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The above visualization is from a Reddit thread, Almost all men are stronger than almost all women. It’s based on grip strength, and basically reiterates my post from last year, Men Are Stronger Than Women (On Average). The same metric, grip strength, is highlighted. The plot above shows that the “great divergence” occurs on the cusp of puberty, exactly when secondary sexual characteristic of males and females become much more pronounced. In my post I pointed out that the Olympic caliber female German fencers were on the lower end of the male distribution.

This came to my mind when reading this nice piece in The New York Times Magazine, The Phenom: The most dominant swimmer in the pool this summer is 19-year-old Katie Ledecky. The question isn’t whether she’ll win, but by how much:

It’s not unusual for men and women swimmers to train together, but being in the pool with Ledecky is something that many men can’t handle. In April, Conor Dwyer, a 6-foot-5, 27-year-old American swimmer who won a gold medal in the 4-by-200 freestyle relay in London, gave a revealing interview posted online by USA Swimming. In it, he talked about male swimmers being “broken” by Ledecky when they practiced together at the Olympic Training Center in Colorado Springs.

Ledecky’s ability to crush men in practice does not necessarily mean she would defeat them in competition. There’s a difference between imposing her will, and perhaps superior conditioning, over the course of a two-hour practice and doing it in a shorter race in which men’s generally greater strength provides an advantage. Her best chance would probably be in the 1,500 freestyle, which women race at the FINA World Championships but not at the Olympics. (The men don’t swim the 800 in the Olympics, so there are the same number of events for male and female swimmers.) Ledecky’s best time in the event would put her among the dozen or so top American men and is 25 seconds faster than their qualifying time at the United States Olympic trials — but it is much too slow to earn a medal at the Games. On the other hand, because no other woman offers a real challenge to her, she is never pushed in that event. I asked Andrew Gemmell, who specializes in the 1,500 free, a hypothetical question: What if, in some dystopian swim universe, Ledecky was told that there would be no women’s events and that she would have to try to make the American team by competing with the men in the 1,500?

His father, who trains her, had told me that he did not think she could qualify, a feat that under current rules would require her to finish first or second at the trials. Andrew, who trains side by side with her, had a different answer. “It would be really difficult, but I would never bet against her,” he said. “I don’t think anybody knows yet what she’s capable of.”

9781440838101 I’m a little surprised honestly that the term “dystopian” got in there, because there are now people with academic appointments arguing for the ending of sex segregation in sports. Often they are sociologists, who believe all things are socially constructed, and take some element of non-binary aspect to gender to meaning that the distribution of possibilities are entirely flat and arbitrary.

Katie Ledecky has preternatural gifts, as well as opportunities afforded to her by her class status. The whole piece highlights Ledecky’s exceptional physical abilities and mental attributes. But even it acknowledges she would likely not beat the top men in her events.

One of the authors of the above book, Sex Segregation in Sports: Why Separate Is Not Equal, Adrienne Milner, was interviewed last year on NPR about the thesis. The interviewer was polite, but a little incredulous. When he brought up biological differences, her response was illuminating, after a fashion.

First, she argued that sex segregation in sport denoted women’s inferiority, and that was a problem. The fact is that when it comes to strength, especially upper body strength, all the data do suggest that women, on average, are markedly inferior to men. This is a fact. This fact causes problems. But the fact that this fact causes problems does not entail that we literally deny the fact. At least that’s my opinion.

Second, she analogizes sex and gender as social constructs to race as a social construct. I knew she was going to go there, because this is a rhetorical nuclear option which is going to quickly defenestrate interlocutors. She observes that:

“We look at race as a social construction. It is not genetic, it is not biological, and we believe the same is [true] for sex … The male-female dichotomy doesn’t cover everyone, right? We have trans people, intersex people.”

As I said above, the reporter was incredulous, but he had a hard time responding after Dr. Milner explicitly connected race and sex, because it is the mainstream position now that race is a social construct and lacks any biological basis. The facts may not be on Milner’s side, but she has the theory and the “moral arc of history” backing her. It would take great courage to still dig in and defend reality as it is, as opposed to her preferences.

The reality is that race and sex/gender are social constructs. The atom is a social construct. Matter and energy are social constructs. Cities are social constructs. Everything is a social construct, as we look through the glass darkly. But social constructs operate on various levels of clarity and distinctiveness and exhibit different levels of pliability and utility. Dalton’s atomic model is profoundly wrong. It has long been superseded by quantum physical models, which have the utility of making correct predictions, whatever their correspondence to reality on a metaphysical level might be. But the Daltonian model is still often implicitly the one introduced to children to allow them to gain some intuition as to the nature of how matter is constituted. In contrast, the metaphysical ideas of the ancients as to the material nature of the universe are both wrong, and, lacking in utility.

All models are wrong, but there are still superior and inferior models. Their measure is in how they correspond to, and predict, reality. Not how they correspond to our ethical judgements of how the universe should be.

Many sociologists dissent from this position. They’ve marched into the academy and taken it over. Because of their ideology that all things are social, they believe they can reshape the fabric of the universe through their own normative preferences. To me this is a problem. I struggle against it. Our deep human intuitions often reject, and recoil, against fragments of reality. But to successfully grapple with reality we need to attempt to understand reality on its own terms, not our own.

I may struggle in vain. Could it be the liberal Whiggish scientific moment in history is over? History is written by the winners, but perhaps in the future science will also be written by the winners. I’m not sure that the truth will win out. Perhaps the glass will become darker, rather than clearer. There are genuine difficult empirical questions about the nature of human variation and our dispositions, and how it relates to the values that we hold to be true. The fact that we’re still discussing sex segregation in sports and how it is unjust illustrates how far we’ve come in the solipsistic and socially constructionist direction.

Imagine that in the end of days all the mandarins will be sociologists, who come not to bring illumination of the truth, but to determine the nature of the truth for us to agree upon. Perhaps this is the true end of history, as humanity returns to an equilibrium where the bracing aspects of reality are shielded from the masses, which lay indolent in their delusions, while the technocrats and artificial intelligences confront the outside.

• Category: Science • Tags: Science, Sex 
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Citation: Figure adapted from Lumley, Alyson J., et al. “Sexual selection protects against extinction.” Nature (2015).

9780198503361_200 Sex is a big deal. William Hamilton spent a significant part of his career on the topic, and the second volume of his collected papers, The Narrow Roads of Gene Land, is focused on this issue. Whenever I talk about sex in an evolutionary biological context one thing that always pops up is why males? In other words, why do so many complex organisms have a whole sex which does not bear offspring? Parthenogenetic lineages of organisms where females can reproduce asexually have double the per generation reproductive output as sexual lineages. And yet over evolutionary history it seems clear that in lineages where sexual and asexual species coexist, the latter are always novel derived lineages. In other words, asexual lineages have a high extinction rate. Sex, and more specifically males, must be good for something. What then?

One hypothesis is that males are good for purging genetic load via sexual selection. On a genetic level all individuals carry deleterious mutations, which they pass on to their offspring. But, because of sample variance in transmission, there will be a distribution of outcomes in any given set of offspring. By chance some individuals will exhibit a higher load of deleterious alleles, while others will carry fewer alleles. If this load is correlated to traits which are visible to the opposite sex, then excess load every generation can be purged through reproductive skew. In other words, one might envisage a situation of sexual selection-mutation balance, where de novo mutations introduced every generation are balanced against deleterious alleles purged from the population through selection of more fit males.

330px-Tribolium_castaneumAll good in theory. But is this empirically true? A new paper in Nature suggests it is. At least for the red flour beetle. The paper is titled Sexual selection protects against extinction. Recall that asexual lineages seem to be more likely to go extinct when one examines them with comparative phylogenetic methods (i.e., with in a clade asexual lineages are invariably young in evolutionary time scales, implying that they do not last long).

