Catastrophic Earthquake Drove Fish Evolution In Just 50 Years

A marine Alaskan threespine stickleback fish. The bone is stained with alizarin red and imaged using a fluorescent microscope. Mark Currey/University of Oregon, Eugene
Janet Fang 14 Dec 2015, 21:38

Evolution doesn’t have to take thousands of years. We know that from artificial selection and observations with human-altered landscapes. But how rapidly can it happen in the wild? Researchers studying little fish called threespine sticklebacks in Alaska after a catastrophic earthquake have discovered evidence for evolution within the span of just 50 years. The findings are published in Proceedings of the National Academy of Sciences this week.

On March 27, 1964, the south coast of Alaska was struck by the largest earthquake ever recorded in North America. In just a few minutes, the 9.2-Richter-scale event uplifted islands in Prince William Sound and the Gulf of Alaska south of Anchorage, and freshwater ponds were created from formerly marine habitat, which spurred the diversification of the resident threespine stickleback fish, Gasterosteus aculeatus.

To see how much freshwater and marine populations have since diverged, University of Oregon’s William Cresko and colleagues analyzed variation in both the appearance (or phenotype) and the genetics of sticklebacks on three seismically affected islands: Middleton, Montague, and Danger. On Middleton Island, for example, an uplift of 3.4 meters (11 feet) created a new terrace with ponds from a previously submarine platform. 

They found that freshwater sticklebacks on uplifted islands have changed dramatically from their oceanic ancestors: In the past half century, they’ve become phenotypically and genetically distinct from marine sticklebacks. Since freshwater fish living in ponds that existed before the earthquake were genetically distinct from those living in ponds created by the earthquake, the post-earthquake populations weren’t derived from preexisting freshwater populations. Furthermore, the team identified five genetically distinct groups among freshwater populations – which implies multiple, independent colonization events by marine sticklebacks. 

But not only did these newish freshwater populations evolve repeatedly from their oceanic ancestors, they also differentiated nearly as much as populations that were founded thousands of years ago. After reconstructing the evolutionary relationships between the freshwater populations, the team discovered that the amount of change in earthquake-derived ponds is about the same as that of much older, mainland freshwater populations from Cook Inlet. 

Most of the evolution that occurred when oceanic sticklebacks invaded freshwater ponds took place within 50 generations – not thousands of years. But how so fast? 

"Part of what we’ve found is that there is enough movement of genetic material between ocean and freshwater populations (and vice versa) that the genetic material necessary for such rapid evolution already exists in the oceanic populations," Cresko explains to IFLScience. "This ‘standing genetic variation’ can be utilized very quickly during evolution in the novel freshwater habitats – much faster than if evolution needed to wait for new mutations." 

William Cresko checks his collection of threespine stickleback. Charlie Litchfield

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