Time to take another look at your (third) cousin?
I’ll admit it: I’m fully baffled by the recent report by the cats at Decode that marriages between third and fourth cousins tend to be more fertile than other relationships. But it’s a pretty fascinating observation–using their extensive Icelandic genealogies, they compile the figure on the right: on the x-axis is a measure of the relatedness of a couple, and the y-axis various measures of fertility. For the most relevant measures–the number of children that reproduce and the number of grandchildren–you see a sort of n-shaped curve, such that very closely-related and very distantly-related couples are less fertile than couples related at the level of third or fourth cousins.
Given the amount of data that they have, they’re able to see this pattern is robust across historical periods, during which average relatedness of couples has decreased by an order of magnitude. And since Iceland is relatively socially homogeneous, the authors suggest there really is biology underlying this phenomenon. But how? I’ve no idea.
(See also John Hawks).
Labels: Biology
1) lower rate of miscarriage because of lower risk of mother-child immune conflict?
2) more sex for reasons of physical compatibility?
3) Environmental hardship forces migration, simultaneously reducing fertility and the relatedness of potential mates?
btw, wuz talking to outrageous p-vaues with some people the other day. see one on the order of exp(-129). lol.
Perhaps people who have to settle for close relatives or travel extensively to find a mate are less fit?
Sailer has a post on it here where he gives some guesses as to what causes it.
I was watching arrested development on joox and it occurred to me how little Alia Shawkat (funny name cause she’s part Iraqi Kurd), who plays Maebe, looks like Michael Cera’s George Michael. I know in the end they turn out not to actually be cousins, but you would think they’d cast someone who would have seemed more credible as a relative.
like ann?
my thoughts: http://scienceblogs.com/gnxp/2008/02/why_cousin_lookin_fertile.php
The underlying assumption that makes this baffling is that the more closely related the pair, the less fertile they somehow should be. This is based on a recently overthrown pervasive notion that sexual reproduction exists to foment genetic diversity. In fact, sexual reproduction it turns out is highly preservative due to the sexual filter (see article for details).
The following is speculation in light of the above. Too closely related means too little diversity and pathology creeps in (the standard answer). But too distant, perhaps this is tantamount to trying to mate with a different species. One would expect a continuous (not discrete) boundary between species anyway. And we must never forget that species are defined by whether individuals from that group can successfully mate, and not some subjective measure such as morphological similarity.
btw, wuz talking to outrageous p-vaues with some people the other day. see one on the order of exp(-129). lol.
seen this curmudgeon?
The earth is round (p
This is based on a recently overthrown pervasive notion that sexual reproduction exists to foment genetic diversity.
don’t use hyperbolic language like “overthrown” when talking about an enormous area like the research of sex if you want to be taken seriously. one paper in genome does not a paradigm shift make.
And we must never forget that species are defined by whether individuals from that group can successfully mate, and not some subjective measure such as morphological similarity.
no, there’s no specific way to define a species. the biological species concept as defined by pre- and post-zygotic mating barriers is one way to define species, but there are many different methods preferred by different groups of biologists. in any case, a tendency toward infertility often exhibits a continuous probabilistic distribution (that is, even mules have turned out to be fertile on rare occasion). stop joking.
Graphs A and D indicate two mechanisms at work:
1- Closer couples have more kids
2- Kids from closer couples die off earlier.
The “n-shaped graphs” seem to result from the conjunction of the two. Point 2 can be explained by inbreeding.
Point 1 can be caused by many factors, but there are a couple obvious possibilities. One is that people are more attracted to their cousins in general. This might be tested by measuring whether close-cousin weddings are disproportionately common – Decode have the data for this.
Another possibility is socio-economical: maybe poorer people (who tend to have more kids) are more likely to marry with their cousins. I’m not sure Decode have socio-economical data.
Of course, you might have behavioral causes: “sex-mad” people will tend to clinch it with the closest person they find that is legally available, i.e. their cousins, and will also have more kids (due to being sex-mad in the first place). Of course the above scenario works just as well if you replace “sex-mad” with “family-oriented”.
Another possibility is socio-economical: maybe poorer people (who tend to have more kids) are more likely to marry with their cousins. I’m not sure Decode have socio-economical data.
I would go with something like this. I know, they think they controlled for everything, but I’d bet that they forgot something, or didn’t control well enough.
My ancestors were rather fond of cousin marriage (they lived in an isolated part of the Appalachians and it was hard to find anyone you weren’t related to) — so fond, in fact, that there are probably less than a dozen surnames on my family tree for the past three hundred and fifty years, and I can count most of the population of eastern Kentucky as kinfolk.
Most of the marriages were second or third cousins — I think there was only one first cousin marriage that I am aware of.
They were also a remarkably fecund lot; average family size was between eight and twelve living children. Surprisingly, they were also a long-lived clan; several of my direct ancestors (including my great-grandma) lived to see their hundredth birthdays.
For the time and place, they were well-educated; lots of clergymen, schoolteachers, and businessmen. Not really your stereotypical hillbillies at all, except for the cousin-marrying thing!
amnestic, when I saw your link I was expecting something like this.
Marginal Revolution just linked here, but I don’t really see how it supported their point (which is not to say I dispute it).
Razib, if you look past what you consider to be hyperbole in my response, I think you will see what amounts to a credible hypothesis. That is, if you assume that sex is advantageous because it is preservative of adaptive structure, then it makes sense that you’d want to breed with those as close to you as possible. With the caveat that too close yields diminishing fitness since some amount of genetic diversity is good.
As for the species issue, I agree with your statement, and further believe it supports my argument.
If you would be interested in vetting the iconoclastic claim made by the paper I referenced, I think it would make for a good post and a lively thread. The claim is for a paradigm shift, and the evidence is compelling. It should be discussed separately and thoroughly.
I seem to remember reading a theory once that while close inbreeding is undesirable because of the increased likelihood of receiving homogeneous deleterious recessives, that distant outbreeding might also be undesirable because it might break up groups of genes that only confer a fitness advantage if you have the full set (although, this is not likely the mechanism at work here, but anyway…)
“Reader”, I think this is a plausible explanation, why do you say it’s not likely the mechanism at work?
Can this be generalized to other populations? Icelanders who are 3rd cousins are in effect already much more related to each other than 3rd cousins would be in more genetically diverse populations.
The entire population of Iceland is decended from a few thousand Norse settlers in the 9th-11th centuries who apparently brought a few hundred Irish captives or brides with them. And the ancestral population from the west coast of Norway was itself not very diverse.
Furthermore, Iceland has had several population bottlenecks in very recent history. In the 18th century, a smallpox epidemic wiped out a third of the population. Later, around 1783, a volcanic eruption devastated much of the island’s livestock, causing a severe famine.
So it is reasonable to speculate that a 3rd cousin in Iceland might be equivalent to a 1st or 2nd cousin in many other societies.
However, because of the long genetic isolation of Iceland, the entire population may have moved towards an equilibrium in which there is a lower genetic load of deleterious mutations. This would make marriages between close relatives much less hazardous than in more outbred societies.
Thus, these results may not mean very much for other human populations. Happy Valentines Day!