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.
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This is what I get for procrastinating. I've been meaning to do a post on this for a couple of days. I was going to go at it from a different angle so I may still write it...
Royal fratricide, called "bloody tanistry", was the primary way the steppe peoples decided succession. In the Ottoman empire the system was bureaucratized, with all brothers of the heir being systematically killed once he becomes Khan.
In more fluid situations the results you mention could also be the outcome of game-theoretic principles of alliance. Groups of brothers tended to form into competing factions under the leadership of one of them.
Genghis Khan killed a half-brother and came near to killing two full brothers (out of four total). He was responsible (diretly or indirectly) for the deaths of perhaps half of his first or second cousins listed in the histories.
afarensis, i'd be curious as to your take. your interests intersect strongly with mine so feel free to duplicate link & comment since the latter will always offer a different perspective (same with evolgen & i).