David discovered that selecting for smaller bodied fish changes more than just the fishes’ body sizes; lots of other traits “come along for the ride.” At the end of his experiment, the small-bodied fish population had smaller body sizes, as predicted, but they also grew slower, produced fewer and smaller eggs, and had fewer vertebra. These other traits are known as correlated traits — they are correlated with body size: big fish tend to grow fast, produce many big eggs, and have more vertebra. The opposite is true of small fish. So when David selected his fish based on body size, he ended up causing a variety of other characteristics to evolve as well.

And those other traits are sometimes unhealthy for the fish population. Laying fewer eggs, for example, means that the population has more trouble bouncing back after being over-fished.

Of course, these results are not what fisheries managers had in mind by requiring fishermen to “throw the little ones back.” That rule was intended to allow the little fish to reach spawning age and to reduce the competition they faced, giving them more resources to grow and reproduce. The approach sounds reasonable and yet it doesn’t seem to work. Why not? As David explains, such rules account only for ecology and ignore evolution: “We are getting the opposite effect from what we would have predicted based on ecology alone. That happens because we have selected for fish that grow slow, that eat less food, that don’t use that food very efficiently, that produce fewer eggs, the eggs they produce were smaller in size, which means that the larvae they produced were smaller when they hatched from the egg — and we changed the behavior so that those fish were more hesitant to feed in the presence of predator — they were more shy about feeding. All of those characteristics reduce productivity — and we can actually drive a population into lower yields instead of higher.” So because they ignore an evolutionary perspective, such regulations may do more harm than good.