Evolution is usually thought of as occurring over long time periods, but it also can happen quickly. Consider a tiny fish whose transformation after the 1964 Alaskan earthquake was uncovered by University of Oregon scientists and their University of Alaska collaborators.
The fish, seawater-native threespine stickleback, in just decades experienced changes in both their genes and visible external traits such as eyes, shape, color, bone size and body armor when they adapted to survive in fresh water. The earthquake -- 9.2 on the Richter scale and second highest ever recorded -- caused geological uplift that captured marine fish in newly formed freshwater ponds on islands in Prince William Sound and the Gulf of Alaska south of Anchorage.
The findings -- detailed in a paper available online in the Proceedings of the National Academy of Sciences -- are important for understanding the impacts of sudden environmental change on organisms in nature, says UO biologist William Cresko, whose lab led the National Science Foundation-funded research.
Though present in more than 6,000 living species of fish, the adipose fin, a small appendage that lies between the dorsal fin and tail, has no clear function and is thought to be vestigial. However, a new study analyzing their origins finds that these fins arose repeatedly and independently in multiple species. In addition, adipose fins appear to have repeatedly and independently evolved a skeleton, offering a glimpse into how new tissue types and structural complexity evolve in vertebrate appendages.
Adipose fins therefore represent a unique example of convergent evolution and new model for exploring the evolution of vertebrate limbs and appendages, report scientists from the University of Chicago in the Proceedings of the Royal Society B on March 5.
"Vertebrates in general have conserved body plans, and new appendages, whether fins or limbs, evolve rarely," said senior author Michael Coates, PhD, chair of the Committee on Evolutionary Biology at the University of Chicago. "Here, we have a natural experiment re-run repeatedly, providing a superb new system in which to explore novelty and change."
Usually small and structurally simple, adipose fins tend to get attention only when they are clipped from farm-raised trout and salmon as a tag. Despite their presence in thousands of fish species, they have been dismissed as a remnant of a once-functional fin. This assumption puzzled Coates and co-authors, as they saw no evidence of deterioration in adipose fin structure or function in the fossil record.
To study the evolutionary origins of this fin, Coates and lead author Thomas Stewart, graduate student in organismal biology and anatomy at the University of Chicago, turned to a technique known as ancestral-state reconstruction. With co-author W. Leo Smith, PhD, from the Biodiversity Institute at the University of Kansas, they created an evolutionary tree describing the relationships between fish with and without adipose fins, using genetic information from more than 200 ray-finned fish and fossil data from known time points. They then used statistical models to predict when and in what species the adipose fin might have first evolved.
They found that adipose fins originated multiple times, independently, in catfish and other groups of ray-finned fishes—a striking example of convergent evolution over a vast range of species.
"It's pretty incredible that a structure which is incredibly common could be so misunderstood," Stewart said. "Our finding, that adipose fins have evolved repeatedly, shows that this structure, long assumed to be more-or-less useless, might be very important to some fishes. It's exciting because it opens up new questions."
