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Compared to clearwater lakes (left), the light regime in blackwater lakes (right) is red-shifted.
Since the end of the last ice age about 12,000 years ago, and perhaps earlier, threespine sticklebacks (Gasterosteus aculeatus) have populated hundreds of freshwater lakes in Canada and evolved independently in each environment. In the lakes on the islands in the Haida Gwaii archipelago, off the coast of British Columbia, results of a 19-year study show that, in addition to other adaptations, the color vision of threespine sticklebacks has adjusted to the various light environments of their respective lakes.
Some lakes have clear water with full-spectrum light that shifts to blue light with increasing depth. Others, termed “blackwater” lakes, are so darkened by dissolved tannins from plant matter that the visual environment is perpetually almost nocturnal and red light dominates. Researchers led by evolutionary biologist David Marques of the University of Victoria, British Columbia, have been studying how the color vision genes of threespine sticklebacks have evolved. In a selection experiment and in multiple wild populations, they found that threespine sticklebacks have been able to adapt to a red-shifted environment by the mutation of a single amino acid on a blue light–sensitive visual pigment gene.
At the start of the 19-year selection experiment, 100 adult threespine sticklebacks were transplanted from the blackwater Mayer Lake—Haida Gwaii’s largest lake—into an uninhabited clearwater pond. At the end of the experiment, individuals were resampled. After about 13 generations in the clearwater environment, the frequency of the red-shifted mutation decreased and the frequency of the blue-shifted version of the gene increased significantly, indicating a reversal in the evolutionary process. Researchers were surprised by the speed of the reversed adaptation.
Even more surprising, as Marques and colleagues discovered, is that these color vision changes had independently happened before, around 200 million years ago. The researchers compared the sticklebacks’ genome to previously published results of retinal protein in bluefin killifish and black bream. Though evolutionarily divided from threespine sticklebacks for millions of years, these fish showed the same alterations, which have likewise allowed them to adapt to clearwater and blackwater habitats. “Evolution may often go the same way of solving the same issue,” says Marques, “even at very different time scales.” (PLOS Biology)