Most regulations on fishing require that only fish above a certain size are “keepers.” Smaller fish must be thrown back. So large fish are selected out of populations while small fish survive to reproduce. In this scenario, a fish with genes for smaller size would have a better chance of surviving and reproducing than a fish with genes for large size — because the large fish is more likely to wind up on someone’s dinner plate. David hypothesized that this form of artificial selection would cause fish populations to evolve smaller body sizes.

It works like this:

1. Imagine a population of fish with variation in body size. Some have genes for large size and some have genes for small size. This represents genetic variation in the population.

2. When the population is fished, many of the largest fish are removed, so more of the small-bodied fish (and their small-bodied genes) remain.

3. The population reproduces. Each individual passes their genes on to their offspring — however, since there are more small-bodied parents, there are also more small-bodied offspring.

4. In the next generation, the population has evolved: average body size in the population is smaller than it used to be and small-bodied genes are more common than they used to be.

5. This downward trend in body size will continue so long as the largest fish are harvested and there is genetic variation in the population.