here’s some really good, but technical, stuff (i’ve spared us all the formulae**) from “Inbreeding and the evolution of social behavior” by richard michod. the essay can be found in “The Natural History of Inbreeding and Outbreeding.”
“Social evolution is interpreted here in a narrow but important sense as the evolution of behaviors that benefit the group but are costly to the individuals that perform them. The term altruism has been applied to such behaviors. However, it should be realized that most of the theory to be discussed applies to the evolution of intraspecific interactions generally. In other words, the effects of the behavior under study, on the fitnesses of both the donor and the recipient, can take any sign.“
i mentioned this in my recap post of a couple of months ago. inbreeding and outbreeding (should) not only affect altruism, but all sorts of social behaviors. i’m also particularly interested in how mating patterns affect the social control of reproduction in human societies (think: burkas), but that’s just me.
“Kin selection can be viewed, and explicitly modeled, as a selection process involving within-family and between-family selection….
“By definition, altruism is selected against by within-family selection (individual selection) and favored by between-family selection, since the average fitness of a family is directly proportional to the frequency of altruists within the family. Indeed, it can be shown that Hamilton’s rule translates into the condition in which between-family selection overrides within-family selection (Wade 1980)….”
this makes intuitive sense — at least it does to me. if you only had one human family on the planet — mom, pop, sis and junior — they would only be in competition with each other for successfully reproducing their genes. however, throw the jones family in next door, and our first family, who are all more related to each other than they are to the joneses, are now all in competition with the joneses. thus, altruism genes are more likely to spread within our first family when they have some competition from outside.
“Breden and Wade pointed out that inbreeding eventually fixes the population for two family types resulting from matings between like homozygotes (AA x AA, aa x aa). Matings between like homozygotes result in families that have no genetic variation in them. Consequently, they concluded that inbreeding generally decreases the average within-family variance. The two types of families produced by matings between like homozygotes are also maximally different from each other in frequency of the altruistic allele and in average fitness. Consequently, inbreeding increases the between-family variance. Both these factors, that is, the decrease of within-family selection and the increase of between-family selection, have the effect of making the evolution of altruism easier. This effect was also emphasized by Boorman and Levitt (1980: 350). They concluded that ‘…a high degree of inbreeding may be expected to favor sib altruism over the random mating case….’
“Conclusions. In conclusion, the evolution of altruism is favored by between-family selection and disfavored by within-family selection. Inbreeding can make the evolution of altruism easier by increasing the between-family variance and decreasing the within-family variance….”
so, inbreeding makes “the evolution of altruism easier.” that’s ’cause, for instance, if two members of a family have altruism genes and they inbreed, then the altruism genes are more likely to be passed down to the next generation. more so than if only one parent had an altruism gene. then there would only be a 50-50 chance that any of the kids would get that altruism gene. the odds of passing on any altruism genes are, obviously, higher if both parents have them. rinse, lather, and repeat for future generations within an inbred family and you can see how the evolution of altruism is made easier.
but note that it’s “sib altruism” we’re talking about here, not a broad, all-encompassing altruism towards all of mankind. inbreeding leads to insular altruistic sentiments.
“In family-structured populations…
*think: the arab world*
“…there is a direct relationship between the inbreeding process and the foundation of the groups within which social interactions take place (families). Family groups are a direct product of a single mating. To the extent that matings are between relatives, the resulting offspring arrays can be expected to be less variable. However, this direct relationship between inbreeding and the foundation of groups in family-structured population is absent in group selection in hierarchically structured populations….
*definition of a hierarchically structured population: “A population is hierarchical, if it can be divided into a certain amount of subpopulations in such a way that the set may naturally break into classes (levels).”*
“In hierarchically structured populations, interactions take place within groups (subpopulations) that include many families. The subpopulation in a hierarchically structured species is more analogous to the total population of a family-structured population, in which the total variance does increase with inbreeding. By studying inbreeding models of multifamily groups (Wade and Breden 1987), the apparent difference between kin selection and group selection concerning the prospects of altruism within groups should be resolved. The beneficial effect of inbreeding on the prospects of altruism due to individual selection should decrease as more and more families contribute to a group, so long as the offspring of families within the same group interact at random.“
translation (i think): in a society which is not based upon families (a society unlike the arab world, iow), if people mate more-or-less randomly within their sub-population (say, the middle class), then there will be less “sib altruism” within that sub-population and more “broad but shallow regional blood ties” as steve sailer described it.
**you’re welcome! (^_^)
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