New research published in the journal Science reveals what scientists previously suspected – that commercial harvesting of fish can cause rapid evolutionary change in those fish stocks.
“Most people think of evolution as a very slow process that unfolds over millennial time scales, but evolution can, in fact, happen very quickly,” said lead author Nina Overgaard Therkildsen, in a news release issued by Cornell University, where Therkildsen is an assistant professor in the Department of Natural Resources.
The research reported in the Aug. 2 issue of the journal was based on a pioneering experiment published in 2002 by David Conover, now the UO’s vice president for research and innovation, and Stephan Munch, now with the National Oceanic and Atmospheric Administration. Both are co-authors on the new paper.
In the new research, researchers identified genetic changes at the molecular level that cause rapid fish evolution brought about by intense harvesting.
Conover made the critical decision nearly two decades ago to freeze thousands of fish from that experiment, which also was published in Science. Those preserved fish became the subjects of the new study, which involved DNA sequencing of the full genome of 900 fish from frozen and archived populations.
In the 2002 paper, Conover, also a professor in the UO’s Department of Biology, and Munch examined what happened to six captive populations of Atlantic silverside fish subjected to harvesting. By selectively harvesting the largest fish in some populations, as most fisheries do, the fish evolved to become smaller and less productive over multiple generations.
The paper created controversy. It challenged previously held beliefs on fisheries management and led to an explosion of interest in fisheries-induced evolution. It also raised the question of whether humans need to consider the evolutionary consequences of harvesting wild fish or other resources.
While the initial research revealed that genetic changes had occurred, the technology to analyze how the genes themselves changed at the level of DNA, however, was not then fully developed.
“I always knew that we were going to look at the changes in DNA of these fish, but at the time, the techniques for doing genetic sequencing and for really understanding the genetic architecture were just beginning to emerge and were extremely expensive,” Conover said. “In the end, we decided to freeze everything because that seemed to be the type of preservation that would most likely enable a study to take place in the future.”
Conover froze thousands of the tiny Atlantic silverside fish, which grow no larger than 6 inches long, in a pair of large freezers set up in his lab at Stony Brook University where he worked prior to coming to the UO in 2016. Meanwhile, he waited for the technology to arise and the right collaborator to emerge.
That person turned out to be Stephen Palumbi, a professor of marine sciences at Stanford University who had an interest in conservation genetics. Palumbi put together a research team that included Therkildsen, then a postdoctoral researcher in his lab.
Therkildsen identified hundreds of different genes across the genome that changed consistently between populations selected for fast and slow growth. The team also observed large linked blocks of genes that changed in concert, dramatically shifting the frequencies of hundreds of genes all at the same time.
Surprisingly, however, these large shifts only happened in some of the populations, which meant that there were multiple genomic pathways for the fish in this experiment to get either larger or smaller.
“Some of these changes are easier to reverse than others, so to predict the impacts of fisheries-induced evolution, it is not enough to track growth rates alone, we need to monitor changes at the genomic level,” Therkildsen said.
The research, he added, has potential to help assess human impacts and improve humanity’s understanding of the speed, consequences and reversibility of complex adaptations as scientists continue to sculpt the evolutionary trajectories of various species.
When asked about his decision to put his research on ice in order for it to be brought back to life years later, Conover said he was only doing what researchers have done for generations.
“There are lots of archived specimens sitting in museums or in people’s laboratories,” he said. “This study illustrates why we need museums and other repositories of archived material. Freezing as a method of preservation is probably not sustainable over the long term, but it was the right thing to do for us because it maximized the value of these specimens and allowed us to do this kind of work.”
In addition to Conover, Palumbi and Munch, contributors to the paper were former Cornell postdoctoral researcher Aryn P. Wilder, now a researcher at the San Diego Zoo Institute for Conservation Research, and Hannes Baumann of the University of Connecticut.
The work was funded by the National Science Foundation.