41SSqWzJIGL._SY344_BO1,204,203,200_ The adapted figure above shows the experimental results which support the proposition that sexual selection purge deleterious alleles. These experiments ran for ~10 years, and consisted of varying primary treatments which differed in terms of intensity of sexual selection in red flour beetles. In panel A you see a comparison between a male and female skewed sex ratios (9:1), red and blue lines respectively. In a male skewed ratio the males are competing for the attention of a few females, and in a female skewed ratio the situation is the reverse. To test for the fitness of the lineages the researchers took the outcomes of long term breeding in these scenarios (fixing the effective population sizes to be comparable) and then forced them to engage in sibling matings. This would “expose” deleterious recessive alleles because of the nature of inbreeding. As is evident above in the female skewed (blue) lineages there is a much quicker extinction rate as inbreeding begins to expose deleterious alleles in the recessive phenotype. In the second set of experiments the authors compared polyandrous (5 males to 1 female) and monogamous lineages. Again, you see that the polyandrous lineages are much more robust to inbreeding, suggesting that sexual selection driving reproductive skew correlated with mutational load is resulting in a lower population wide genetic load.

41czavSUnNL._SY344_BO1,204,203,200_ There are many arguments for why sex persists (though many of them do not seem to directly address the cost of males, since sexuality does not necessarily entail two different sexes where one does not bear offspring or produce eggs). I don’t think that sexual selection needs to be the explanation as such. Additionally, I think there is the problem that extremely skewed sex ratios as is the case above does not seem biologically plausible in many organisms. In big and slow breeding organisms, such as humans, extreme sex ratios are not typically common. It seems unlikely that sex is maintained purely through purging of deleterious alleles via a “good genes” model of sexual selection. But then to truly test this hypothesis it strikes me that some sequencing methodologies could be brought to bear. For example, do individuals with lower load have a higher realized reproductive fitness? This is entirely testable.

Citation: Lumley, Alyson J., et al. “Sexual selection protects against extinction.” Nature (2015).

• Category: Science • Tags: Sex, Sexual Selection 
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Pioneer_plaque We live in a Whiggish age, and on the Left side of the cultural spectrum there is always the demand for the eternal revolution to march forward and transform our society. Three years ago The New Republic published America’s Next Great Civil Rights Struggle, about transgender individuals. In some ways this makes logical sense, as arguably transgender individuals are the next step beyond equal rights for people of homosexual orientation. I have argued before that scope of “social justice” in terms of personal liberation is becoming narrower, and narrower. Expanding the possibilities for women affected half the population. Expanding the possibilities for African Americans affected 10 percent of the population. Expanding the possibilities for gays and lesbians affected somewhere around 5 percent of the population, being generous with the numbers. Transgender individuals are an even smaller proportion of the population, and yet a great deal of cultural energy is now going into addressing their concerns. Some of them, such as basic safety and protection from violence, are common sense matters of human decency which I believe most people can support wholeheartedly. But other aspects of acknowledging the transgender experience require a re-evaluation of basic elements of language and social expectation. Not only will many Americans resist, I believe it is a quixotic enterprise, because though a small proportion of individuals express disjunction between their sex and gender, the vast majority do not. Many social norms did not emerge to impose a binary of sexuality, but reflect it.

To illustrate the problems, an NPR piece The End Of Gender?

To chronicle her adventures in gender-neutral parenting, Arwyn Daemyir writes a blog called Raising My Boychick. She describes herself as “a walking contradiction: knitting feminist fulltime parent, Wiccan science-minded woowoo massage therapist, queer-identified male-partnered monogamist, body-loving healthy-eating fat chick, unmedicated mostly-stable bipolar.”

She describes her boychick, born in March 2007, as a “male-assigned at birth — and so far apparently comfortable with that assignment, white, currently able-bodied, congenitally hypothyroid, cosleeper, former breastfed toddler, elimination communication graduate, sling baby and early walker, trial and terror, cliched light of our life, and impetus for the blog. Odds are good he will be the most privileged of persons: a middle class, able bodied, cisgender, straight, white male.”

The adjective cisgender — as opposed to transgender — describes someone who is at peace with the gender he or she was assigned at birth.

Daemyir lives in Portland, Ore. She and her straight male partner are expecting another baby in September.

For Daemyir, gender-neutral parenting is not an attempt to eliminate gender, “because the 70s’-era gender neutral parenting movement proved that’s not possible.”

The last is important. The late 1960s to early 1980s witnessed a great deal of experimentation in the counter-culture movements. In some ways the bounds of sexuality were more expansive than today, witness the sexual abuse at Horace Mann, which many admit were less shocking during that period when people were experimenting with the bounds of sexual behavior. Or, the support for pedophilia in some mainstream circles (see the arc of the expulsion of NAMBLA from association with mainstream gay rights organizations). We have plenty of empirical evidence of what doesn’t, and does, work (there was a similar period of experimentation in the early 19th century in Upstate New York, out of which came the Mormons).

Pieces which espouse the end of gender reflect a cultural mood in certain sectors, no more. The reality is that there are strong biological parameters to human cultural expression. There are differing levels of sexual egalitarianism across societies, but some sexual differentiation is obligate by dint of our biology, which expresses in concrete physical terms (pregnancy) as well as behavioral tendencies. Culture can both increase and dampen these differences, but they can not abolish them. A small number of individual are transgender in a straightforward sense. But the vast majority of humans don’t benefit from lack of acknowledgement of their biological realities. Or least the tendency to pretend as if the dominant modes of expression are just one among equal modes.

300px-Tiresias_striking_the_snakes A genuinely radical take on the idea of gender and sex would be to attack the biology directly: the Tiresias option, where individuals can choose their sex facultatively over their lifetime. That is, one could “experiment” with living a life as the opposite sex, and then also have the option of “changing back.” This model does not negate biology, but embraces it. And, I think it would be a much more genuine challenge to the two sex binary which is universal in human societies.* But I see few radicals talking about this realistically.

* I am aware of “third sexes/genders” in many cultures, but these are marginal to the two primaries.

• Category: Science • Tags: Sex 
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I was recently reading Sexual Behavior in the United States: Results from a National Probability Sample of Men and Women Ages 14–94. At N ~ 6,000 it’s a large sample of American sexual behavior around 2010. There was one descriptive result which I thought was interesting, though not surprising. Before the age of 25 it seems that women are more likely to have sex in a given year than an equivalent age man. After the age of 25 this starts to reverse, and men are more likely to be having sexual intercourse in a given year. The dynamics underlying this phenomenon seem to be easily subject to various speculations, so I’ll leave that to readers. Rather, I offer the graph (data drawn from the paper linked above):

• Category: Science • Tags: Data, Data Analysis, Sex 
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My post below elicited this response:

Here are a couple of cases which seem to defy easy classification.
A “chimera”. This is a person who has cells derived from two zygotes. It can happen if two fertilized eggs merge very early in development. The individual may appear entirely normal (there may be chimeras reading this who are unaware of their condition); but the cells in their body will come from two quite distinct origins. If the original zygotes were male and female, then the adult individual will have some cells in their body with the XY (male) chromosomes, and others with the XX (female) chromosomes. There may be no external sexual ambiguity as long as the sex organs all come from the one lineage; in general all kinds of sexual ambiguity might arise.

Second case; more common (though still unusual) is where an individual is genetically of one gender, and phenotypically of the other. This can be either an XX individual who develops with external male genitalia; or XY who develops with female genitalia. This is usually caused (I think) by excess or deficit of the appropriate hormones during fetal development.

For most people, gender is unambiguous. But there is no sharp dividing line or easy way of classifying that covers all individuals.

The examples I’m considering are entirely independent of psychology or choice. They are real physical conditions in which the conventional physical basis for determining gender becomes ambiguous.

First, I didn’t really need that lecture. My post actually linked to androgen insensitivity syndrome. And I’m aware of other forms of inter/ambiguous sex, as noted above. And of course there are species where sex, as opposed to gender, is more fluid and facultative. I’m aware of all that.

Rather, the issue hinges around the assertion that we must have a category system which covers all individuals. If that’s your criterion than the vast majority of scientific concepts are social constructions with imperfect mapping. So, for example, people should legitimately attack the Endangered Species Act as being grounded in fuzzy science (the species concept). We shouldn’t talk of planets and asteroids, where do you draw the line? And so forth.

If you assume that sex determination occurs through inheritance of sex chromosomes, the distribution above for sex diagnostic characteristics should be expected. You can generate a synthetic metric, and males and females should cluster together. But you’re going to have developmental and chromosomal abnormalities, with the latter leading to a third mode in between the male and female.

