inbreeding and the evolution of altruistic behavior ii

in Understanding Human History, michael hart did a real nice job of explaining how kin selection or inclusive fitness works and how “genes for altruism” could be selected for [pgs. 37-38]:

“For about a century after Darwin proposed his theory of evolution, the origin of altruistic behavior in animals remained a puzzle. It was not until the 1960s, when William D. Hamilton proposed his theory of kin selection, that a satisfactory explanation was given. That theory can perhaps best be explained by an example:

“Suppose a man sees his identical twin drowning in a river, and estimates (correctly) that if he were to jump in and try to save his brother the probability of success would be 80%, while the probability that he would die in the attempt would be 20%. Consider these two alternatives:

“a) Some of the man’s genes strongly dispose him to rescue his brother, and he therefore jumps in and tries to save him (‘altruistic behavior’).

“b) The man does not have genes that dispose him to rescue his brother, and he therefore stays on the shore and lets his brother drown (‘selfish behavior’).

“In case (b), exactly one copy of the man’s genes survives, and may later be replicated. However, in case (a), if the rescue attempt is successful, two copies of the man’s genes survive (one in his own body, one in his brother’s). As this will happen 80% of the time, on average 1.6 (= 0.80 × 2) copies of the man’s genes will survive. In this situation, therefore, genes that dispose a person to altruistic behavior will — on average — have more surviving copies than genes that dispose a person to act selfishly and will be favored by natural selection.

“Now consider a slightly different example. Suppose that the man on shore is a brother — but not a twin — of the person who is drowning. Case (b) will still result in one copy of his genes being preserved. However, since ordinary siblings share only 50% of their genes, if the man on shore succeeds in rescuing his brother then (on average) 1.5 copies of the man’s genes will survive. Since 80% of the attempts will be successful, case (a) will on average result in 1.2 (= 0.80 × 1.5) copies of the altruistic genes surviving. Since 1.2 is greater than 1.0, the altruistic genes will be favored by natural selection in this case too.

“Suppose, however, that the two men were not brothers, but merely first cousins. First cousins, on average, share only one-eighth of their genes. In this case, altruistic behavior results in only 0.9 (= 0.80 × 1.125) copies of the man’s genes surviving, and natural selection will therefore favor the genes for selfish behavior.

“The upshot is that a gene that disposes its bearer to behave altruistically toward a close relative can have a selective advantage over one that disposes its bearer to act completely selfishly. Furthermore, this can occur even though the relative never returns the favor, and even if the survival of the relative does not increase the group’s chances of survival. It is not necessary that either reciprocal altruism or group selection operate for kin selection to result in the spread of genes that dispose their bearer to act altruistically toward close relatives.”

what’s missing from these examples is, of course, inbreeding. and depth of time.

take michael’s second example up there…

“Suppose that the man on shore is a brother — but not a twin — of the person who is drowning.”

…but let’s add that the parents of these brothers were first-cousins. that makes these two guys: brothers AND second-cousins (i.e. the children of two first-cousins). so they probably share not only 50% of their genes in common as brothers, but also 3.13% of their genes in common as second-cousins. so the “push” to jump in the water to save the brother/cousin must be somewhat stronger in the inbred pair than for the brother to save just a plain ol’ brother.

now let’s take this example of michael’s…

“Suppose, however, that the two men were not brothers, but merely first cousins. First cousins, on average, share only one-eighth of their genes.”

…but let’s make them double first-cousins rather than just first-cousins. what happens then?

well, while first-cousins probably share 1/8th or 12.5% of their genes in common, double first-cousins share … well, double that! … or 1/4 or 25% of their genes in common.

what happens to michael’s calculation then?

