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.
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 low-armored variants survive and reproduce at a higher rate than the fully armored individuals. Here's why:
- 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 survival.
Evolution may have happened in the past, but it's no longer happening today.
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 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.