I wouldn’t make much of this argument, but I’ve encountered a rejection of the sex concept from biologists. Which begs the question: what the hell are evolutionary biologists studying then when it comes to sex? Obviously the last is a silly question, everyone knows what biologists are studying. It doesn’t matter that sex is not a concept which exhibits the perfect clarity and precision of a mathematical proof. It’s about as good as it gets in biology for an abstract category. We aren’t sure how to define life itself! That means biology in its own foundation doesn’t exhibit the stringency some require from sex.

• Category: Science • Tags: Gender, Sex 
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The New York Times has an article up on a new I.O.C. ruling on who can compete as a woman. Basically they look at testosterone levels. This seems a different tack than cases where women were banned from competing as women because they had a male karytoptype (AIS). This article came on my radar because I had already read this op-ed from about a week ago, You Say You’re a Woman? That Should Be Enough. This sentence jumped out at me:

Second, when it comes to sex, sports authorities should acknowledge that while science can offer evidence, it cannot dictate what evidence we should use. Scientifically, there is no clear or objective way to draw a bright line between male and female.

What do people think of this assertion? I’m aware of intersex individuals. But if we start to assert that dioecy is just a “social construct” then let’s revisit species concepts. I’m sure there are some farmers and loggers who might assert that one can’t draw objective bright lines between populations. Distinctions between male and female in most species is much more clear and distinct than various taxonomic categories.

• Category: Science • Tags: Bioethics, Sex 
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I’ve been commenting on internet porn for nearly 10 years. One reason is that as someone who graduated high school in the spring of 1995 I’m probably in the very last cohort of American males for whom pornography was an item subject to scarcity. Those who are 2-3 years younger already experienced a totally different world. The furtive quest to find a friend of a friend whose dad was less than vigilant in guarding his porn stash was a rite of adolescent male passage in my cohort, but would seem totally laughable by 1997. There’s a lot of commentary on the effect of porn on society and sexual relations, but from what I can tell nothing much has really changed between then and now, except that hardcore porn has become harder. Before I see hard data I’m skeptical that American males accept more perversion because of watching porn. Read the Kinsey Reports; farm boys long knew some farm boys lost their virginity to animals.

All this must be kept in mind when reading pieces tinged with moral panic, such as this one in The New York Times, So How Do We Talk About This?, which details the reaction of parents to their children discovering porn. There are few specific elements which strike me as manifestly stupid. For example:

Bonnie, a university administrator in North Carolina with a teenage son and two stepdaughters, realized only after discussing the matter that she and her husband had been sending unintended messages by emphasizing safety and self-protection with the girls and limits with her son.

“Later, we realized how terribly, albeit unconsciously, sexist that was,” she said.

I recently argued to Chris Mooney that the cultural Left in the United States has still not reconciled itself to sex differences,* and that is one of the clear areas where liberal ideology trumps the science. Now, I can understand the reasons why some might object to the idea that males and females have different cognitive profiles, which might result in differential representation in different fields (e.g., more men in engineering, more women in medicine). But at some point if you are going to hold the position that the Left has made its peace with human nature you have to be open to sex differences, there are compelling biological reasons why a species which exhibits physical sexual dimorphism would also exhibit behavioral sexual dimorphism. Do people truly entertain the idea that adolescent boys and girls react to visual pornography in the same way, if not for social or cultural pressures? If so, that’s really stupid, and reiterates why I refuse to identify myself as a liberal.

But more generally, I wonder if our perception of the development and maturation of children and their encounters with sexuality is ahistorical. From what I have read before the modern era private rooms were not common. Peasants lived in an open house, with children sleeping in a corner, and parents in the other corner. During winter the family might sleep together in one bed. Obviously sexual relations did occur, and there were a set of norms which governed this so that everyone could continue on their way.

My point is that for most of human history children have had some exposure to sex, whether because of lack of privacy in the home, or through observation of farm animals. I personally do not think this is optimal. For most of human history infant mortality was rather high too. But, it does remind us that the rapid rise of awareness of sexually explicit material by children younger than puberty, or just on the cusp, is not a totally novel phenomenon.

* Actually, Althouse’s Rule, you can discuss sex differences if it is unflattering to males.

• Category: Science • Tags: Culture, Porn, Sex 
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The magazine Foreign Policy recently had a “sex” issue out. This issue is particularly famous for Mona Eltahaway’s jeremiad against Arab male culture, and their attitudes toward women. Over at Charli Carpenter expresses some concern that the issue seemed so singularly focused on Arabs, as if women’s rights is a problem with particular salience for Arab Muslims. As it is, she admits that as a matter of truth it may be so, but still has qualms about essentialization.

Now, I like to think in terms of distributions, and don’t find essentialization particularly useful on a fundamental level. But, my personal observation is that the term ‘essentialization’ tends to be used when there are phenomena brought to light which make people uncomfortable. For example, I rarely hear essentialization being nearly a great a problem when talking about Republicans or Western Christian conservatives.

But it does make to wonder: how bad are Arab countries when it comes to women’s rights? Let’s look at the World Values Survey. There are two questions in the survey which have a lot of normative baggage:

– If jobs are scarce: men should have more right to a job than women

– It is an essential characteristic of democracy that women have the same rights as men

As a matter of pedantic accuracy obviously it is not an essential characteristic of democracy that women and men have the same rights. Ancient Athens and America before the 1920s are generally considered democracies. But, the question is a rough gauge of attitudes toward male and female legal equality. I relabeled these questions as “economic” and “legal” equality. They are given as percentages, so I converted the categories into numbers, and then took the weighted average. These two indices measure the two dimensions of equality, with higher values favoring more equality. Below I generated a scatterplot showing the relationship between the two (naturally positively correlated). I’ve also attached table data after the figure.

