Razib Khan One-stop-shopping for all of my content

June 24, 2012

It’s science, not math

Filed under: Gender,science,Sex — Razib Khan @ 12:30 pm

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 ...

June 23, 2012

Sex & sport

Filed under: Bioethics,Sex — Razib Khan @ 9:26 pm

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.

May 10, 2012

Porn, a new age, an old age, and all that

Filed under: Culture,Porn,Sex — Razib Khan @ 9:53 pm

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 ...

April 28, 2012

Iraq: the model that wasn’t

Filed under: Arabs,Data Analysis,Sex — Razib Khan @ 10:13 pm

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 bloggingheads.tv 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 ...

April 8, 2012

Understanding ethnic biased sex slavery

Filed under: Sex,Women — Razib Khan @ 12:12 pm

Recently on the NPR show Here and Now I was introduced to the concept of ‘loverboys.’ This is a specific phenomenon in the Netherlands were older males target schoolgirls, and entrap them in relationships where these girls are prostituted out (there are repeated references to group sex). Even before it got to that part I had a suspicion that these men were from ethnic minorities. The reason is that I figured I’d have heard of the phenomenon by now, because of the extreme details (e.g., ‘gang bangs’). Rather, there must have been an extenuating circumstance, and that was most likely to be the ethnic angle. And sure enough most of the loverboys are from families with roots in Morocco and Turkey, and their targets are girls from ethnic Dutch backgrounds.

I can outline the various sociocultural factors at work here. For example, Dutch do not have the same concept of ‘honor’ as many ‘traditional’ Islamic and South Asian societies, so their young girls are likely perceived as vulnerable. And the lower socioeconomic status of Muslims in most of European makes it entirely plausible that this population would be involved in prostitution. On the other hand, their social conservatism and generally barbaric attitudes toward female honor would probably mitigate against the involvement of Muslim women in the practice.

But what I’m curious about attitudes from the inside. I’m wondering if some readers could place themselves in the psychology of the pimps here, at least at some point in the past, and explain what’s going on. I’m not really interesting in judging at this point, but deeply understanding. In particular, I’m curious about non-economic angles.

December 17, 2011

Promiscuity and vaginal bacterial diversity

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 ofPeromyscus 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.

July 7, 2011

Everything I didn’t know about sex

Filed under: Evolution,Genetics,Sex — Razib Khan @ 11:19 pm

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 ...

May 31, 2011

The Rights of Woman

Filed under: Sex — Razib Khan @ 10:53 pm

All the talk about the ladies, I poked around The World Values Survey. I was going to post the results in relation to “women” related questions, but I’ll leave it to you. I will say this:

- Muslim and East Asian societies are the least forthrightly supportive of female political and social equality

- There doesn’t seem to be much difference between Muslims and non-Muslims in conservative non-Western nations (e.g., Egypt or Ghana)

- There’s a huge difference between Muslims and other groups in European nations

January 8, 2011

More than “just one nation”

Filed under: crime,Pathology,Sex,Society — Razib Khan @ 11:24 am

Jack Straw: Some white girls are ‘easy meat’ for abuse:

But he said: “I certainly don’t think it’s just a Pakistani thing. My staff would say there is an over-representation of people from ethnic minority groups among perpetrators – Afghans, people from Arabic nations, Pakistanis. But it’s not just one nation.”

January 5, 2011

Cultural differences

Filed under: homosexuality,Pederasty,Sex — Razib Khan @ 9:16 am

Speaking of sex, DADT (Afghanistan Edition)

And, the impact of those experiences is already being felt in portions of Afghanistan, putting American forces squarely in the middle of complex moral, social and sexual issues. A source at Army Special Operations command tells In From the Cold that Afghan women, emboldened by the presence of U.S. troops. have complained about beatings they’ve suffered at the hands of their husbands. The domestic violence reportedly stemmed from the inability of the women to become pregnant and produce sons, highly valued in Afghan society.

When U.S. civil affairs teams (and other special forces units) quietly investigated the problem, they quickly discovered a common denominator. Virtually all of the younger men who beat their wives (over their inability to become pregnant) had been former “apprentices” of older Afghan men, who used them for their sexual pleasure. Upon entering marriage, whatever the men knew of sex had been learned during their “apprenticeship,” at the hands of the older man. To put it bluntly, some of the younger Afghans were unfamiliar with the desired (and required) mechanics for conception.

October 18, 2010

Sex with thee and the last woman

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?

ResearchBlogging.orgThe 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

October 1, 2010

Polygamy and human evolution: maybe it’s agriculture

Filed under: Anthroplogy,anthropology,History,Sex — Razib Khan @ 11:11 am

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.

May 18, 2010

An umbrella against the mutational showers

Filed under: Biology,Genetics,Good Genes,Selection,Sex — Razib Khan @ 2:41 am

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

March 31, 2010

When sickliness is manliness

Filed under: Behavior,Disease,Evolution,Evolutionary Genetics,Genetics,Immunity,Sex — Razib Khan @ 2:25 am

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 computae 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

March 30, 2010

The sexual straightjacket

Filed under: Biology,Evolution,Evolutionary Genetics,Genetics,Sex,Sex Differences — Razib Khan @ 5:21 pm

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.

Powered by WordPress