The relentless advance of urban sprawl across the planet is forcing plants to spread fewer seeds which could place them at even greater risk of extinction.

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Cities are now home to half of the world's 6.6 billion humans and by 2030 that urban fraction will rise to 60 per cent as the sprawl continues to eat up the countryside.

As the spread of motorways and towns cuts up rural areas into smaller fragments, a study has now revealed that plants are evolving quickly to cope with the change.

Dr Pierre-Olivier Cheptou of the Centre d'Ecologie Fonctionnelle et Evolutive, Centre National de la Recherche Scientifique, Montpellier, and colleagues show that increasing areas of concrete found in urban centres worldwide are forcing urban plants to change the type of seeds they produce.

The researchers studied Crepis sancta, a weed that grows in wasteland and in the small plots surrounding trees planted on pavements in southern France. The weed produces two kinds of fruit: one that is light and blown by the wind, and another that is heavier and falls to the nearby ground.

The team took Crepis sancta seeds from around trees on several locations in the city of Montpellier, France, and grew the plants in a greenhouse, observing what fraction of seeds produced were of the light, easily dispersed type.

Compared with plants from the countryside, they report in the Proceedings of the National Academy of Sciences that plants from urban patches consistently produced fewer light seeds: evolutionary pressure favoured those that dropped heavier seeds on the small patch of nearby soil than spread them further afield, when they were more likely to fall on pavements and roads where they cannot germinate.

Based on a mathematical model of breeding, the researchers estimate that the current version of urban Crepis sancta took approximately 12 years to evolve.

They report that plants in a fragmented urban setting thus become doubly isolated, both in terms of being hemmed in by urban wasteland and unable to spread their genes afar, cutting the chances of species survival.

Dr Cheptou adds that the same effect could be seen in other creatures. "For instance, you may imagine that insects will loose their wings when habitats become fragmented and distances among patches become too large."

The team says that many other studies have also found a reduction in dispersal in organisms that live on islands, such as plants or insects. He adds: "This is however the first study measuring selection pressure on dispersal (small seeds lost) which allows us to quantify the rate of genetic changes."