Democracy: Women have the same rights as men.
Country Not essential 2 3 4 5 6 7 8 9 Essential
Malaysia 4% 2% 2% 5% 18% 14% 14% 15% 12% 15%
Thailand 2% 1% 2% 2% 11% 10% 14% 17% 18% 22%
Iraq 10% 5% 6% 7% 15% 7% 8% 9% 7% 27%
Rwanda 2% 2% 3% 4% 9% 8% 9% 14% 19% 32%
Iran 5% 2% 3% 3% 9% 7% 8% 13% 14% 35%
South Africa 2% 3% 2% 3% 7% 7% 9% 12% 15% 41%
South Korea 4% 1% 1% 2% 9% 6% 7% 13% 15% 42%
Japan 2% 1% 1% 1% 4% 12% 8% 15% 13% 43%
Zambia 3% 3% 4% 4% 10% 9% 5% 7% 11% 45%
Morocco 8% 2% 4% 4% 10% 2% 5% 10% 10% 45%
Indonesia 6% 1% 1% 2% 7% 4% 8% 12% 11% 47%
Ukraine 1% 1% 1% 3% 6% 7% 8% 11% 14% 48%
Ghana 3% 2% 2% 1% 4% 4% 4% 12% 19% 49%
France 1% 1% 1% 2% 6% 3% 4% 15% 17% 50%
Moldova 1% 0% 1% 1% 6% 4% 7% 10% 20% 51%
Jordan 11% 3% 3% 4% 6% 5% 4% 9% 6% 51%
Egypt 7% 1% 3% 4% 9% 6% 7% 8% 4% 51%
Mali 8% 2% 1% 3% 9% 4% 8% 6% 9% 51%
Ethiopia 1% 0% 0% 0% 1% 1% 9% 11% 25% 52%
Chile 2% 1% 1% 1% 8% 6% 6% 10% 10% 55%
Serbia 3% 1% 1% 2% 4% 4% 5% 11% 13% 56%
Brazil 4% 1% 2% 2% 6% 3% 5% 9% 13% 56%
Turkey 1% 1% 1% 2% 4% 6% 8% 13% 9% 57%
United States 2% 1% 1% 1% 11% 4% 5% 8% 11% 57%
Bulgaria 2% 2% 2% 2% 4% 3% 3% 9% 16% 57%
Mexico 8% 2% 2% 1% 5% 3% 3% 9% 11% 57%
Burkina Faso 4% 1% 2% 2% 7% 4% 5% 6% 9% 59%
Taiwan 0% 0% 0% 1% 3% 5% 6% 11% 15% 59%
Netherlands 1% 0% 0% 1% 2% 3% 5% 12% 17% 60%
Uruguay 2% 0% 1% 1% 8% 5% 6% 9% 9% 60%
China 2% 1% 1% 0% 2% 3% 3% 8% 19% 61%
Peru 2% 1% 1% 1% 4% 3% 4% 10% 14% 61%
Great Britain 1% 1% 1% 1% 3% 3% 3% 12% 14% 62%
Slovenia 2% 1% 1% 1% 4% 2% 4% 11% 12% 63%
Spain 1% 0% 1% 1% 5% 4% 7% 7% 11% 63%
Finland 0% 0% 1% 1% 2% 2% 4% 10% 17% 64%
Trinidad and Tobago 3% 1% 1% 1% 4% 3% 3% 8% 12% 65%
Poland 1% 0% 1% 2% 4% 2% 4% 10% 11% 65%
Canada 1% 1% 1% 1% 4% 2% 3% 11% 12% 66%
Viet Nam 0% 0% 0% 0% 1% 3% 4% 10% 14% 68%
Cyprus 2% 1% 1% 1% 5% 3% 3% 7% 8% 69%
Georgia 1% 0% 0% 1% 3% 2% 5% 9% 9% 70%
Norway 1% 0% 1% 1% 2% 1% 2% 9% 15% 70%
Australia 2% 1% 0% 0% 4% 1% 4% 7% 10% 72%
Romania 0% 0% 0% 1% 3% 1% 3% 7% 11% 74%
Russian Federation 2% 0% 1% 1% 3% 2% 3% 6% 9% 74%
Germany 1% 1% 1% 1% 2% 4% 4% 6% 7% 74%
Switzerland 2% 0% 1% 0% 2% 1% 2% 4% 10% 78%
Argentina 1% 0% 0% 0% 3% 1% 3% 5% 6% 80%
Sweden 0% 0% 0% 0% 0% 0% 0% 0% 16% 84%
Andorra 2% 0% 0% 0% 2% 1% 1% 3% 5% 88%
Men more right to jobs
Country Agree Neither Disagree
Egypt 89% 7% 4%
Jordan 88% 4% 8%
Iraq 84% 0% 16%
Iran 69% 14% 17%
Mali 62% 15% 23%
Indonesia 55% 8% 36%
Ghana 54% 9% 37%
Turkey 53% 17% 30%
Georgia 53% 21% 26%
Burkina Faso 52% 13% 35%
India 51% 28% 21%
Morocco 51% 16% 33%
Malaysia 49% 36% 15%
Taiwan 44% 20% 36%
China 42% 25% 33%
Viet Nam 41% 22% 38%
Moldova 38% 23% 39%
South Africa 37% 13% 50%
Russian Federation 37% 20% 44%
South Korea 37% 37% 26%
Cyprus 37% 17% 46%
Romania 35% 24% 41%
Zambia 34% 15% 52%
Ukraine 33% 23% 45%
Thailand 32% 27% 41%
Poland 31% 18% 51%
Chile 30% 24% 46%
Argentina 28% 12% 60%
Japan 27% 55% 18%
Mexico 25% 7% 68%
Trinidad and Tobago 25% 9% 66%
Rwanda 25% 11% 64%
Bulgaria 24% 23% 53%
Brazil 22% 14% 64%
Switzerland 22% 15% 63%
Italy 22% 19% 59%
Uruguay 22% 9% 69%
Hong Kong 22% 34% 44%
Guatemala 19% 9% 72%
France 18% 8% 74%
Germany 18% 15% 67%
Peru 18% 10% 73%
Spain 17% 7% 76%
Great Britain 16% 8% 76%
Canada 14% 8% 78%
Australia 14% 21% 65%
Slovenia 14% 13% 74%
Netherlands 13% 6% 81%
Serbia 13% 24% 63%
Finland 10% 9% 82%
New Zealand 8% 19% 73%
United States 7% 27% 66%
Norway 7% 5% 89%
Ethiopia 6% 8% 86%
Andorra 4% 6% 90%
Sweden 2% 4% 94%



• Category: Science • Tags: Arabs, Data Analysis, Sex 
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It’s a fun fact that there are an order of magnitude more bacterial cells in your body than your own cells. Not only that, it’s well known that we wouldn’t flourish, let alone survive, without our gut “flora,” which digest material which would otherwise pass through out system. Not only are microbes good for us, but they’re also bad for us. The evolutionary flexibility of microbial pathogens is one of the major arguments for why sex exists among multiceullar species: it allows them to adapt to rapidly fluctuating disease pressures. Therefore, obviously the ecology of multicellular organisms’ microbial flora is essential to properly characterize. One element of the project involves genomics. This is not so easy for microbes because we don’t have the reference sequences of most of these organisms. We rely mostly on species which are easy to culture, and that does not include most lineages in the wild. That being said, there are workarounds, such as looking at the 16S rNA sequence, which is strongly constrained in bacterial lineages (i.e., it can serve as a “clock” to measure divergence of very deeply separated lineages).

With that, a new paper, Promiscuity in mice is associated with increased vaginal bacterial diversity:

Differences in the number of sexual partners (i.e., mating system) have the potential to exert a strong influence on the bacterial communities present in reproductive structures like the vagina. Because this structure serves as a conduit for gametes, bacteria present there may have a pronounced, direct effect on host reproductive success. As a first step towards the identification of the relationship between sexual behavior and potentially pathogenic bacterial communities inhabiting vital reproductive structures, as well as their potential effects on fitness, I sought to quantify differences in bacterial diversity in a promiscuous and monogamous mammal species . To accomplish this, I used two sympatric species of Peromyscus rodents—Peromyscus californicus and Peromyscus maniculatus that differ with regard to the number of sexual partners per individual to test the hypothesis that bacterial diversity should be greater in the promiscuous P. maniculatus relative to the monogamous P. californicus. As predicted, phylogenetically controlled and operational taxonomic unit-based indices of bacterial diversity indicated that diversity is greater in the promiscuous species. These results provide important new insights into the effects of mating system on bacterial diversity in free-living vertebrates, and may suggest a potential cost of promiscuity.

These two species are sympatric and exhibit very different behaviors. Sympatric means that they aren’t geographically separated, so they are subject to the same environmental conditions. Rather, their distinctions on the species level seem to be due to behavior, in this case, the number of sexual partners of females. This then is a nice test for assessing the relationship of microbial diversity in the vagina as a function of partners. I suspect a priori you’d expect a positive relationship. And that’s what the author found. He presented a diversity index, but the results are rather intelligible visually. You can see clearly that the promiscuous species is characterized by a greater range of species richness than the monogamous one.

There are some studies of metagenomics of bacterial communities in humans. But to my knowledge it doesn’t look like there are any which have attempted to correlate number of sexual partners to diversity of vaginal flora. This is possible very important as a long term issue. The evolutionary biologist Paul Ewald has been reporting that there is a connection between history of infection and many late in life diseases, such as cancers. Mike Snyder had Stanford has been tracking his own biomarkers in extensive detail for several years, and has indicated that his own onset of Type II Diabetes was probably triggered by an earlier infection. These inferences were only possible because of his extremely rich personal data set, part of a broader project in his laboratory. But, it might give us a window into the more precise individual etiologies of diseases.

Citation: Naturwissenschaften. 2011 Nov;98(11):951-60. Epub 2011 Oct 1

Image Credit: Wikipedia, Wikipedia.

• Category: Science • Tags: Anthropology, Genomics, Medicine, Sex 
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ResearchBlogging.orgThe Pith: The primary reason for the pervasiveness of sex among complex organisms is to maintain genomic integrity, not to increase genetic variation..

I just read a very strange article in the journal Evolution, Sex reduces genetic variation. In it the authors argue that contrary to conventional wisdom and evolutionary orthodoxy the rationale for the prevalence of sex amongst eukaryotic organisms is not maintenance of genetic variation, but rather a constraint upon genetic variation! This is a very peculiar view, and as someone not immersed in the literature on sex totally surprising to me.

