The Great Wall of China, built to hinder marauding tribes, hampers the flow of plant genes too. Members of the same species growing on either side are genetically quite different, indicates a new study of the Juyong Guan region. The alterations in the plants' genetic make-up are likely to have occurred over the past 600 years. "The differences show how rapidly changes in plant populations occur,' points out Peter Raven of the US-based Missouri Botanical Gardens.

A team from the China-based Peking University studied plants at three sites located 70 kilometres north of Beijing. At two of the sites, the terrain is separated by the wall, which is six metres tall and just as wide. At the third site, a narrow mountain path runs through the vegetation. In total, the team analysed 416 dna samples from six species with different habitats, pollination styles and reproductive systems.

To their utter surprise, the researchers found that plants isolated by the Great Wall hardly shared any genes.