Imagine a simple ecosystem with mice that feed on the beetles. As the mouse population grows, any beetles that develop better defences - a thicker carapace, perhaps - will have a much better chance of escaping being eaten. Result: the proportion of beetles with thick exoskeletons will rapidly increase. That is, the beetles will evolve thick shells.
Now suppose a few mice have stronger jaws and teeth that enable them to eat the thick-shelled beetles. As the proportion of beetles with thick shells increases, the strong-jawed mice will thrive and they will come to dominate the mouse population. Now the beetles are back where they started.
So far this is classic co-evolution, an arms race in which two species have to evolve rapidly just to keep pace with each other. But what happens next?
Growing thick carapaces is a costly strategy. When thick exoskeletons no longer offer better protection, beetles with thin shells will grow faster and produce more offspring. So evolution will go into reverse, with the beetles evolving thin shells again. That mean mice with strong jaws will no longer have an advantage, and their evolution will go into reverse.
That means, as you've guessed, that the mice and beetles are right back where they started.
And so things could go on, with the flip-flopping of the selective pressures on the populations - called fluctuating selection - resulting in evolution constantly reversing direction.
It was first demonstrated to be possible in 2003 by lab experiments involving algae and an algal predator known as a rotifer. Now a paper in Science details the same phenomenon occurring in soil bacteria and the viruses that infect them...
(More on this soon)