The standard model is simple: sex allows organisms to swap genetic material and generate new combinations. This is at a particular premium for large, complex, and slow-breeding lineages, as is the norm amongst eukaryotes. In contrast, bacteria and their ilk have huge population sizes to draw from, and are quite literally protean in their ability to shift strategies to climb whatever adaptive landscape nature throws at them. Carl has a nice review of a paper in Science which reported just this finding in keeping with expectation. Increase the pathogen pressure, and eukaryotes which exchange genes marginalize those which do not because they can dodge the punches that their evolutionary adversaries throw in their direction.

The authors of the Evolution paper don’t deny that this is part of the story. Rather, they propose that this is a secondary part of the story. They acknowledge the power of microevolutionary pressure to modulate the basic genomic template on the margins where variation at the level of the gene is the target of selection. But, their contention is that by and large sexual reproduction’s primary value, which overrides the two-fold cost of sex, is to maintain the integrity of the genome of the parent. In other words, at the scale of the whole genome sex facilitates homogenization and stabilization of the basic ancestral template.

There seem two primary issues. Both of which I can not evaluate with any degree of confidence because of my lack of deep familiarity with the literature (no, I don’t check every single citation in a paper before reviewing it!). First, there’s the empirical issue, where the authors argue that in contrast to expectation asexual lineages are not more homogeneous or less diverse than sexual ones. The assumption has been contradicted, the authors assert, by new genomic techniques, though they grant that more work needs to be done. This very diversity and tendency to radiate in all directions argues against the stability of these asexual lineages. They also suggest that cancer, an asexual process, is a proper analogy for the riotous directional diversity of reproduction without sex (mitosis vs. meiosis in this case).

But perhaps more importantly within the paper there are some theoretical considerations. They argue that there is evidence that sex, as defined specifically by the process of meoisis and fusion of gametes, has a role to play in encouraging DNA repair, preventing the replication of chromosomal abnormalities, and dampening epigenetic variation. Let me quote their abstract:

The common thread among many of these disciplines is that sex acts like a coarse filter, weeding out major changes, such as chromosomal rearrangements (that are almost always deleterious), but letting minor variation, such as changes at the nucleotide or gene level (that are often neutral), flow through the sexual sieve. Sex acts as a constraint on genomic and epigenetic variation, thereby limiting adaptive evolution. The diverse reasons for sex reducing genetic variation (especially at the genome level) and slowing down evolution may provide a sufficient benefit to offset the famed costs of sex.

Since the authors grant that sex does provide benefits in maintaining additive genetic variation at the level of the gene, it stands to reason that the mainstream research program should yield positive results. Rather, the thrust of this paper seems to be that this might be a case of looking for lost keys underneath the lamp-light. Ultimately I think this argument ties back into the unit of selection argument, though they seem to avoid that explosive issue in any explicit sense.

I’m somewhat a loss to really tote up the plausibility of this heterodox proposition with any clarity of conscience or confidence. Rather, I am curious as to what readers think? This did get published in Evolution, which from what I recall was created to midwife the Neo-Darwinian Synthesis. But I also know there are people who read this weblog who know quite a bit about DNA repair or epigenetics….

Citation: Gorelick R, & Heng HH (2011). Sex reduces genetic variation: a multidisciplinary review. Evolution; international journal of organic evolution, 65 (4), 1088-98 PMID: 21091466

• Category: Science • Tags: Evolution, Genetics, Sex 
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Male_and_female_pheasantA quintessentially sexy topic in biology is the origin of sex. Not only are biologists interested in it, but so is the public. Of Matt Ridley’s older books it is predictable that The Red Queen has the highest rank on Amazon. We humans have a fixation on sex, both in our public norms and our private actions. Why?

Because without a fixation on sex we would not be here. Celibates do not inherit the earth biologically. This answer emerges naturally from a Darwinian framework. And yet more deeply still: why sex for reproduction? Here I allude to the famous two-fold cost of sex. In dioecious species you have males and females, and males do not directly produce offspring. The increase of the population is constrained by the number of females in such lineages (male gametes are cheap). There is no such limitation in asexual lineages, where every individual can contribute to reproductive “primary production.” Additionally, the mating dance is another cost of sex. Individuals expend time and energy seeking out mates, and may have to compete and display for the attention of all. Why bother? The answer on the broadest-scale seems to be variation. Variation in selective pressures, and variation in genes. Sex famously results in the shuffling of genetic permutations through recombination and segregation. In a world of protean change where one’s genes are critical to giving one the edge of fitness this constant flux of combinations results in more long term robusticity. What clones gain in proximate perfection, they lose when judged by the vicissitudes of the pressures of adaptation. In the present they flourish, but in the future they perish. Sex is the tortoise, clonal reproduction is the hare.

And yet science is more than just coarse generalities; biology especially so. The details of how sex emerges ad persists still remains to be fleshed out. The second volume of W. D. Hamilton’s collected papers, Narrow Roads of Gene Land, is the largest. Mostly because it was not edited appropriately (he died before it could be). But also perhaps because it is the volume most fixated upon the origin and persistence of sex, which is a broad and expansive topic.

A new paper in Nature tackles sex through experimental evolution. In may ways the answer it offers to the question of sex is old-fashioned and straightforward. Higher rates of sex evolve in spatially heterogeneous environments:

The evolution and maintenance of sexual reproduction has puzzled biologists for decades…Although this field is rich in hypotheses…experimental evidence is scarce. Some important experiments have demonstrated differences in evolutionary rates between sexual and asexual populations…other experiments have documented evolutionary changes in phenomena related to genetic mixing, such as recombination…and selfing…However, direct experiments of the evolution of sex within populations are extremely rare…Here we use the rotifer, Brachionus calyciflorus, which is capable of both sexual and asexual reproduction, to test recent theory…predicting that there is more opportunity for sex to evolve in spatially heterogeneous environments. Replicated experimental populations of rotifers were maintained in homogeneous environments, composed of either high- or low-quality food habitats, or in heterogeneous environments that consisted of a mix of the two habitats. For populations maintained in either type of homogeneous environment, the rate of sex evolves rapidly towards zero. In contrast, higher rates of sex evolve in populations experiencing spatially heterogeneous environments. The data indicate that the higher level of sex observed under heterogeneity is not due to sex being less costly or selection against sex being less efficient; rather sex is sufficiently advantageous in heterogeneous environments to overwhelm its inherent costs…Counter to some alternative theories…for the evolution of sex, there is no evidence that genetic drift plays any part in the evolution of sex in these populations.

sexthee0I’m not too familiar with B. calyciflorus, but it seems that it is facultatively sexual. Given the appropriate environmental cues (high densities, quorum sensing) some females can produce offspring which can have sex. The image to the left is from supplements, and shows the potential life cycles of this organism. Amictic in this context means individuals who produce diploid eggs which can not be fertilized. These eggs give rise to females parthenogenetically. The divergence between the two is when amictic females produce mictic females. These females produce eggs which are haploid, and can be fertilized. Those which are fertilized produce amictic females. Those which are not fertilized produce males.

Apparently in this species a propensity toward producing mictic females under stress conditions is heritable. Therefore, a propensity toward greater or less sexuality is heritable. There are within a give population both sexually and asexually reproducing individuals. Unlike humans, or bdelloid rotifers, B. calyciflorus is not locked into a particular style of reproduction, but can shift its strategy conditionally upon changes in the environment. Therefore it is an ideal organism upon with to test theories of the origin and maintenance of sex. For them sexual reproduction is a option, and insight can be gained by exploring the conditions under which that option is exercised.

The two parameters they shifted in this experiment was the quality of nutrition (high vs. low) and the rate of migration within a set of populations (~1% vs. ~10%), for which the N was ~10,000. There were two treatments:

– Homogeneous environments of high-quality and low-quality food

– Heterogeneous environments where high and low-quality food zones existed adjacent to each other with two populations

The populations within these treatments were derived from wild lineages with a relatively high proportion of sexually reproducing individuals. Previous work confirmed that sexual reproduction, or propensity to reproduce sexually, was heritable. So if the environment favored sexuality or asexuality the frequencies should change over time as there is heritable variation for the trait within the rotifer populations. In other words, sex could be a target of natural selection.

