds had a better chance of germinating in urban areas
Crepis sancta growing in urban areas produces heavy seeds that fall to the ground rather than lighter seeds that are dispersed by the wind.
Wind-blown seeds are less likely to germinate because most end up on concrete surfaces, scientists say.
The findings appear in Proceedings of the National Academy of Sciences.
Researchers from the Centre d'Ecologie Fonctionelle et Evolutive (CEFE), based in Montpellier, estimate that the change in the way the plant disperses its seeds has taken place in as little as five to 12 generations (five to 12 years).
Co-author Pierre-Oliver Cheptou said the team was surprised by the speed of the change.
"Logic would assume that this sort of evolutionary trait would develop more slowly, which is probably the case in less fragmented situations," he told BBC News.
"However, at the same time, it is consistent with the estimation we had for the trait in a fragmented urban situation."
The team collected seed samples from the weed, which grows on wasteland or next to roadside trees, from various locations around the French city.
They then grew them in a greenhouse to see what fraction of the resulting plants' seeds were of the light, wind-dispersible variety.
Compared with specimens taken from the countryside, the urban samples produced far fewer of these seeds.
The researchers said "dispersing" seeds had a 55% lower chance of germinating because the majority ended up on concrete surfaces.
The heavier seeds were at an evolutionary advantage because they would fall down into the patch of soil that had supported the previous generation of the plant.
They added that their findings supported "cost of dispersal" theories.
"When dispersal is passive (wind or water transport) and habitat choice is random, the probability of settling in a suitable site is positively dependent upon the frequency of suitable sites in the landscape," they wrote.
"Many empirical studies have reported a reduction in dispersal structures in organisms that live on islands, such as plants or insects."
Dr Cheptou said that their study showed the same also applied to plants in urban areas, where suitable soil was widely fragmented by buildings, pavements and roads.
He explained that such a strategy, while increasing the odds of survival for the next generation, could have drawbacks.
"We can hypothesise that in fragmented situations, the evolution towards lower dispersal leads to more isolated populations, increasing the risk of extinction."