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'Polygamy Hurt 19th Century Mormon Wives' Evolutionary Fitness'

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Polygamy practiced by some 19th century Mormon men had the curious effect of suppressing the overall offspring numbers of Mormon women in plural marriages, say scientists from Indiana University Bloomington and three other institutions in the March 2011 issue of Evolution and Human Behavior.

Simply put, the more sister-wives a Mormon woman had, the fewer children she was likely to produce.

"Although it's great in terms of number of children for successful males to have harems, the data show that for every new woman added to a male's household, the number each wife produced goes down by one child or so," said IU Bloomington evolutionary biologist Michael Wade, whose theoretical work guided the study. "This regression is known as a 'Bateman gradient,' named after the geneticist who first observed a similar phenomenon in fruit flies."

The paper's coauthors were Jacob Moorad (Duke University, Indiana University Ph.D. 2005), Daniel Promislow (University of Georgia), and Ken Smith (University of Utah).

The researchers' survey of birth, marriage and death records from the Utah Population Database covers nearly 186,000 Utah adults and their 630,000 children who lived or died between 1830 and 1894. This period marked an important transition for the nascent Mormon Church, as polygamy began to be phased out in deference to U.S. laws banning the practice but also via internal pressure from the Mormons themselves.

The scientists' study confirmed their expectation that a moratorium on Mormon polygamy would have the effect of decreasing the intensity of sexual selection among males and ultimately bringing the strength of reproductive selection on men closer to that acting on women. With fewer polygamous marriages, more males had access to wives, which led to a decrease in the variation in Mormon males' mating and reproductive success. The scientists estimate that ending polygamy reduced the strength of sexual selection on males by 58 percent.

"This study was very exciting for us, in large part because you just don't get to see the demographic effects of dramatically changing a mating system within a single population -- in any organism," Wade said. "It's an added bonus that this change from polygamy to monogamy just happened to involve people who kept such thorough records of the marriages, births and deaths at that time."

Wade, who specializes in the evolutionary biology of mating systems, says much of his work has elucidated and expanded on the ideas of Angus Bateman. Bateman, a prolific theorist, was unable to empirically test all his theories about mating and mating fitness before he died in 1996. Last year Wade and Northern Arizona University biologist Stephen Shuster co-wrote a retrospective on a classic paper Bateman wrote for the journal Heredity in 1948. Wade and Shuster extolled Bateman's vision, in particular the way in which Bateman thought sexual selection should be quantified. Bateman's critics thought his reductions of biology were too simplistic, yet Wade says Bateman's simple formulas are often dead-on.

"Bateman's ideas still are very much alive, the present study included," Wade said. "It was also his idea that selection could be stronger on males than on females, that what can be an advantage to males can be a disadvantage to females of the same species. And the advantage isn't just in having more mates. You may simply produce more offspring, than the average, if you're a male successful in reproductive competition against other males."

Which isn't to say systems of polygamy in humans or elsewhere in nature are necessarily good for all the males involved. Indeed, Wade says, polygamy is a bad thing for most males of a species.

"When the ratio of sexes is about equal, for every male that has three mates, there must be two males that have none," Wade said. "If a male has even more mates, then the disparity among male 'reproductive' haves and have-nots can become quite great."

So if polygamy (or the female equivalent, polyandry) is disadvantageous to most of the sequestered sex and most of the mate-sequestering sex, why should such systems survive?

"The complete answer is still forthcoming," Wade said. "One thing we know now, based on rigorous studies in many species, particularly the fruit fly, is that selection can be so strong on males that it can drag the entire species off of a naturally selected viability optimum."

Wade points to a familiar example.

"Take the peacock," Wade said. "Its tail is magnificent for attracting females and bad for attracting predators. It is believed that in some situations there is a "predator hard cap" on the fitness of sexual characteristics. But there's also research suggesting even the predator hard cap can be overpowered if sexual selection on males is strong enough. That is, males trade high risks to their lives in order to gain large numbers of mates and thereby offspring."

This research was funded by grants from the National Science Foundation (DEB-0717234 and DEB-0614086) and the National Institutes of Health (RO1GM065414-06 and P30-AG013283). The coauthors also thank the Pedigree and Population Resource, funded by the Huntsman Cancer Foundation, for helping to maintain the Utah Population Database.

"Mating system change reduces the strength of sexual selection in an American frontier population of the 19th century," Evolution & Human Behavior, vol. 32, iss. 2, pp. 79-156 (March 2011)

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What exactly do they mean when they say that the intensity of sexual selection was decreased and how exactly did that have a negative affect?