In the figure below you see two panels. The first, a, shows populations subject to 10% transfer per generation. The second, b, 1% transfer per generation. This is the migration parameter, which is an order of magnitude higher in the first than the second panel. Triangles are heterogeneous environments, while circles represent homogeneous ones. The x-axis is the time parameter. At weak 14, the vertical line, all populations were mixed together and reassigned.


It’s immediately obvious that the proportion of sexually reproducing organisms is dropping rapidly in the homogeneous environments vis-a-vis the heterogeneous environments. Interestingly the shift in the migration parameter does not have much of an effect. In the first 14 weeks the propensity for sex drops even in the heterogeneous environment from the wild-type baseline. But once the lineages are mixed together and allowed to evolve from their laboratory baseline you see that sex has a positive benefit in the heterogeneous environment, shifting back up to an equilibrium state.

The authors note that the equilibrium propensity for sexual reproduction of rotifers seems higher in the wild than in the laboratory. That doesn’t seem so surprising, presumably there are many more variables which shift in the wild than in the laboratory, where conditions are consciously controlled to tease out independent predictors. The most common model for the maintenance of sex today in terms of the ultimate driver is host-pathogen co-evolution. Sex being the only way that slow-reproducing complex organisms can keep up with prolific asexual pathogens. The rotifers may be subject to this dynamic, as well as spatial heterogeneity. It does not seem to me that nature should be in the business of enforcing a monopoly on the supply of proteanism.

The_Madonna_in_SorrowWhat does this mean in the long-term? Well, it may be that sex, and males, are adaptations to an unpredictable and wild world whose caprice we can not account for. As humanity, or perhaps more generally sentient beings, begin to control nature and buffer themselves artificially from the volatile fluctuations, will we need sex and males? At the end of history when conditions are stable, and all that is before us is the terminus of heat death, perhaps what awaits us are a series of mindless and boring clonal lineages, perfectly adapted to turning nutrients into flesh, generation to generation.

Citation: Becks L, & Agrawal AF (2010). Higher rates of sex evolve in spatially heterogeneous environments. Nature PMID: 20944628

Image Credit: ChrisO, Wikimedia Commons

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Eric Michael Johnson has a fascinating piece in Psychology Today, Sex, Evolution, and the Case of the Missing Polygamists. I want to spotlight a few paragraphs:

Keep in mind that in terms of interpreting such genetic evidence we are of necessity confined to a fairly recent time depth (and remember, by “recent” someone like me means the last 10,000 years or so). For this time period multiple lines of evidence do indeed suggest that humans were moderately to extremely polygynous and that women were moving between groups more than men were.

However, humans have been around for far longer than 10,000 years, with conservative estimates placing the emergence of modern Homo sapiens at about 200,000 years ago. A genetic record extending back 10,000 years is remarkable, but it’s essentially adding only three more novels to our existing timeline. There is also something very important to consider that dramatically influenced human behavior within the last 10,000 years: the invention of agriculture. Prior to about 12,000 years ago all humans were hunter-gatherers and lived a migratory existence. With the advent of farming some human societies began to remain sedentary for the first time in our history. This change had serious impacts on human life and behavior. Just as Alzheimer’s dramatically altered the content of Agatha Christie’s work, so agriculture radically transformed human society and, by consequence, sexual behavior.

Cultural norms can be protean, but we humans have short time horizons. One model of human history which I am convinced of is that many “traditional” social arrangements which we view as old-fashioned and timeless are actually innovations which arose during the shift to agriculture, which allowed for the birth of complex societies in many regions of the world. Not only did customary norms shift, but the nature of customary norms varied across the society as class stratification emerged. Solemnized marriage between elite lineages may seem normative through much of the history of civilization, but this was obviously going to be much less of a factor for most of the peasantry.

Both hunger-gatherers and farmers lived on the Malthusian margin. Excess population swallowed any gains in economic productivity. But obviously the change in population density and mode of production resulted in qualitative differences between the two classes of societies, especially for agriculture elites who could much more efficiently extract rents to sustain a more affluent lifestyle of leisure. Now much of the world is moving to the next stage: agricultural values combined with the reality of post-Malthusian consumer societies. Institutionalized marriage between a man and a woman in classic a bourgeois sense in the modern West is a hybrid of of values. No longer so much a bond between family lineages, as it was for pre-modern elites, nor is it an ephemeral and common law arrangement as it may have been for the masses (in part because of shorter life expectancies).

Finally we should look past the West, and see that changes are occurring all the across the world as cultures are generating values “mash-ups.” Over the 20th century in South Asia “Sanskritization” has taken hold and practices such as dowry which were normative among upper class groups have now spread throughout the culture. The decline of matrilineal social structures in parts of Southern India in favor of more conventional national implicit or explicit patrlineage is part of the horizontal homogenization through space which is concomitant with the vertical integration of values across class lines. Similarly, the rise of “Islamic Orthodoxy” and adherence to sharia can only exist in a regime of economic surplus, as punctilious attention to religious law is often not possible for families on the boundary between subsistence and starvation.

Our “software” is in many ways a compound of deep evolutionarily encoded instincts, along with more recently crystallized norms and values which “have always been.” But we’ve gotten a massive upgrade in the “hardware,” and the software is adjusting and expanding.

• Category: History, Science • Tags: Anthropology, Sex 
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Mutations are as you know a double-edged sword. On the one hand mutations are the stuff of evolution; neutral changes on the molecular or phenotypic level are the result of from mutations, as are changes which enhance fitness and so are driven to fixation by positive selection. On the other hand mutations also tend to cause problems. In fact, mutations which are deleterious far outnumber those which are positive. It is much easier to break complex systems which are near a fitness optimum than it is to improve upon them through random chance. In fact a Fisherian geometric analogy of the affect of genes on fitness implies that once a genetic configuration nears an optimum mutations of larger effect have a tendency to decrease fitness. Sometimes environments and selection pressures change radically, and large effect mutations may become needful. But despite their short term necessity these mutations still cause major problems because they disrupt many phenotypes due to pleiotropy.

But much of the playing out of evolutionary dynamics is not so dramatic. Instead of very costly mutations for good or ill, most mutations may be of only minimal negative effect, especially if they are masked because of recessive expression patterns. That is, only when two copies of the mutation are present does all hell break loose. And yet even mutations which exhibit recessive expression tend to generate some drag on the fitness of heterozygotes. And if you sum small values together you can obtain a larger value. This gentle rain of small negative effect mutations can be balanced by natural selection, which weeds does not smile upon less fit individuals who have a higher mutational load. Presumably those with “good genes,” fewer deleterious mutations, will have more offspring than those with “bad genes.” Because mutations accrue from one generation to the next, and, there is sampling variance of deleterious alleles, a certain set of offspring will always be gifted with fewer deleterious mutations than their siblings. This is a genetics of chance. And so the mutation-selection balance is maintained over time, the latter rising to the fore if the former comes to greater prominence.

The above has been a set of logic inferences from premises. Evolution is about the logic of life’s process, but as a natural science its beauty is that it is testable through empirical means. A short report in Science explores mutational load and fitness, and connects it with the ever popular topic of sexual selection, Additive Genetic Breeding Values Correlate with the Load of Partially Deleterious Mutations:

The mutation-selection–balance model predicts most additive genetic variation to arise from numerous mildly deleterious mutations of small effect. Correspondingly, “good genes” models of sexual selection and recent models for the evolution of sex are built on the assumption that mutational loads and breeding values for fitness-related traits are correlated. In support of this concept, inbreeding depression was negatively genetically correlated with breeding values for traits under natural and sexual selection in the weevil Callosobruchus maculatus. The correlations were stronger in males and strongest for condition. These results confirm the role of existing, partially recessive mutations in maintaining additive genetic variation in outbred populations, reveal the nature of good genes under sexual selection, and show how sexual selection can offset the cost of sex.

mutAdditive genetic variance just refers to the variation of genes which affect the phenotype by independent and usually small effects which sum together to produce the range of variation of the trait. Imagine for example that the range of variation in height within the population was 10 inches, and that there were 10 genes which varied, and that each gene exhibited co-dominance. One could construct a model where every gene pair could add 0, 0.5 or 1 inch to the height independently, so that the maximum height could be constructed by adding 10 inches to the baseline and 1 inch per locus, and the minimum height by adding no inches to the baseline when each locus is homozygous for null alleles.

