Aaron J. Louie
BI 399
Discussion: Wed 1300
Position Paper

Hamilton's Rule

In a world of fierce competition for resources, animals must fight to survive. It seems only logical that many animals would pool their resources to co-operate in defense, food gathering, and parental care. In explaining the evolution of this "eusocial" behavior, W.D. Hamilton (1964) hypothesized that the more closely related an altruist was to its benefactor, the more likely it would exhibit eusocial behavior towards that animal. Consider, for instance, honeybees. What mechanism causes the extreme eusocial behavior in worker bees that sacrifice their reproductive abilities and even their lives to defend and nurture their colony? Hamilton's rule explains that this is due to relatedness between individuals in the hive. Hymenoptera exhibit haplodiploidy, where males are haploid and females are diploid. The workers can thus be 3/4 related to their sisters, who they rear and defend so selflessly. Normal diploid siblings would be only 1/2 related. However, it has been shown that, when there are several fathers inseminating the reproducing female or when there are more than one reproducing females, the workers may be only 1/8 related. Nevertheless, these populations continue to exhibit altruistic behavior. Hamilton's rule does not adequately explain the presence of eusocial behavior in this case. H. K. Reeve (1994), on the other hand, devised a hypothesis to explain this discrepancy. He dubbed it the "protected invasion model," which basically states that eusocial behavior in haplodiploid insects evolved because "the haplodiploid genetic system ensures that dominant alleles for these traits are especially protected from random loss (female co-operation) or are especially vulnerable to random loss (male parental and alloparental care) when such alleles are rare" (Reeve 1994). For the purposes of this paper, I will argue in favor of Reeve's protected invasion model.

The protected invasion hypothesis is based on two components. First, Reeve shows that the probability of fixation of a dominant eusocial allele is greater for females than for males in a haplodiploid population. Secondly, he demonstrates that the probability of fixation of a mutant female eusocial allele is greater in haplodiploids than in diploid populations. This, he asserts, is the mechanism for the prevalence of both eusocial behavior and female dominance in Hymenoptera. Since an altruistic trait would be conserved only in females in a haplodiploid population, protected invasion hypothesis predicts that eusociality would be more compatible with queen polyandry in the Hymenoptera, a condition not satisfied by Hamilton's rule.

On the other hand, Hamilton's rule, also referred to as kin selection, suggests that an altruistic gene's success depends on the gene's benefit to itself (Drickamer, Vessey & Meikle 1996). The way that an altruistic gene can most efficiently succeed, then, is if the benefactor of the altruist also carries that gene, thereby increasing the likelihood that that gene will be passed on. This hypothesis seems to be at work in many species, such as ground squirrels, rhesus monkeys, lions, black jackals, and white-footed mice (Drickamer, Vessey & Meikle 1996). Trivers and Hare (1976) also found that, in social insects, kin selection could be used to explain the overwhelming presence of altruistic behavior towards the queen, the young, and other workers. However, Reeve (1994) pointed out that eusocial behavior would not have evolved as a result of kin selection, but the evolution of sex-ratio control most likely evolved after the appearance of eusocial behavior.

Hamilton's model, which attributes the presence of altruistic behavior to the relatedness of animals, does not take into account the possibility of queen polyandry, multiple queens, or the evolution of sex-ratio control. Reeves concludes that the protected invasion model explains these discrepancies and provides a more concrete genetic model for the inheritance of genetic traits, at least in the Hymenoptera.

Bibliography

Drickamer, L. C., Vessey, S. H. & Meikle, D. 1996. Animal Behavior: mechanisms, ecology, evolution. 4th edn. Dubuque: Wm. C. Brown.

Reeve, H. K. 1993. Haplodiploidy, eusociality and absence of male parental and alloparental care in Hymenoptera: A unifying genetic hypothesis distinct from kin selection theory. Phil. Trans. R. Soc. Lond., B, 342, 335-352.

Trivers, R. L. & Hare, H., 1976. Haplodiploidy and the evolution of the social insects. Science 191, 249-263.