Yet the stone wall that runs thousands of kilometres across northern China failed to fulfil its early promise. Northern armies broke through the defence and overran the region regularly.

Yet new scientific research has discovered that the wall, now in disrepair, played a major part in hampering the flow of plant genes.

Genetic invaders

The research reveals that certain plants of the same species growing on either side of the Juyongguan section of the Great Wall in the northern suburbs of Beijing are genetically different. They differ so much that it cannot be explained simply by the influence of natural elements, according to Gu Hongya, chief scientist who lead the research.

Gu, a biologist from the College of Life Science at Peking University, said the influence of the Great Wall as a physical barrier to the gene flow of plants is evident when comparing the genetic structure of plants growing at the Juyongguan with those of the same species in nearby areas without the Great Wall.

"They look little different from each other," Gu said. "But analysis of their DNA patterns revealed genetic variations that might have arisen from many factors - a major one being the Great Wall."

Gene flow helps with evolution, as does natural selection and mutation. Yet each of these processes have different impacts on the evolution of organisms. Gene flow from one population to another tends to make the two populations similar, whereas natural selection does the opposite.

"It (gene flow) happens every day in nature," Gu said.

She added that the study of gene flow and the factors that affect it would not only contribute to our understanding of biological evolution, but also help prevent so-called biological pollution by way of genetic transfer.

Gene flow occurs when the allele - one member of a pair or series of genes that occupy a specific position on a specific chromosome - is transferred from one individual to another.

It has long been known that geographic isolation can have significant effects on the genetic structure of populations. Previous research has indicated that a population may be physically separated when its original habitat becomes divided by a natural barrier such as river, shoreline, or mountain range.

These barriers can restrict or prevent gene flow and result in the genetic differentiation of isolated subpopulations, according to Gu. But she noted that the mating system, pollination biology and life history of a species can also have a critical role in the genetic differentiation of its populations.

What surprised Gu and her colleagues in their study is the genetic variation among the plant species caused by the barrier to gene flow developed so quickly - about 600 years after the Juyongguan was built - in places divided by a wall only 5.8 metres wide.

"Previous research suggests that the impact of blocking gene flow becomes evident over a much longer period of time and in distant places," Gu said. "We are really a bit surprised that it could happen on both sides of a stone wall."

Seeds of an idea

Peter Raven, a biologist from the Missouri Botanical Gardens of the United States, said the differences "show how rapidly changes in (plant) populations can occur."

In fact, it was Raven who inspired the research.

Gu said she was attending an international symposium on biology three years ago when she met with Raven, who came up with the idea during a conversation with Gu.

It was such a good idea that, once proposed, it had to be explored, Gu said.

Gu's team collected the plant samples at the Juyongguan Great Wall in September 1999 and April 2000.

The Juyongguan Great Wall, some 70 kilometres north of Beijing, was originally built in the Ming Dynasty (1368-1644).

Gu chose the place because it is off the beaten track and was subject to less human interference than other sites like Badaling.

Six species of plants were collected from both sides of the Great Wall at two sites and five species were collected from along a mountain path about 2 kilometres away from the control sites. The Great Wall is 6 metres high and 5.8 metres wide on average at the sampling sites, and the mountain path is approximately 1.5 metres wide.

Genetic analysis of the 416 individual plants collected showed that plants inside the Great Wall are genetically more diverse than their counterparts on the other side.

"The same plant species on either side of the Great Wall are significantly different in genetic terms," Gu said.

The differentiation between the same species growing on the two sides of the Juyongguan Great Wall was also significantly greater than that of the control site - the mountain path area, indicating gene flow between the plant populations separated by the Great Wall was disrupted to a certain extent.

"If the gene flow, either through pollens or seeds, or human activities, occurred without interruption, the plants would become similar over time, as they do in other areas," Gu added.

The team also found that wind-pollinated species were more genetically similar than insect-pollinated plants.

On the same side of the wall, insect-pollinated plants such as the woody shrub Vitex negundo are genetically more varied than wind-pollinated plants, such as the Siberian elm (Ulmus pumila), the study found.

The result indicates that gene flow between the wind-pollinated plant populations separated by the Great Wall is more similar than that for insect-pollinated plants.

"The spring wind in Beijing is so strong that the Great Wall is not so great a barrier," Gu said.

(People’s Daily May 28, 2003)