“In this case, altruistic behavior results in only 0.9 (= 0.80 × 1.125) copies of the man’s genes surviving, and natural selection will therefore favor the genes for selfish behavior.”

in the case of double first-cousins the calculation becomes 0.80 x 1.25 = 1.0. that’s just breaking even using michael’s example, but what if the odds of saving the cousin from drowing are better than 80%?

or what about the depth of time i mentioned above? what if the family of my double first-cousins has been inbreeding for a very long time. a very, very long time. like for fifty generations or more. then the relatedness between all the family members, including these double first-cousins, will be even closer. natural selection ought, then, to favor such double first-cousins jumping in to save each other.

as wade and breden showed (see also previous post), inbreeding can help to accelerate the rate of the evolution (or frequency in a population) of altruism genes [pg. 846]:

[T]he increase in matings between homozygous parents decreases the genetic variance within families, because these matings produce genotypically homogeneous arrays of offspring.”

repeated inbreeding in a family reduces the diversity (whoa!) of the allele types within that family, and if we’re talking about “genes for altruism” here, then the variety of those must get reduced within inbred families, too. in a population that consists of, say, ten inbreeding families, the one that has super-duper altruism genes that lead all of its family members to help each other out more than the members of the other families will have the advantage (provided selection favors that advantage for whatever reasons). and those super-duper altruism genes will no doubt eventually spread to the other families since, in reality, no family groups inbreed 100% of the time anywhere — there will pretty definitely be gene flow between families. so then you’ll get a whole population of super-duper family altruists (note that these people are NOT altruistic to unrelated individuals).

the human populations on earth today that inbreed most closely (within patrilineages) and often practice double first-cousin marriage — AND have been doing this for prolly at least a couple of thousand years (time depth) — are the arabs (who later spread these mating practices to the maghreb, the mashriq and far off places like iraq and afghanistan and all the other ‘stans) and some peoples in the levant like the druze. i think that, because of their long-standing mating practices, they are the prime human examples of wade and breden’s accelerated evolution of altruism thanks to inbreeding.

previously: inbreeding and the evolution of altruistic behavior and more on inbreeding and the evolution of altruistic behavior

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hgdp samples and relatedness

**update 03/22: see follow up post — more on the hgdp samples — and just ignore what i said about the french samples below.**

**update 08/28: ignore what i said about ignoring what i said about the french samples. see here.**

i had a post up back in january about some cool research that looked at what runs of homozygosity (roh) in samples from the human genome diversity project (hgdp) can tell us about the inbreeding or outbreeding of different human populations.

but i’ve been bothered by the thought of how the hgdp samples were gathered. as professor harpending said:

“No one knows, by the way, how sampling was carried out for this nor for any of the HGDP populations.”

ugh. the hgdp is really, really cool — but not having info on where the samples came from — like genealogical info — poses a problem if you want to use this data to look at recent inbreeding/outbreeding or, i think, even the sort of thought experiment that prof. harpening conducted a couple of weeks ago, however cool that was, too.

here’s an example of what i mean.

prof. harpending compared the relatedness or kinship of the individuals in a couple of sets of samples from the hgdp: the french, the japanese, and the druze. he found that the kinship of indviduals in both the french and japanese populations to their nearest “relatives” (i presume two individuals who had the most similar genomes?) is very similar. as he said: “from the viewpoint of kinship, one person is not very different from another person.” the druze, otoh, are very dissimilar and the good professor thinks that this is a population in which “opportunities for discord and clannishness are high as individuals able to discriminate kin would ally against the ‘others.'”

i’m not going to argue with that! the druze, like the arabs, regularly practice father’s brother’s daughter (fbd) marriage, the most incestuous form of cousin marriage around, so i’m not surprised that their genomes reflect this fact. (fbd marriage probably originated in the levant, so it could be that the people who are today known as the druze are the product of one of the longest running close-inbreeding projects in humans around.) amongst the druze, each extended-family or clan must’ve become, over time, it’s own little semi-isolated sub-group. like the arabs, i’d expect a lot of clannishness and infighting.

however, wrt to the french and japanese samples: the ceph folks do have some information on the hgdp samples, and one point of difference between the french and japanese samples is that the french samples are described as having been drawn from relatives whereas the japanese samples were not.

there are 29 french samples described as: French (various regions) relatives, and there are 31 japanese samples described as just Japanese, so i assume that means the japanese samples do not include relatives.

so what does French (various regions) relatives mean? i guess that the samples were drawn from different regions of france, but we don’t know which regions or how many. (which is too bad because different regions of france have, historically, had different inbreeding rates.) and how many relatives? who knows? i’m going to presume all 29 are not relatives from one family living scattered across the country, although i suppose that could’ve been the case. what seems more likely to me is that we’re looking at groups of samples from a number of different families, but how many? two, three, four … ten? again, who knows?

