Neurological Correlates

Polygyny gets quite a bit of attention — not surprisingly, but does it explain the observation of excess “effective femaleness” in the gene pool? Two recent PLoS Genetics papers provide potential explanations based on population genetics and social organization.

What everyone seems to agree upon: Women pass along more genes (in a population) than do men.

What seems to be up in the air: Why this is so — or maybe what is going on

Among possible explanations:

(a) As Hammer et al. (below) hypothesize, a higher percentage of X-linked diversity (relative to autosomal chromosomes) is explained by polygyny — males mating with multiple females.

(b) Ségurel et al show that the social organization of patrilineal populations, in which genetically related men (but not women) tend to cluster together, reduces the number of “effective males” — and, correspondingly increases the “effective female” proportion. In essence, there are many males, but they are genetically similar — a sort of pseudo polygyny.

Loretta Lynn/Conway Twitty cover, “You’re The Reason Our Kids Are Ugly” (by Kelly Hogan w/ Pardner via Youtube)(See 3:35 for the discussion of inheritance)

Full post and discussion with a consanguinity twist:

What everyone seems to agree upon: Women pass along more genes (in a population) than do men.

What seems to be up in the air: Why this is so — or maybe what is going on

Among possible explanations:

(a) As Hammer et al. (below) hypothesize, a higher percentage of X-linked diversity (relative to autosomal chromosomes) is explained by polygyny — males mating with multiple females. Because there are fewer reproductively-successful males than females, the females all are stuck with more or less the same male genes to reproduce with (to oversimplify without getting into genes, non coding regions, markers, etc., but background was all taken into account, see below). Same boyfriend among multiple pregnant girlfriends. A few men having lots of babies. Many women having a few babies.

Dr. Hammer’s group analyzed data and found that polygyny was really the only behavior which could account for the relatively high rate of X-linked mutations, although many behaviors which can alter genetic heritability in populations.
(As an aside, Dr. Hammer graciously replied to an e mail question I sent, as follows “[In response to your query], Does the higher relative genetic diversity seen in the X chromosome have anything to do with inherent instability of regions on the X chromosome?[the answer ] is no. This is because we correct for different mutation rates by dividing human diversity by human-orangutan divergence. Moreover, we avoided regions of simple repeats, and focused on point mutational changes.”);

(b) Ségurel et al (below) point out, and expand upon the concept that there can be a larger “effective number of females” (or lower “effective number of males”) for various reasons, including polygyny. (See the summary below and the paper for the full story). Ségurel et al show that in patrilineal herder groups of Central Asia, in contrast to bilineal agriculturalists, the effective number of women is higher than that of men. This can be explained by the social organization of patrilineal populations, in which genetically related men (but not women) tend to cluster together. This study suggests that differences in sex-specific migration rates may not be the only cause of contrasting male and female differentiation in humans, and that differences in effective numbers do matter.

Evolution is premised on fertility. Human fertility increases the further away your mate’s hometown. Labouriau, Rodrigo, Amorim, Antonio, “Human Fertility Increases With Marital Radius,” Genetics 2008 178: 601-603. (See here, “Why not to marry your cousin“). Let’s say both the male and female move to California, away from both sets of intrusive, lecherous in-laws — now there are equal numbers of “effective” males and females. Fertility increases. There is among offspring a decreased consanguinity and its deleterious effects. The population multiplies.

Where related males cluster, and there are fewer “effective males” (all being genetically alike for practical purposes), the relative X-linked chromosomal diversity is the only thing which can save the offspring from deleterious effects of marrying their cousins or uncles. So Uncle Warren is doing no favor to evolution, not that he believes in it anyway. (OK, I editorialize).

I’m late in writing this up, and there have been lots of great articles already, and here are some good ones:

The Impact Of Polygyny On Human Genetic Variation, from Anthropology.net

The X Chromosome and the Case against Monogamy, from Scientific American

The papers:

Hammer MF, Mendez FL, Cox MP, Woerner AE, Wall JD (2008) Sex-Biased Evolutionary Forces Shape Genomic Patterns of Human Diversity. PLoS Genet 4(9): 1000202. doi:10.1371/journal.pgen.1000202 http://dx.plos.org/10.1371/journal.pgen.1000202
Author’s Summary:
Like many primate species, the mating system of humans is considered to be moderately polygynous (i.e., males exhibit a higher variance in reproductive success than females). As a consequence, males are expected to have a lower effective population size (Ne) than females, and the proportion of neutral genetic variation on the X chromosome (relative to the autosomes) should be higher than expected under the assumption of strict neutrality and an equal breeding sex ratio. We test for the effects of polygyny by measuring levels of neutral polymorphism at 40 independent loci on the X chromosome and autosomes in six human populations. To correct for mutation rate heterogeneity among loci, we divide our diversity estimates within human populations by divergence with orangutan at each locus. Consistent with expectations under a model of polygyny, we find elevated levels of X-linked versus autosomal diversity. While it is possible that multiple demographic processes may contribute to the observed patterns of genomic diversity (i.e., background selection, changes in population size, and sex-specific migration), we conclude that an historical excess of breeding females over the number of breeding males can by itself explain most of the observed increase in effective population size of the X chromosome.

Laure Ségurel, Begoña Martínez-Cruz, Lluis Quintana-Murci, Patricia Balaresque, Myriam Georges, Tatiana Hegay, Almaz Aldashev, Firuza Nasyrova, Mark A. Jobling, Evelyne Heyer, Renaud Vitalis, Molly Przeworski (2008). Sex-Specific Genetic Structure and Social Organization in Central Asia: Insights from a Multi-Locus Study PLoS Genetics, 4 (9) DOI: 10.1371/journal.pgen.1000200
Author’s summary:
Human evolutionary history has been investigated mainly through the prism of genetic variation of the Y chromosome and mitochondrial DNA. These two uniparentally inherited markers reflect the demographic history of males and females, respectively. Their contrasting patterns of genetic differentiation reveal that women are more mobile than men among populations, which might be due to specific marriage rules. However, these two markers provide only a limited understanding of the underlying demographic processes. To obtain an independent picture of sex-specific demography, we developed a new multi-locus approach based on the analysis of markers from the autosomal and X-chromosomal compartments. We applied our method to 21 human populations sampled in Central Asia, with contrasting social organizations and lifestyles. We found that, in patrilineal populations, not only the migration rate but also the number of reproductive individuals is likely to be higher for women. This result does not hold for bilineal populations, for which both the migration rate and the number of reproductive individuals can be equal for both sexes. The social organization of patrilineal populations is the likely cause of this pattern. This study suggests that differences in sex-specific migration rates may not be the only cause of contrasting male and female differentiation in humans, and that differences in effective numbers do matter.