American Scientist

To the Editors:

The article "Cheating Viruses and Game Theory" (September-October) by Paul E. Turner was intriguing and enlightening to read. However, I noticed a hidden—yet very significant—weak point. In the last paragraph of the "Cheaters Sometimes Prosper" section, the author mentions an apparent conflict between the prediction of evolutionary game theory in the prisoner's dilemma and the theory of evolution. Cheaters can lower the average fitness of the population. On the other hand, paraphrasing the author's statement, Darwin's theory of evolution suggests that the population becomes better adapted to its environment over time.

As far as I know, evolutionary theory does not claim that populations are driven toward higher fitness. Evolutionary pressure acts at the level of individuals. As a result, the average fitness of the population usually increases, but it can decrease in some cases, such as when cheaters have an advantage over cooperators and can take over the population.

When resources are plentiful, the presence of the cheaters increases—maybe temporarily—the fitness of the population. When there are too many cheaters, the resources become scarce and the fitness of the population drops. The whole scenario does not contradict the theory of evolution, it supports it!

Dmitri E. Kourennyi
Case Western Reserve University
Cleveland, OH

Dr. Turner responds:

Darwin's theory of natural selection states that individuals in a population will vary, and that those individual variants that are best suited to exploit the environment will on average leave more offspring. All else being equal, the implicit assumption is that better performance of average individuals in the population will be reflected by an increase in the mean fitness of the population, a direct result of selection acting at the level of individuals. Thus, the article goes on to read "...which steers the population to become better adapted to its environment over time."

In my opinion, Dr. Kourennyi is misreading this sentence in at least two ways. First, his literal reading is that "steers" is equivalent to "guides," where the population is purposefully taken in the direction of increased fitness. As he correctly indicates, natural selection is blind and the process does not drive populations to increased fitness through time. As for Dr. Kourennyi's conclusion, I wrote that "[Prisoner's dilemma] is somewhat counter to Darwin's theory by natural selection." Selection can favor takeover by cheaters, leading to a surprising (in a Darwinian sense) decrease in mean fitness of the population.