what difference would this make? well if the kinship in the french set of samples and the japanese set of samples look to be around the same, i.e. “one person is not very different from another,” BUT the french samples are from relatives and the japanese samples are not, then that would mean that the individuals in the broader french population must be even more like one another than the individuals in the broader japanese population since french family members have the same kinship to one another as japanese strangers do.

to put it more simply, comparing the french and japanese samples is like comparing apples and oranges because, if the ceph information is correct, the french samples include family members whereas the japanese ones do not.

the druze samples, too, are described as coming from relatives — again no info as to how many families/relatives — so the broader druze population should prove to be even more dissimilar to one another than these family members are.

i would love to see lots more studies done on inbreeding/outbreeding (and possible inclusive fitness-related behaviors) in human populations from a genetics p.o.v. — like what prof. harpending did in his recent post. but afaics, using the hgdp data is problematic. i look forward to when there are more whole genome sequences available out there WITH accompanying genealogical/pedigree information.

previously: runs of homozygosity and inbreeding (and outbreeding)

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more roh

in “Genomic Runs of Homozygosity Record Population History and Consanguinity” that i posted about yesterday, kirin, et. al., say:

“Europeans and East Asians have very similar ROH profiles in all but the shortest category (0.5-1 Mb). There are no significant differences between either the percentage of individuals with ROH of different lengths or sum length of ROH above different length thresholds (>1.5 Mb) for these two continental groupings (File S1). This is not surprising because both of these groups are mainly represented here by fairly large populations with no documented preference for consanguineous marriage.

ehhhhhhh … well … if they’re talking about now, i.e. in the present, then yeah — that’s probably pretty right. but many of the european populations that they looked at (i.e. from the human genome diversity project [hgdp]), regularly practiced some to quite a lot of consanguineous marriages up until fairly recently. (i haven’t checked into the asian populations that they looked at.)

the european populations that they looked at are: the adygeis, the basques, french folks, italians, orcadians, russians, sardinians and tuscans.

the adygeis are the circassians and it’s my understanding that they have avoided cousin marriage for quite some time, although they are endogamous (obviously). the russians — religious russians, anyway — avoid first- and second-cousin marriage. but the basques and the french have had some signficant amounts of consanguineous marriage up until quite recently. and the italians and sardinians?! holy toledo! of all of these groups, it’s probably the tuscans that have avoided cousin marriage for the longest. (dunno about the orkney islanders.)

like i said yesterday, if anything, kirin, et. al., have probably got some of the most inbred europeans in their sample.

anyway … i took at look at their supplemental info [opens pdf] and found that they’ve included data for the proportion (percentage) of the genomes from each group that are covered in “runs of homozygosity” (roh). the more roh in your — or your population’s — genome(s), the more inbred you (all) are (or maybe the smaller your gene pool is — see yesterday’s post). when i took out just the europeans plus the han chinese and japanese and a couple of other interesting groups, here’s what i got:

most of the european groups have the least number of roh (these are roh of all different lengths). the han chinese are like the italians or the sardinians, who have a long and recent history of close marriages (not so much the northern italians) — and the japanese even more so. wikipedia tells us that cousin marriage was preferred in china until the mid-twentieth century, so there you go.

and the father’s brother’s daughter’s marriage groups? their roh are higher than the inbred europeans, the han chinese and the japanese.

you can see here, too, that the japanese have greater numbers of longer roh than french people (the black circles are the japanese, the orange circles are the french) — that means more recent inbreeding amongst the japanese (click on image for LARGER view – should open in new tab/window):

interestingly, many balochis (green circles) have fewer and shorter roh than the french — many have more and longer. dunno what that tells us about the balochi. new blood? tribes merging with (fairly) unrelated tribes? just plain ol’ out-marriage?

here are the percentages of the genomes covered by roh for each of the populations in the study in ascending order. i tried to match the colors for the continental groupings from the chart in yesterday’s post — dunno if i succeeded?:

previously: runs of homozygosity and inbreeding (and outbreeding)

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