Using the example of white clover, researchers have shown for the first time on a global scale how life in an urban environment can change living beings. The widespread plant shows a similar pattern of genetic adaptation in many cities around the world: in the urban environment, the clover often does not invest in a repellent against predators. This example illustrates the complex way in which man-made environmental changes can change the characteristics of living beings.
Where once pristine meadows, forests and waters shaped the earth, man has created drastically new living conditions: Large parts of the world have been transformed into cultural landscapes to varying degrees. The city represents the most extreme form. As is well known, many original animal and plant species cannot cope with the very special conditions in the environment characterized by concrete and asphalt. However, certain species found opportunities to exist in niches. In some cases, it is already known that urban life has led to adaptive evolution in these species: there is evidence of genetically determined trait changes in urban populations.
It is a process based on selection: if a trait, such as color or the formation of certain substances in body tissues, is advantageous in an urban environment, a specimen equipped in this way can survive better and provide more offspring. Over generations, the trait can then establish itself more and more firmly in the genome of the urban population of a species. So far, however, such effects have only been documented regionally in certain cities. The international team of scientists from the research project “Global Urban Evolution” (GLUE) is now investigating this effect for the first time on a broader scale.
Stadt-Klee is different
As a model organism, they chose a plant that is distributed all over the world and can be found in both rural and urban areas: white clover (Trifolium repens). The focus of their current study was the ability of this plant to form hydrocyanic acid (hydrogen cyanide). The production of this substance is based on a genetic basis, the inheritance of which can be easily traced. The main purpose of hydrocyanic acid is to deter predators such as snails, rabbits and the like. However, the production is associated with costs for the plant – the production of hydrocyanic acid requires resources that cannot be used for growth and seed yield. There are therefore also plants in the populations of white clover that, as it were, take the risk of forgoing production – their predisposition to the production of hydrocyanic acid is inactivated. It is obvious that when the importance of defense dwindles, plants with such a disposition increasingly have an advantage.
This trend appears to be common in urban clover populations around the world, according to the results of the GLUE researchers. The results are based on research data from 26 countries – including Germany. Scientists have recorded and analyzed the genetic characteristics of thousands of white clover plants in a total of 160 cities and their surrounding areas. As they report, their results uncovered a widespread pattern: Compared to rural stocks, the urban clover populations in many cases have particularly high proportions of plants that no longer produce hydrocyanic acid. A gradual effect also became apparent: the more pronounced the urban character of a living space, the less frequently the clover has the predisposition to produce the substance.
Model plant for urban evolution
In addition to other possible factors, the scientists attribute the effect primarily to the lower incidence of herbivores in urban areas: Where white clover grows in the city, it is therefore less threatened by snails, rabbits and the like than in a more rural environment. For the reproductive success of the urban clover, therefore, in many cases it seems to be cheaper to forego the expensive production of hydrocyanic acid than to maintain it. In this respect, the inner cities of Toronto, Tokyo, or Munich are comparable, and apparently this is how the widespread parallelism in this evolutionary development came about.
“This study is a model for understanding how humans are changing the evolution of life around us,” says co-author Rob Ness of the University of Toronto. The results clearly document how the city as a habitat can influence the course of evolution. This can be important for ecologists and evolutionary biologists, as well as for society,” says the scientist.
For GLUE, however, the current release is just the beginning, writes the University of Toronto. The extensive genetic data on white clover that the more than 280 participants in the project have collected can now serve as a basis for further investigations for a long time to come. “There has never been a field study of evolution on this scale, or a global study of how urbanization affects evolution,” says Marc Johnson University of Toronto, who is coordinating the project. “Almost everyone we asked to collaborate agreed – the researchers recognized the importance of this project and this is what made our global project possible,” said Johnson.
Source: University of Toronto, professional article: Science, doi: 10.1126/science.abk0989