Universiteit Leiden

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Cities have a direct influence on evolution

A global biological study has provided the most direct evidence to date that humans, and specifically cities, are the drivers of evolutionary change on Earth. Leiden University, Naturalis and the Municipality of Leiden worked on and helped fund the study.

What is the influence of cities on evolution? That is the question that the Global Urban Evolution Project (GLUE), an international research project by evolutionary biologists, hopes to answer. The white clover, an inconspicuous plant that grows in almost every city in the world, has now provided them with an answer. By studying white clovers from 160 cities and nearby rural areas in all kinds of climates, the researchers have reached a remarkable conclusion: the white clover in most of the urban areas studied has developed different genetic traits from its rural counterpart.

Less poison secreted

The researchers saw this by looking at the amount of hydrocyanic acid (hydrogen cyanide) secreted by urban and rural white clovers. Hydrocyanic acid is a poison used by the white clover as a defence against herbivores such as caterpillars. The clover is also more resistant to moisture deficiency thanks to this poison secretion. The urban white clover, it now appears, secretes less hydrocyanic acid. The difference in the secretion of hydrocyanic acid has been demonstrated down to a genetic level. The researchers say it is mainly because cities have fewer herbivores and are less likely to suffer from drought. The white clover in cities has adapted to its environment. This demonstrates the influence of cities – and therefore of humans – on evolution. GLUE has published its findings in the leading scientific journal Science.

‘Based on the knowledge we have now acquired, we can develop strategies to conserve rare species and allow them to adapt to urban environments.’

Astonishing response

Not only are the conclusions of the study remarkable, but so too is the method. To collect the white clover samples, GLUE enlisted the help of scientists around the world. The response was astonishing, with over 280 researchers from 160 cities in 26 countries answering the call. Researchers Menno Schilthuizen and Iva Njunjić from Naturalis and Leiden University also provided samples of white clovers from Leiden and the surrounding area. Schilthuizen and Njunjić sampled 800 Leiden clover plants along a straight eight-kilometre line from the city centre to the countryside. ‘It was a really fun project. Because we had drawn the line more or less at randomly on the map, we ended up in parts of the city we had never been to before,’ said Schilthuizen. Apart from providing samples, Leiden also contributed financially to the GLUE research via Evoscope, a project by Leiden University, Naturalis and the municipality of Leiden.

Field research on an unprecedented scale

‘There has never been field research into evolution on this scale before,’ said Marc Johnson, one of the GLUE project leaders. He also called the study a model for inclusive science. This is research in which all groups in society participate.

Protect species, ward off diseases

GLUE sees the results of the study as an important basis for further research. The knowledge acquired will enable us to develop strategies to conserve rare species and help them adapt to urban environments. It will also improve our understanding of how to prevent diseases and pests from adapting to environments where people live.

Text: Jan Joost Aten

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