Discovery News has a fascinating review of new research which suggests that royal fratricide tended to follow Hamiltonian principles, that is, cousins were killed so that nearer relations could prosper. Hamilton's Rule states that an "altruistic" behavior is genetically beneficial if Cost < Benefit to Other X Coefficient of Relationship to Other (C < B*r). Roughly speaking, if you have a coefficient of relation to a siblling of 0.5, then for every unit of fitness you sacrifice for a sibling they need to increase their own fitness by over two units. The logic is simple: imagine you carry a gene which states "be altruistic to your siblings!" To make this simple, assume that it was a de novo mutation in one of the parents. Well, there is a 50% chance that your sibling is carrying this gene (assume that the parent is heterozygous, so expectation is half of the offspring should carry a copy and half should not). For the altruism mutation to spread it needs to recoup the decrease of its own propogation directly via its host organism in altruistic acts by increasing the fitness of others who carry the allele. Since a host of siblings will only have a 50% chance of carrying the allele to come out ahead you need to more than double their fitness to break even. But, on a different note, my understanding is that recent research coming out of eusocial insects suggests that Hamiltonian principles might not apply there as easily as we thought. Many Hymenoptera are haplodiploid, resulting in rather close relationships between sisters (closer than between the potential mother and offspring assuming no inbreeding), so it seems a perfect situation where inclusive fitness and kin selection should result in cooperation (eusocial insects were Hamilton's biological illustration of his model). The problem is that genetic testing shows that the coefficient of relation in many colonies is far less than assumed, and C < B*r seems to be violated quite often. As a result more complex models which suggest that kin selection and inclusive fitness were the sufficient preconditions for exploration of new biosocial lifestyles, but not necessary conditions to perpetuate them, have emerged. We'll see, interesting times.... Via Steve.