To test his hypothesis that fish populations evolve smaller body sizes when they are harvested based on size, David set up an experiment in his lab.

He and colleague Stephan Munch set up enough aquaria to hold 6000 Atlantic silversides! The fish were divided up into six populations of 1000 each. They raised these fish populations through four generations — but with a twist. Each generation before spawning, the researchers removed some of the fish from each tank — simulating what would happen in the wild when fish are harvested. In Population A1 (shown below), they removed the 900 biggest fish from the tanks. In Population C1 (shown below), they removed the 900 smallest fish from the tanks. And in Population B1 (shown below), they randomly selected 900 fish and removed them from the tanks. As a safeguard against the possibility of an accident or illness affecting one of the populations and ruining the four-year-long experiment, they ran duplicate populations with the other 3000 fish: Population A2 was treated identically to Population A1, B2 was treated identically to B1, and C2 was treated identically to C1. Luckily, none of the fish populations had any unusual problems!

David’s lab, basically a small fish hatchery, requires lots of maintenance. Everyday, over the four years that it took to obtain the initial results from this experiment, either he or one of his students and colleagues spent the day taking care of the fish in the experiment: feeding them, cleaning tanks, and checking for sick fish. As David puts it, “I run my lab as a team, because all of the work we do really cannot be done by a single person working in isolation.”

At the beginning of the experiment, all the tanks had fish of about the same size. Can you predict what happened over the next four generations? Click on the button below to see the results.

Body size evolved exactly as David had expected, as shown in the graph below. Populations in which large fish were removed evolved smaller body sizes, and populations in which small fish were removed evolved larger body sizes. The body size of random-harvested populations did not change.