The researchers sequenced the genome of the extinct whitefish species Coregonus gutturosus from historical scale samples, as well as historical and contemporary samples of the three surviving Lake Constance whitefish species. All four species have emerged since the last ice age and are endemic to Lake Constance. All across Switzerland, there were more than 30 endemic whitefish species in the large pre-alpine Swiss lakes. During the eutrophication period of the lakes in the last century, however, this diversity decreased dramatically. Around a third of the Alpine whitefish species went extinct within just a few decades. Species that had adapted to deep water, such as C. gutturosus, were most affected. Because of the anthropogenic eutrophication and the associated oxygen depletion, the populations became continuously smaller and, at the same time, successful reproduction was only possible at shallower depths, where co-occurring species usually spawned. In consequence, the species started to admix with co-occurring relatives. This process is called speciation reversal, because species that had once diverged genetically in consequence of adaptation to different ecological niches can lose their differentiation through genetic admixture induced by environmental change, such as anthropogenic lake eutrophication. Speciation reversal contributes to extinction, as the already reduced population can become completely replaced by hybrids and thus merges with the predominant species.