The results of David’s experiment showed not only that body size in fish can evolve in response to harvesting — but also that it can evolve rapidly. The change that David observed in fish populations happened over just four generations and four years. Furthermore, in those four years, the fish evolved a lot! At the end of the experiment, the mass of the fish harvested from the populations where only the smallest fish were removed was almost twice as large as that of the fish from the population where the biggest fish were removed.

If the same sort of evolution is going on in wild populations (as it seems to be), it’s bad news for both the fishermen and the fish. From the fisherman’s perspective, it means that his or her catch (the mass of fish harvested) is going to decrease. The fisherman may be able to remove the largest fish from the sea, but over time, the population will evolve such that even the largest fish will be smaller and smaller. And from the fish’s perspective, evolving smaller body sizes has some pretty drastic side effects…