Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [50]
E. coli meets these spiteful standards. It competes in the crowded confines of the intestines for a limited supply of sugar. An individual microbe sacrifices its own reproductive future by committing suicide, but its colicins destroy many competitors, allowing the microbe’s own close relatives to thrive. As with persistence, becoming a colicin maker is a matter of chance. The noisy production of proteins determines which few individuals will respond to starvation by switching on their colicin-producing genes. The burden is shared by all.
Spite, some experiments now suggest, may also drive E. coli to become more diverse. Margaret Riley, a biologist at the University of Massachusetts, Amherst, and her colleagues have observed the evolution of this arms race in experiments on E. coli in both petri dishes and the guts of lab mice. Once in a rare while, an antidote gene may mutate into a more powerful form. Instead of just defending E. coli against its own colicin, it can also defend against the colicins made by other strains. This mutation gives a microbe an evolutionary edge, because it can survive enemy attacks that kill other members of its strain.
This powerful antidote opens the way for another advance. A second mutation strikes the colicin-producing gene, causing it to make a new colicin. Its relatives, which still carry an antidote for the old colicin, are killed off by the mutant toxin. But thanks to its powerful antidote, the microbe that makes the new colicin can survive while its relatives die. Its spite becomes intimate.
The emergence of new colicins drives the evolution of new antidotes in other strains. Likewise, new antidotes drive the evolution of new colicins. But E. coli has to pay a price for this weaponry. It has to use energy to make colicins and antidotes, which are particularly big as bacterial molecules go. A new colicin may be even deadlier than its predecessor, but it may also become a drain on a microbe. If a mutation leaves a microbe unable to make colicins—but still able to resist them—it may be able to channel the extra energy into reproducing. A colicin-free strain will spread, outcompeting the colicin makers.
If colicin makers are driven to extinction, their colicins no longer pose a threat to neighboring bacteria. Now antidotes become a waste of effort, since there is nothing for them to protect E. coli against. Natural selection can begin to favor pacifists—microbes that make neither colicins nor antidotes. Once the pacifists come to dominate the population, colicin producers can invade the population once more, killing off the vulnerable strains and getting the food for themselves. And so the journey comes full circle.
These sorts of cycles emerge spontaneously from evolution. You can think of them as games in which players use different kinds of strategies to compete with other players. In the case of E. coli, a strategy might be to make a particular colicin or to do without colicins and antidotes altogether. In the case of a male elephant seal, strategies might include fighting with other males for the opportunity to mate with females or sneaking off with a female without the big male on the beach noticing. In some cases, one strategy may prove superior to all the others. In other cases, two strategies may coexist. Fighting males and sneaker males can coexist in many species, for example. In still other cases, the success of strategies goes up and down over time.
Scientists sometimes call this cycling evolution a rock-scissors-paper game. In the game, each player can make a fist for a rock, extend two fingers for scissors, or hold the hand flat for paper. A player wins or loses depending on what the other players do. Rock beats scissors, but scissors beats paper, and paper beats rock. If a population of organisms is dominated by one strategy—call it paper—then natural selection will favor scissors. But once scissors takes over, rock is favored, then paper, and so on.
The common side-blotched lizard of coastal California plays a colorful version of rock-scissors-paper. The male lizards