Brain Worm
This is an imaginary example; but it looks fairly realistic, in that it can show a kind of parasite with a certain life cycle can be subject to group selection, and altruistic behavior can evolve in this species.
The brain worm (represented by red) is a mutant of this parasite; it behaves as an altruist (i.e. acts so as to increase the fitness of other members but to decrease its own fitness). At the earliest stage of its life cycle, it reproduces in the same way as the other members. But when it gets into an ant, it affects the nerve of the host and controls the host's behavior (the host camps out on grass blades); but this mutant parasite dies without offspring. How can such an altruistic behavior evolve?
Now, here is how the altruistic mutant behaves; among the ten mutants in the mixed group, one and only one becomes the brain worm, and other members (including 9 other mutants) become "free riders". Thus the brain worm increases the group fitness at the cost of its own life.
Back to Evolution of Altruism
Last modified Oct. 4, 2005. Soshichi Uchii
