XVI.4.2 Spiteful behavior is not very widespread in nature
It could be expected that spiteful behavioral patterns will become fixed in evolution in the same way as selfish behavioral patterns because, in sexually reproducing species, the spreading of a biological trait in the population is determined by how much it increases the effectiveness of spreading the allele responsible for this trait (compared to the other alleles of the same gene) and, in asexually reproducing organisms, it is decided by how much the trait heightens the individual fitness of the carrier of a particular allele (compared to the average carriers of other alleles in the population) (Hamilton 1970).At first sight, it seems that it does not matter whether the individual achieves an increase in its relative fitness by increasing its absolute fitness or by lowering the absolute fitness of its competitors, other individuals in the population. Nonetheless, observations in nature show that spiteful behavioral patterns are quite rare (Dobson, Chesser, & Zinner 2000; Foster, Wenseleers, & Ratnieks 2001). I will intentionally ignore the most trivial explanation that the individuals “selflessly harm” other members of the population so skillfully and inconspicuously that, in most cases, a naive and idealistic biologist cannot observe it. In any case, it would be appropriate to mention that our shared experience with the behavior of the representatives of a certain well-studied primate species indicate that this possibility should not be ignored.
The simplest explanation of the evident absence of spiteful behavior is that all three mechanisms of the evolution of altruistic behavior mentioned in the previous section, i.e. group selection, kin selection and reciprocal behavior, simultaneously act as a barrier against the egression and spreading of spiteful behavioral patterns. A technically different, but at least equally important, reason for the absence of these behavioral patterns is that, in consequence, not only the bearer of the trait, the Vandal, but also other individuals in the population (more specifically those who are at the particular moment not directly affected by the spiteful behavior) profit from it. These “innocent bystanders”, whose relative fitness increases thanks to lowering the absolute fitness of the individual affected by vandalism, are moreover at an advantage in relation to the bearer of spiteful behavior. They do not have to expend any strength on spiteful behavior and do not expose themselves to the risk of possible revenge from the vandalized victims.
Theoretic models show that spiteful behavior can spread in a population mainly when vandals can recognize the degree of their genetic affinity to the victims and direct the spiteful behavior primarily towards unrelated individuals. In this context, it is sometimes discussed whether certain elements of behavior of individuals infected by some parasites could be interpreted and their origin be explained as spiteful behavior of the infected host (Rozsa 1999; Rozsa 2000). If the individual’s fitness is lowered, because it has been infected with a parasite, then the best thing it can do is to infect other individuals in the population. Theoretical models show that, if it infects other individuals in the population regardless of the degree of their genetic relationship, i.e. regardless of the probability of their sharing the copies of the same alleles, this behavior will be selectively neutral, i.e. it will not lead to any change in the inclusive fitness of the individual. If the infected organism preferentially harms the individuals that are not related to it, the gene for this behavior may spread in the population.
It is well known that individuals infected by certain kinds of parasites have higher motility and are able to migrate over longer distances (Poulin 1994a). According to some concepts, this may be a result of manipulative activity of the parasite, which is favored by higher motility of the infected individuals, because motile hosts can infect more so-far healthy organisms (Randolph 1998). According to another hypothesis, it can be an expression of spiteful behavior of the infected host, which is trying to infect as many still healthy non-relatives in the population as it can, thus heightening its own inclusive fitness.