Mutations can be conceived of in the same manner, with each mutation being a new variant which changes trait value. Even if most of the impact of a mutation is masked there is a small effect in the heterozygote state, and this may serve as a fitness drag. The range in mutational load can then naturally be analogized to additive genetic variance, in this case the trait under consideration ultimately being fitness, mediated through life history and morphological phenotypes.

In this report they focused primarily on the weevil’s ability to obtain resources and transform those resources into size, which correlates with greater sexual access for males and fecundity for females (ergo, greater fitness). They bred various outbred and inbred lineages across families of these weevils, because these sorts of crosses gauge the impact of masked deleterious alleles, which will manifest in homozygote state more often between related pairs who share mutations than unrelated ones. They found a correlation of -0.24 between inbreeding and breeding value; in other words the more inbred the pair the fewer offspring. The impact of these recessively expressed alleles is mitigated in heterozygous individuals, but because of the non-trivial impact the number of these alleles within an individual will determine its fitness all things equal.

328_892_F1Interestingly when background variables were controlled males tended to show the greatest fitness drag due to inbreeding depression. This would comport with models of sexual selection where males justify their expense (because they can not bear offspring) within the population by serving as the perishable dumping grounds of bad genes. In particular in a polygynous population a few healthy males with good genes could give rise to most of the next generation, and so providing the balance of selection to the background mutational rate.

Of course mating patterns vary between taxa. The more reproductive skew there is, in particular for males, the more recourse selection has every generation to dump deleterious alleles via selection. In contrast monogamous populations will have less power to expunge mutations in this fashion because there is more genetic equality across males, the bad will reproduce along with the good, more or less. Therefore a breeding experiment of weevils may have more limited insight than these authors may wish to admit. Geoffrey Miller’s The Mating Mind attempted to take the insights of sexual selection and develop a model of human evolutionary history, but it does not seem that this theory has swept all before it. Only time will tell, but until then more breeding experiments can’t help but clarify where theory goes wrong or right.

Citation: Tomkins, J., Penrose, M., Greeff, J., & LeBas, N. (2010). Additive Genetic Breeding Values Correlate with the Load of Partially Deleterious Mutations Science, 328 (5980), 892-894 DOI: 10.1126/science.1188013

• Category: Science • Tags: Biology, Genetics, Good Genes, Selection, Sex 
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ResearchBlogging.orgBelow I note that sex matters when it comes to evolution, specifically in the case of how sexual reproduction forces the bits of the genome to be passed back and forth across sexes. In fact, the origin of sex is arguably the most important evolutionary question after the origin of species, and it remains one of the most active areas of research in evolutionary genetics. More specifically the existence of males, who do not bear offspring themselves but seem to be transient gene carriers is a major conundrum. But that’s not the main issue in this post. Let’s take the existence of males as a given. How do sex differences play out in evolutionary terms shaping other phenotypes? Consider Bateman’s principle:

Bateman’s principle is the theory that females almost always invest more energy into producing offspring than males, and therefore in most species females are a limiting resource over which the other sex will compete.

Female ova are energetically more expensive, and scarcer, than male sperm. Additionally, in mammals and other live-bearing species the female invests more time and energy after the point of fertilization but before the young exhibit any modicum of organismic independence (the seahorse being the exception). And, often the female is the “primary caregiver” in the case of species where the offspring require more care after birth. The logic of Bateman’s principle is so obvious when its premises are stated that it easily leads to a proliferation of numerous inferences, and many data are “explained” by its operation (in Mother Nature: Maternal Instincts and How They Shape the Human Species the biological anthroplogist Sarah Hrdy moots the complaint that the principle is applied rather too generously in the context of an important operationally monogamous primate, humans).

But the general behavioral point is rooted in realities of anatomy and life-history; in many dioecious species males and females exhibit a great deal of biological and behavioral dimorphism. But the direction and nature of dimorphism varies. Male gorillas and elephant seals are far larger than females of their kind, but among raptors females are larger. If evolution operated like Newtonian mechanics I assume we wouldn’t be theorizing about why species or sex existed at all, we’d all long ago have evolved toward perfectly adapted spherical cows floating in our own effluvium, a species which is a biosphere.

Going beyond what is skin deep, in humans it is often stated that males are less immunologically robust than females. Some argue that this is due to higher testosterone levels, which produce a weakened immune system. Amtoz Zahavi might argue that this is an illustration of the ‘handicap principle’. Only very robust males who are genetically superior can ‘afford’ the weakened immune system which high testosterone produces, in addition to the various secondary sexual characteristics beloved of film goers. Others would naturally suggest that male behavior is to blame. For example, perhaps males forage or wander about more, all the better to catch bugs, and they pay less attention to cleanliness.

But could there be a deeper evolutionary dynamic rooted in the differential behaviors implied from Bateman’s principle? A new paper in The Proceedings of the Royal Society explores this question with a mathematical model, The evolution of sex-specific immune defences:

Why do males and females often differ in their ability to cope with infection? Beyond physiological mechanisms, it has recently been proposed that life-history theory could explain immune differences from an adaptive point of view in relation to sex-specific reproductive strategies. However, a point often overlooked is that the benefits of immunity, and possibly the costs, depend not only on the host genotype but also on the presence and the phenotype of pathogens. To address this issue we developed an adaptive dynamic model that includes host–pathogen population dynamics and host sexual reproduction. Our model predicts that, although different reproductive strategies, following Bateman’s principle, are not enough to select for different levels of immunity, males and females respond differently to further changes in the characteristics of either sex. For example, if males are more exposed to infection than females (e.g. for behavioural reasons), it is possible to see them evolve lower immunocompetence than females. This and other counterintuitive results highlight the importance of ecological feedbacks in the evolution of immune defences. While this study focuses on sex-specific natural selection, it could easily be extended to include sexual selection and thus help to understand the interplay between the two processes.

The paper is Open Access, so you can read it for yourself. The formalism is heavy going, and the text makes it clear that they stuffed a lot of it into the supplements. You can basically “hum” through the formalism, but I thought I’d lay it out real quick, or at least major aspects.

This shows the birth rate of a given genotype contingent upon population density & proportions of males & females infected with a pathogen


These equations takes the first and nests them into an epidemiological framework which illustrates pathogen transmission (look at the first right hand term in the first two)


And these are the three models that they ran computations with


There are many symbols in those equations which aren’t obvious, and very difficult to keep track of. Here’s the table which shows what the symbols mean….


If you really want to understand the methods and derivations, as well how the details of how they computa e evolutionarily stable strategies, you’ll have to go into the supplements. Let’s just assume that their findings are valid based on their premises.


– They assume no sexual selection
– They assume unlimited male gametes, so total reproductive skew where one male fertilizes all females is possible
– Fecundity is inversely correlated with population density
– Total population growth is ultimately dependent on females, they are the “rate limiting” sex
– Total population growth is proportional to density
– There is no acquired immunity
– There is no evolution of the pathogen in this model

Basically the model is exploring a quantitative trait which exhibits characteristics in relation to resistance of acquiring the pathogen and tolerance of it once the pathogen is acquired. In terms of the “three models,” the first is one where there is resistance to the pathogen, individuals recover from infection and decrease pathogen fitness. The second is one of tolerance, individuals are infected, but may still reproduce while infected. Note that the ability to resist or tolerate infection has a trade off, reduced lifespan (consider some forms of malaria resistance). The third model shows the trade off of tolerance and resistance.

