LETTERS TO THE EDITORS
On With the Game
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
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.