Using decades-old frozen fish, researchers have discovered roots of rapid evolutionary adaptation to human activity in the Anthropocene. Similar traits - at least in the fish they studied - can arise through highly divergent genetic pathways, the researchers say. "Interestingly," write Christian Jørgensen and Katja Enberg in a related Perspective, "the genes that strongly increased in one population did not change in parallel populations subject to the same treatment, supporting the view that genetically speaking there are multiple ways to Rome." Mounting evidence shows that the myriad of human-induced environmental change creates strong selective pressures, leading to rapid, yet significant, evolutionary change in many species. For example, selective pressures set in motion by the activities of global fisheries have resulted in marked changes in growth rates and reproductive timing of many commercially fished species. Often, these changes occur within a handful of generations. However, the underlying genetics that makes this type of rapid adaptation possible remains poorly understood. And there are two schools of thought that purport to predict evolutionary change - one suggesting DNA in a species has come about through ages of careful evolutionary tinkering, and the other suggesting traits in a species today, based on its genetics, are influenced by the joint effects of maybe hundreds of genes. Nina Therkildsen and colleagues revisited a near twenty-year-old study, which simulated intensive fishing on thousands of small Atlantic silversides over several generations, and found pronounced evolutionary shifts in overall growth rate. Therkildsen et al. sequenced the genomes of the since-frozen experimental silversides and tracked genomic changes that correlated to changes in fish body size. According to the results, the observed rapid decline in body size occurred through two contrasting polygenic mechanisms. The authors observed many small parallel changes in hundreds of unlinked genes related to growth variation in the wild. Others, however, showed big shifts in large blocks of tightly linked genes, which can result in big changes at some genomic loci. In a related Perspective, Christian Jørgensen and Katja Enberg explain how this finding fits within the two schools of thought that purport to predict evolutionary change.
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