Perspective in Science

Threespine stickleback, the protagonists of the research article that we wrote a perspective piece for,
are a model for understanding how repeatable evolution is. This is because populations of these little fish have independently evolved to live in freshwater lakes and streams all over the Northern Hemisphere.

Repeated adaptations to life in freshwater range from anatomical features like losing armoured anti-predator defences, to growing and reproducing faster, to digestive enzymes that compensate for the dearth of fatty acids in freshwater diets.

Life in freshwater does not just involve new predators and new food sources, but also new parasites. Below is an illustration (not to scale) of the life cycle of some of the key tapeworm parasites than infect freshwater stickleback. Eggs and larvae are eaten by little crustaceans such as copepods, as depicted on the bottom of the figure. Stickleback, such as this male guarding his nest, become infected when they eat copepods full of tapeworms. Birds, such as loons and mergansers, distribute tapeworm eggs across different watersheds when they eat infected stickleback and poo out tapeworm eggs. As a result, while the stickleback in each watershed are on isolated evolutionary trajectories, they are constantly coevolving against parasites like tapeworms, with a much more global distribution. This system provides a perfect natural experiment for understanding how host immune systems evolve different ways to combat a single parasite.