The evolutionary response of a fly species to global warming has accelerated at an unprecedented rate

Although this is a good thing in principle, the phenomenon raises fears that the pace of global warming is exhausting the evolutionary capacity for adaptation in many species.

Researchers from the Autonomous University of Barcelona (UAB) have observed how rising temperatures and heat waves over the past two decades have accelerated the presence of genetic variation that increases tolerance to high temperatures in populations of a species of fly very common in European forests, Drosophila subobscura. The evolutionary response has occurred through an increase in the relative frequency of genetic variations that already existed in individuals of the species, not through new variations, which implies a rate of warming that may have put species with less genetic variability at risk.

Research into evolutionary responses to global warming is a laborious task. Studies are scarce and date back to the end of the last century, but they already showed that these responses were occurring. In order to update the monitoring of evolutionary responses in a context in which global warming has continued to worsen, researchers from the Department of Genetics and Microbiology at the UAB, Francisco Rodríguez-Trelles Astruga and Rosa Tarrío Fernández, have studied the genetic variations of the fly species Drosophila subobscura, much smaller than the house fly and common in the forests of Europe.

In this species, adaptation to the climate occurs through a type of genetic variation known as chromosomal inversion polymorphism. Certain inversions (a type of mutation that changes the orientation of genome segments) confer greater tolerance to cold on their carriers, while others confer greater tolerance to heat. The results obtained by the UAB researchers corroborate those obtained in previous studies, which indicated that the proportion of inversions that favour heat tolerance was increasing and that of those that favour cold tolerance was decreasing. The study shows for the first time how this pattern has accelerated over the last two decades in temperate Europe with respect to the Mediterranean at an unprecedented rate, following the incidence of heat waves that are increasingly longer and more intense.

“The study began in 2015 and involved laborious field and laboratory work. Over the course of four years, we captured samples of Drosophila subobscura in 12 European locations at different latitudes. Local residents told us about their experience as witnesses of heat waves they had never experienced before. Each sample was then genetically characterized upon arrival at our UAB laboratory,” explains Francisco Rodríguez-Trelles. The researchers took samples in Vienna (Austria), Leuven (Belgium), Lagrasse, Montpellier and Villars (France), Tübingen (Germany), Groningen (Netherlands), Leuk (Switzerland), Málaga, Punta Umbría, Riba-roja de Túria and Queralbs (Spain).

Projections indicate that, unless further efforts are made to halt global warming, current Central European populations of this species will become genetically indistinguishable from those in Southern Europe by around the mid-2050s.

For Professor Francisco Rodríguez-Telles, who coordinates the PhD programme in Genetics at the UAB, “this is something unprecedented, since it is a model species that is used in textbooks as an example of how genetic variability confers adaptation to climate at different latitudes.” In the Drosophila subobscura specimens, researchers have not detected any new inversions that confer greater adaptation to heat than the inversions already in place, which means that the rate of appearance of new inversions is too slow to keep pace with the increase in temperature. This prospect is particularly alarming if one takes into account that a large number of insect species are less well equipped to adapt evolutionarily to global warming than Drosophila subobscura.

Drosophila flies, a class of flies widely used in scientific research. (Photo: NASA Ames Research Center / Dominic Hart)

Plasticity and evolution, mechanisms of adaptation to increasing temperature

According to scientific forecasts, man-made global warming continues to worsen and its adverse consequences are accumulating. There are two basic mechanisms of adaptation to rising temperatures: plasticity and evolution. The first operates at the level of the individual, while the second operates at the level of populations and species. Examples of plasticity are physiological acclimatization in the same place or the change from one place to another with a more tolerable temperature. When the increase in temperature exceeds the acclimatization capacity of some individuals and thermal refuges are not found, the survival of the species depends on the genetically most thermotolerant individuals and evolution is triggered, which is the response mechanism analyzed in this study.

“The observation of evolutionary responses to global warming is both good and bad news,” explains Rodríguez-Trelles. “Good news, because it means that there are genetic variants that are tolerant to thermal stress. Bad news, because it means mortality for the unfortunate carriers of genetic variants that are insufficiently thermotolerant. Furthermore, if warming is too fast and continues for too long, even the most thermotolerant individuals can succumb, leading to the extinction of the species,” concludes the UAB researcher.

The study is titled “Drosophila subobscura evolutionary response to global warming in Europe.” It has been published in the academic journal Nature Climate Change. (Source: UAB)