The Evolution of Spite

In a famous letter to Asa Gray on 22 May 1860, Darwin wrote:

I cannot persuade myself that a beneficent & omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars.

The Ichneumonidae are a family within the order Hymenoptera, famous for being parasitoids of other insects. As Wikipedia blandly puts it, a parasitoid “ultimately sterilises or kills, and sometimes consumes, the host. Thus parasitoids are similar to typical parasites except in the more dire prognosis for the host.” In the case of parisitoid wasps, females lay their eggs inside the bodies of other insects. Their larvae then develop, consuming the host from the inside. As Darwin notes, the host may be alive as this happens.

There is a recently discovered twist to the particular horrors of parisitoid wasps¹. In the wasp species Copidosoma floridanum, females are often not the first to deposit their eggs in a host. C. floridanum has evolved a remarkable strategy for this situation²:

Females lay their eggs on the eggs of moth caterpillars, after which the wasp eggs divide asexually and consume the growing caterpillar from the inside. Although most larvae develop normally, a fraction become a soldier morph. Developing as a soldier is costly to the actor, because the soldiers are sterile, and costly to the recipient, because the soldiers seek out and kill other larvae within the host. Finally, soldiers preferentially kill larvae to which they are less related (those that have developed from other eggs), freeing up resources for their clone-mates.

This is a perfect example of what has come to be called spite: behaviour that is costly both to the actor and the recipient. How can spite evolve? Hamilton's rule states that a trait will be favored by selection when \(rb–c>0\), where \(c\) is the fitness cost to the actor, \(b\) is the fitness benefit to the recipient, and \(r\) is their genetic relatedness. One way in which this expression can be positive is if \(r\) and \(b\) are both negative, and their product is greater than \(c\). Negative relatedness means that the recipient is less related to the actor than is an average member of the population. Soldier morphs propagate their genes by killing off larvae that don't contain them, thereby raising the chances that their relatives will survive and reproduce.

Darwin would have been pleased.