A few days ago I mentioned that the story about a bumper crop of twins in a German town in southern Brazil was notable because of the elevated frequency of identical, not fraternal, twins. A reader points out that that was an error, and The New York Times has appended a correction:
The Cândido Godói Journal article on Monday, about the unexplained proliferation of twins born in the farming town of Cândido Godói in southern Brazil, misstated the type of twins usually associated with a genetic tendency of the mother. They are fraternal twins -- like a majority of those born in the town. They are not identical twins, which are generally believed to be conceived by chance.
As I noted before, variation between populations and due to environmental inputs that affect twinning rates affect the rate of fraternal, not identical, twins. Inbreeding* or some environmental parameter, or a combination, probably explain this then.
* Since twinning is somewhat heritable, the alleles which modulate twinning rates could have increased in frequency in an inbred population through drift.
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Razib said: "Since twinning is somewhat heritable, the alleles which modulate twinning rates could have increased in frequency in an inbred population through drift."
Let's see: a putative allele that increases the odds of twinning, meaning that one pregnancy results in two offspring rather than one. Soooo, ... you get two (2?) offspring (double the number?) instead of one (two for one?) So, ... not one offspring; but two? Twice as many offspring? Twokidsnotone?
Hmmmm. Yup, must be drift. (move along now; no natural selection here).
Hmmmm. Yup, must be drift. (move along now; no natural selection here).
this is going to be selected against in a pre-1900 environment since twins are not as healthy, and most families probably didn't have the marginal product to feed both simultaneously when they were very young. so you probably have to have natural selection within the past 4 generations. have you cranked the back of the envelope math for the increase in frequency, or are you just mouthing off?
"this is going to be selected against in a pre-1900 environment ... have you cranked the back of the envelope math ..."
It could theoretically double each generation (although practically it would probably be somewhat less than this) if starting from a low level. So in 5 generations increase by as much as 32 fold. What do we have here? a 5 fold increase? 5th root of 5 is ~1.38. Sounds plausible.
Soooo, ... you get two (2?) offspring (double the number?) instead of one (two for one?) So, ... not one offspring; but two? Twice as many offspring? Twokidsnotone?
Think, man. If that's all there is to it, why aren't most births twin births?
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this is going to be selected against in a pre-1900 environment since twins are not as healthy
Not necessarily in all such environments, don't forget the pre-industrial Finns.
It could theoretically double each generation (although practically it would probably be somewhat less than this) if starting from a low level. So in 5 generations increase by as much as 32 fold. What do we have here? a 5 fold increase? 5th root of 5 is ~1.38. Sounds plausible.
it would only double if any women with the "twinning allele" only gave birth to twins. and, if the number of gestations in independent from the number of offspring (IOW, a woman who can expect to have 4 gestations who will ONLY have twins has the same likelihood as a woman who only has 4 gestations who will expect not to have twins). twinning does run in families, but the effect isn't that strong (the normal chance is 1 in 100, 1 in 10 seems a huge effect). one would have to look at the pedigrees of the specific brazilian case, but windy makes a point that i was thinking of: if it's so easy to increase fitness this shouldn't be a freak occurrence and twinning should be way more frequency among american whites than european whites. among mormons than non-mormons.