Scientists from the USA and Japan have shown that evolution can occur very rapidly in reverse in some small fishes in a study published in the most recent issue of the journal Current Biology.
Studying threespine sticklebacks (Gasterosteus aculeatus) in Lake Washington, the largest of three major lakes in the Seattle area, Jun Kitano and co-authors show that the degree of plating in the fish has changed dramatically over the last 40 years. The authors examined sticklebacks collected from the lake over four periods in time: from 1957, 1968"69, 1976 and 2005.
They write hereas ancestral marine sticklebacks typically have a continuous row of lateral plates (completely plated morph), freshwater sticklebacks usually have a reduction in lateral plates resulting in a gap in the middle part of the plate row (partially plated morph) or a loss of both the middle and posterior plates (low-plated morph).
The authors analysed the fishes for the frequencies at which low-plated, partially-plated or completely-plated morphs appear and found that only 6% of the fishes collected in 1968"69 were completely-plated, compared with 40% of the fish collected in 1976 and 49% of the fish collected in 2005.
The data demonstrate that the frequency of plate-morph phenotypes has changed dramatically in Lake Washington within the past 40 years, which is equivalent to 40 generations in this stickleback population.
The authors then calculated the strength of selection for the completely-plated morph and their result suggests that the complete morph had 58%"72% greater fitness than that of the low-plated morph during this period.
The authors offer an explanation for the rapid reverse evolution of the stickleback: One of the dramatic ecological changes that occurred in Lake Washington during the early 1970s is increased water transparency as a result of the mitigation of eutrophication in the late 1960s Previous behavioral experiments have demonstrated that an increase in water transparency significantly increases the reaction distance of visual predators to their prey, thus leading to increased predation pressure on prey fish.
"Cutthroat trout (Oncorhynchus clarki) are visual predators, extremely sensitive to subtle changes in water transparency, and are the primary predators of threespine sticklebacks in both the littoral and pelagic zones of Lake Washington Predation by toothed predators, such as cutthroat trout, is thought to favor completely plated sticklebacks because the posterior lateral plates can protect the stickleback from being injured and swallowed Consistent with this hypothesis, we have shown that the increase in the frequency of completely plated morphs occurred during the time when the water clarity increased dramatically in Lake Washington
For more information, see the paper: Kitano, J, DI Bolnick, DA Beauchamp, MM Mazur, S Mori, T Nakano and CL Peichel (2008) Reverse evolution of armor plates in the threespine stickleback. Current Biology 18, pp. 769"774.