The “pay off” of the paper is that they show that the male evolutionarily stable strategy (ESS), that is, a morph which can not be “invaded” by a mutation, may be one of reduced immune resistance in certain circumstances of high rates of infection. There is an exploration of varying rates of virulence, but there was no counterintuitive finding so I won’t cover that. In any case, here’s the figure:


The text is small, so to clarify:

1) The two panels on the top left are for model 1, and show variation in male and female recovery from infection left to right (resistance)

2) The two panels on the bottom left are for model 2, and show variation in male and female fecundity when infected left to right (tolerance)

3) The four panels on the right are for model 3, and show variation in recovery in the top two panels and fecundity in the bottom two, with male parameters varied on the left and female on the right

The vertical axis on all of the panels are male infection rate, the horizontal the female infection rate. Circled crosses (⊕) indicate regions (delimited by solid lines) where females evolve higher immunocompetence than males. The lighter shading indicates a higher value of the trait at ESS (recovery or fecundity). Note that the two top left panels show a peculiar pattern for males, the sort of counterintuitive finding which the model promises: when infection rates among males are very high their resistance levels drop. Why? The model is constructed so that resistance has a cost, and if they keep getting infected the cost is constant and there’s no benefit as they keep getting sick. In short it is better to breed actively for a short time and die than attempt to fight a losing battle against infection (I can think of possible explanations of behavior and biological resistance in high disease human societies right now). It is at medium levels of infection rates that males develop strong immune systems so that they recover. The bottom right portion of panel which shows variation in male resistance illustrates a trend where high female infection results in reduced immune state in males. Why? The argument is simple; female population drops due to disease result in a massive overall population drop and the epidemiological model is such that lower densities hinder pathogen transmission. So the cost for resistance becomes higher than the upside toward short-term promiscuous breeding in hopes of not catching the disease. Another point that is notable from the panels is that males seem to be more sensitive to variation in infection rates. This makes sense insofar as males exhibit a higher potential variance in reproductive outcomes because of the difference in behavior baked into the model (males have higher intrasexual competition).

One can say much more, as is said in the paper. Since you can read it yourself, I commend you to do so if you are curious. Rather, I would like a step back and ask: what does this “prove?” It does not prove anything, rather, this is a model with many assumptions which still manages to be quite gnarly on a first run through. It is though suggestive in joint consideration with empirical trends which have long been observed. Those empirical trends emerge out of particular dynamics and background parameters, and models can help us formalize and project abstractly around real concrete biological problems. The authors admit their model is simple, but they also assert that they’ve added layers of complexity which is necessary to understand the dynamics in the real world with any level of clarity. In the future they promise to add sexual selection, which I suspect will make a much bigger splash than this.

I’ll let them finish. From their conclusion:

We assessed the selective pressures on a subset of sex-specific traits (recovery rate, reproductive success during infection and lifespan) caused by arbitrary differences between males and females in infection rate or virulence (i.e. disease-induced death rate). In so doing, we covered a range of scenarios whereby sex-specific reproductive traits such as hormones and behaviour could plausibly affect the exposure to infection…r the severity of disease…First, we showed that changes in the traits of either sex affect the selective pressures on both sexes, either in the same or in opposite directions, depending on the ecological feedbacks. For example, an increase in male susceptibility (or exposure) to infection favours the spread of the pathogen in the whole population and therefore tends to select for higher resistance or tolerance in both sexes if the cost of immunity is constitutive. However, above a certain level of exposure, the benefit of rapid recovery in males decreases owing to constant reinfection (we assume no acquired immunity). This selects for lower resistance in males, ultimately leading to the counterintuitive situation where males with higher susceptibility or exposure to infection than females evolve lower immunocompetence…A similar pattern arises if the cost of immunity is facultative, in the form of a trade-off between rate of recovery and relative fecundity during infection (model (iii)): if males happen to be more susceptible (or exposed) to infection than females, they are predicted to evolve a longer infectious period balanced by higher sexual activity during infection than females.

Restif, O., & Amos, W. (2010). The evolution of sex-specific immune defences Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2010.0188

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Earlier I pointed to the possibility of biophysical constraints and parameters in terms of inheritance shaping the local trajectory of evolution. Today Olivia Judson has a nice post [link fixed] on how the existence of two sexes in many species results in a strange metastable tug-of-war in terms of phenotypic evolution:

In sum, the traits that make a “good” male are often different from those that make a “good” female. (Note: I’m only talking about “good” in evolutionary terms. That means a trait that improves your chance of having surviving offspring.) Since many of these traits have a genetic underpinning, male and female genes are thus being sculpted by different forces.

But — and this is the source of the tension I mentioned — males and females are formed from the same underlying set of genes. After all, in humans, whether you’re a boy or a girl comes down to whether you have a Y chromosome or not: boys do, girls don’t. The rest of the genes occur in both sexes.

The X choromosome in mammals spends about 2/3 of its time in females and 1/3 in males.* And obviously the Y is found only in males. But the rest of the genome is found in both males and females. Judson notes that traits which may be attractive in males may not in females, and which may be attractive in females may not in males. There’s a fair amount of evolutionary psychological work in humans in this vein in regards to the heritability of testosterone and estrogen levels in females and males and how it effects the same and opposite sex (in short, there is suggestive data that “sexy” individuals of one sex, those who exhibit strong secondary sexual characteristics, may be prone to having less sexy offspring of the opposite sex).

Of course you can overcome the balancing tug of war; that’s why you have sexual dimorphism in things like size or facial proportion. But these sorts of traits emerge very slowly because of the equilibrium described above, modifier genes and sex-specific gene expression have to slowly engineer around the overwhelming problem that males and females are genetically no different on a sequence level aside from the Y chromosome. Some estimates put the rate of evolutionary change of sexual dimorphism, that is, trait differences between sexes, between 1 and 2 orders of magnitude slower than conventional population level evolution. Ergo, one would expect that sexual dimorphism differences varying across populations have great time depth, and are probably more interspecific than intraspecific (for example, gorillas vs. humans).

There is naturally a whole field devoted to the study of the origin of sex. But whatever its ultimate rationale and utility an evolutionary context, its existence as a background condition in many taxa may result in a constraint of the exploration of phenotype space, as species divided into two sexes characterized by strong phenotypic differences dance between two sex-specific phenotypic optima.

* Sex determination varies by taxon.

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Tyler Cowen Alex Tabarok points me to this paper, Sociosexuality from Argentina to Zimbabwe. The abstract:

The Sociosexual Orientation Inventory (SOI; Simpson & Gangestad 1991) is a self-report measure of individual differences in human mating strategies. Low SOI scores signify that a person is sociosexually restricted, or follows a more monogamous mating strategy. High SOI scores indicate that an individual is unrestricted, or has a more promiscuous mating strategy. As part of the International Sexuality Description Project (ISDP), the SOI was translated from English into 25 additional languages and administered to a total sample of 14,059 people across 48 nations. Responses to the SOI were used to address four main issues. First, the psychometric properties of the SOI were examined in cross-cultural perspective. The SOI possessed adequate reliability and validity both within and across a diverse range of modern cultures. Second, theories concerning the systematic distribution of sociosexuality across cultures were evaluated. Both operational sex ratios and reproductively demanding environments related in evolutionary-predicted ways to national levels of sociosexuality. Third, sex differences in sociosexuality were generally large and demonstrated cross-cultural universality across the 48 nations of the ISDP, confirming several evolutionary theories of human mating. Fourth, sex differences in sociosexuality were significantly larger when reproductive environments were demanding but were reduced to more moderate levels in cultures with more political and economic gender equality. Implications for evolutionary and social role theories of human sexuality are discussed.

Below is a table of SOIs….

• Category: Science • Tags: Finn Baiting, Sex 
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Doubling Of Sexually Transmitted Infections Among Over-45s In Under A Decade. Dare we say an “epidemic???” If you want to push the envelope of course, She was 82. He was 95. They had dementia. They fell in love. And then they started having sex. In any case:

While the numbers of infections identified in younger age groups rose 97% during the period of the study, those identified in the over 45s rose 127%.

“Indeed, it may be argued that older people are more susceptible [to sexually transmitted infections] as they are less likely to use condoms than younger people,” they say, adding that as successive waves of people with more liberal sexual attitudes and behaviours age, the problem is likely to worsen.

I guess the “safe sex” message just isn’t getting through to the less young.

Related: Your generation was sluttier.

• Category: Science • Tags: Disease, Sex 
Razib Khan
About Razib Khan

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