Download STICKLEBACK EVOLUTION

STICKLEBACK EVOLUTION
Three-spine sticklebacks are small fish that live in oceans, streams, and lakes across the northern hemisphere.
Sticklebacks that live in freshwater lakes often look quite different from their ocean-dwelling cousins. One difference
is the amount of protective armor that covers their bodies: while oceanic sticklebacks have about 30 armor plates
extending from head to tail, most freshwater sticklebacks have just a handful of plates that sit closer to the front of the
body.
Dr. David Kingsley describes the stickleback lifestyle and how it has led to the evolution of different body forms in a variety of habitats.
A fully armored stickleback from the ocean near Loberg Lake (top), and a low-armored stickleback taken from Loberg Lake in 1994
(bottom). The fish have been stained with a dye called Alizarin Red S, which stains bones, in order to highlight their differences.
Photos courtesy Michael A. Bell, Professor of Ecology and Evolution, Stony Brook University.
Less Armor is an Advantage in Freshwater Environments
Low-armored forms of sticklebacks evolve in freshwater environments again and again. Given how quickly these
shifts occur, the freshwater environment is most likely selecting for low-armored gene variants that are already
present at a low frequency in ocean populations. When a group of fish moves from the ocean to fresh water, the lowarmored variants survive and reproduce at a higher rate than the fully armored individuals. Here's why:
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Predation by saltwater fish favors more armor.
Predation by insect larvae that live in fresh water favors faster-moving fish with less armor.
Low-armor forms grow faster, making them (1) too big for predators, (2) reach sexual maturity more
quickly, and (3) able to store more energy reserves, which increases their chance of overwinter
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Studying Sticklebacks to Understand Evolution
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Misconception:
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Evolution may have happened in the past, but it's no longer happening
today.
Reality:
Scientists observe evolution in progress today.
Sticklebacks inhabit hundreds if not thousands of lakes and streams throughout the northern
hemisphere. In addition to changes in body armor, these isolated stickleback populations have evolved a
variety of changes that set them apart from their ocean-dwelling ancestors. In order to figure out what
genes are driving these differences, researchers crossbreed marine and freshwater sticklebacks. By
studying the resulting hybrid offspring, geneticists can see what specific changes to which genes are
causing the differences. In sticklebacks, as in other organisms, small changes to single genes can have
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major effects.
The Ectodysplasin gene appears to be responsible for changes in body armor in many freshwater
stickleback populations. Recessive low-armored gene variations are found in about 1% of marine
sticklebacks. Evidence suggests that it is this variant that is repeatedly being selected for in freshwater
environments.