Back and forth, endlessly

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)

It's happening

“Darwin was rather inclined to exaggerate the necessary slowness of the action of natural selection; but with the knowledge we now possess of the great amount and range of individual variation, there seems no difficulty in an amount of change, quite equivalent to that which usually distinguishes allied species, sometimes taking place in less than a century, should any rapid change in conditions necessitate an equally rapid adaptation.”

These words could easily have been written sometime in the past decade. In fact, they were written in 1889 by Alfred Russell Wallace, the biologist who independently came up with the idea of evolution by natural selection. And how right he is turning out to be.

Of course, like anyone with an interest in evolution, I've long been aware that natural selection can act fast. The classic example is the peppered moth, but many more have emerged over the years.


I've been keeping track of the latest studies for the past couple of years, with a view to writing a feature. I decided from the outset to exclude any examples of rapid evolution involving viruses and bacteria, partly because there are countless examples and partly because people without a background in biology find it hard to relate to this kind of stuff.


Even excluding viruses and bacteria, when I began looking into the topic, I was really amazed by just how many examples of rapid evolution have now been documented. In fact, a growing number of biologists now think that far from being the exception, rapid evolution is the norm.


As a result, my feature has turned into two, the first of which is out today. Even so, there is lots of fascinating stuff I haven't been able to fit in but would like to write about if I find the time. Hence this blog.