Some people say that there were not enough females for all the males becasue of polygamy and others dispute this. I wonder what the truth is. It seems to me that in the present day, there are nearly always more women at church than men. If this was the case in the 19th centruy then I don't think there would have been a proplem with some men taking extra wives.

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It seems to me that in the present day, there are nearly always more women at church than men. If this was the case in the 19th centruy then I don't think there would have been a proplem with some men taking extra wives.

Agreed.

Though I think I should have posted this topic In the News section.

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Seems to me that this is saying two things. Many wives are not good as far as offspring is concerned per individual woman, and that polygamous, men on the whole, do not seem to be endued with an enormous libido.

Glenn

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It has long been known that women in polygamous relationships tend to have less children. No surprise there. Not sure what they mean when it comes to "sexual selection" etc. unless they are simply discussing the genetic implications of such and that they've been able to test what would have been theoretical based on the numbers of offspring in comparison.

In an age where birth control was not really available (save abstinence) and pregnancy and childbirth often meant death, having less children (and therefore more time to recover one's physical health in between) would be a good thing, I would assume (and it is an assumption, I would like to see the death rates compared between polygamous and monogamous wives though this could be difficult to come by since many polygamous wives ended up later in monogamous marriages through the death of a spouse and remarriage or divorce and remarriage).

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Seems to me that this is saying two things. Many wives are not good as far as offspring is concerned per individual woman, and that polygamous, men on the whole, do not seem to be endued with an enormous libido.

Glenn

I wonder if the study took into consideration the reality that men were taught to only have sex once a month. That's pretty scant, I'm not surprised they were not having more kids.

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I wonder if the study took into consideration the reality that men were taught to only have sex once a month. That's pretty scant, I'm not surprised they were not having more kids.

I have never heard that. Do you have a reference?

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It has long been known that women in polygamous relationships tend to have less children. No surprise there. Not sure what they mean when it comes to "sexual selection" etc. unless they are simply discussing the genetic implications of such and that they've been able to test what would have been theoretical based on the numbers of offspring in comparison.

In an age where birth control was not really available (save abstinence) and pregnancy and childbirth often meant death, having less children (and therefore more time to recover one's physical health in between) would be a good thing, I would assume (and it is an assumption, I would like to see the death rates compared between polygamous and monogamous wives though this could be difficult to come by since many polygamous wives ended up later in monogamous marriages through the death of a spouse and remarriage or divorce and remarriage).

I'd like to see numbers on children surviving childhood diseases and such and reaching sexual maturity. Survival rates would necessarily greatly affect fecundity of offspring and, thus, the viability of polygyny as a reproductive strategy. Let's compare polygynous vs. monogamous marriages and their offspring, say, for communities West of the Mississippi from 1850 through 1895. That's a tougher bit of research that would yield a more accurate picture.

Just because a single woman tends to have fewer children doesn't mean her genes have a lesser chance of spreading her dna if her children tend to be more likely to survive to adulthood.

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I'd like to see numbers on children surviving childhood diseases and such and reaching sexual maturity. Survival rates would necessarily greatly affect fecundity of offspring and, thus, the viability of polygyny as a reproductive strategy. Let's compare polygynous vs. monogamous marriages and their offspring, say, for communities West of the Mississippi from 1850 through 1895. That's a tougher bit of research that would yield a more accurate picture.

Just because a single woman tends to have fewer children doesn't mean her genes have a lesser chance of spreading her dna if her children tend to be more likely to survive to adulthood.

Definitely.

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Interesting study. I have an ancestor who was a polygamist, and his wives all had several children. He had three wives, and IMO, that seems to be the number that is the most successful for polygamists. In cases like Brigham Young who had a ridiculous number of wives, I can see this study being applicable. I don't think very many of his wives had many children by him with the exception of the first two. The biggest problem this produces is: who will provide for her when the husband dies? Brigham Young's will was an absolute nightmare, and I have no idea what the fate of his childless wives was after his death. I'm also not sure that a woman's "evolutionary fitness" is tied to a large number of children. A few very healthy children who are raised to respect their mother and take care of her in her elderly years is really all the evolutionary fitness a woman needs.

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I wonder if the study took into consideration the reality that men were taught to only have sex once a month. That's pretty scant, I'm not surprised they were not having more kids.

It doesn't seem that they were listening, does it? :P

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