The climate is always changing, say some skeptics who downplay the urgency and impact of anthropogenic global warming. What they miss is that natural climate change occurs over a much broader timeline, which typically allows species to evolve and adapt to their new environment. The only exceptions are during mass extinctions, and some scientists not only believe that today's climate change is unnatural, but that it's representative of a new mass extinction, the sixth one since complex life appeared on Earth.
Case in point, a new study performed the largest artificial evolution experiment on warming tolerance, showing that the rate of global warming is faster than the rate of evolution in zebrafish. For other slow-breeding animals, global warming may actually be even faster than their innate capacity to adapt.
"The question of evolutionary rescue to climate change impacts is extremely important but very difficult to answer, and we realised that a large artificial selection experiment would be a good start to attempt to get those answers. We managed to see evolution of higher thermal tolerance in fish in the lab, which is amazing in itself. Unfortunately, the rate of evolution we measured is slower than what would have been needed for successful adaptation to climate change," Fredrik Jutfelt, associate professor at the Norwegian University of Science and Technology's (NTNU) Department of Biology, told ZME Science.
Left behind to perish in a world that changes too fast
In order to investigate how zebrafish (Danio rerio) cope and adapt to rising water temperature, Jutfelt and colleagues contrived a huge experiment involving more than 20,000 individuals bred over six generations. The original fish were caught in the wild and then bred by the researchers based on their ability to withstand water of increasing temperature.
The experiments last for three years, during which the researchers were most concerned with preserving the health of fish. Any infection or system failure of the aquarium would have been catastrophic to the study, but luckily the research was completed without any such problems.
The work was tedious nevertheless, as the researchers had to follow thousands of individuals and measure their thermal tolerance. Only those who were in the top third performers were allowed to reproduce the next generation.
"Using this method over generations we can measure the response to selection and the rate of evolutionary change. Simple in theory but requires a lot of work when dealing with larger organisms," Jutfelt said.
According to the results, this artificial evolution increased heat tolerance in the fish by just 0.04 °C per generation. For many species, especially those with long breeding cycles, this is much slower than the rate of warming experienced in their environment.
"Generation time has a huge impact on the evolutionary potential. Some tiny animals have generation times of days or weeks, which means they have the potential to rapidly adapt to environmental changes. Greenland sharks, for example, become reproductive at around 150 years old, so evolution takes millennia. Our study suggests that even fishes with short generation times such as zebrafish will struggle to adapt rapidly enough as the waters warm," Jutfelt said.
The vast majority of global warming (over 90%) is absorbed by the oceans, which act as planetary heat sinks. As a result, the world's oceans were 0.77 °C warmer in 2019 compared to the average for the 20th century -- and this warming has been accelerating. A study published in the journal Advances in Atmospheric Sciences early this year showed that the rate of warming in the oceans has increased by 450% in the last three decades compared to the preceding three decades.
In response, marine species have been forced to change their habits and breeding patterns, as well as to migrate towards the poles, much faster than land animals. Above a threshold, there will simply be no place to hide, and many species of fish and other marine wildlife will perish.
Scientists have known all along that species cannot possibly keep up with the accelerating ocean temperatures, but this new study has finally quantified just how much the evolution of marine species may be lagging behind. Although zebrafish are freshwater fish, marine fish with similar breeding cycles may respond similarly.
In the future, Jutfelt and colleagues plan on investigating the mechanisms, both physiological and genetic, that allowed the zebrafish to adapt to warming.
"Many have hoped that animals can adapt to warming, to change in concert with the increasing temperatures. We show that such evolution is possible but may only be rapid enough in species with short generation times. Many questions about generalisability of this first study still remain so we continue to study adaptation to warming in different species and contexts," he concluded.