Archive for the ‘Descent’ Category

Molecular Variance Across Genetic Systems in Modern Humans and Their Kinship Structures

Saturday, November 2nd, 2013

Global patterns of sex-biased migrations in humans 

Chuan-Chao Wang, Li Jin, Hui Li.

Abstract 

A series of studies have revealed the among-population components of genetic variation are higher for the paternal Y chromosome than for the maternal mitochondrial DNA (mtDNA), which indicates sex-biased migrations in human populations. However, this phenomenon might be also an ascertainment bias due to nonrandom sampling of SNPs. To eliminate the possible bias, we used the whole Y chromosome and mtDNA sequence data of 491 individuals from the 1000 Genomes Project Phase I to address the sex-biased migration dispute. We found that genetic differentiation between populations was higher for Y chromosome than for the mtDNA at global scales. The migration rate of female might be three times higher than that of male, assuming the effective population size is the same for male and female.

Link

The paper contains a rundown of inter- and intragroup diversity values for both mtDNA and Y-DNA on a worldwide scale (see below).

Anthropogenesis-MolecularVariance-Wang2013

As far as I know, this is the only paper that has pooled it all together for the two haploid systems. This is consistent with the classic observation that most of the variation found among modern humans is concentrated within human groups. But it’s noteworthy that the continents differ in the degree in which this pattern is manifested. Some continents and populations buck the general human trend by accumulating more variation between groups than others. The notable pattern in this data, as Wang et al. (2013) point out, is that

“the between-population component of genetic variation was slightly higher for mtDNA than for the Y chromosome in America and Africa (by 5~6 times), but not in Europe and Asia. In Europe and Asia, between-population component of genetic variation was about 10~20 times higher for Y chromosome than for the mtDNA.”

What this pattern likely means is that Amerindian and Sub-Saharan African populations have been more consistently matrilocal/uxorilocal/matrilineal, while populations outside of Africa and America more consistently patrilocal/virilocal/patrilineal. Wang et al. (2013) confirm this interpretation and refute the conclusion reached by Wilder et al. 2004 (“Global Patterns of Human Mitochondrial DNA and Y-Chromosome Structure Are Not Influenced by Higher Migration Rates of Females versus Males,” Nat Genet 3610, 1122-1125) that there is no correlation between social structure and sex-biased migration rates, on the one hand, and molecular variation at haploid loci, on the other.

Wang et al.’s data and interpretations are consistent with the worldwide data from social anthropology on the relative frequencies of matricentric vs. patricentric social structures. Burton et al. (“Regions Based on Social Structure,” Current Anthropology 37 (4), 1996, 93) define matri- and patricentric forms of social organization in the following way:

“Matricentric social organization traits include localized or dispersed matrilineal groups, matrilocal or uxorilocal residence, monogamy and the absence of marriage exchange. Matricentric societies tend to organize kinship groups around women through matrilocal or uxorilocal residence or through matrilineal kinship groups. Patricentric social organization traits include nomadic or seminomadic settlement patterns, clan communities, localized or dispersed patrilineal groups, patrilocal residence, polygyny and bridewealth payments. Hence, patricentric societies tend to organize kin groups around men, through patrilocal residence, patrilineal descent or polygyny.”

The worldwide distribution of matri- vs. patricenrtic structures is skewed between Africa, the Pacific and America, on the one hand, and Eurasia, on the other (see below, from Burton et al. 1996, 109), which is consistent with Wang et al.’s data from mtDNA and Y-DNA. North American Indian societies is the only exception from this pattern – they fall on the patricentric side.

Anthropogenesis-MatriPatricentric

My own kin-terminological data shows the paucity of “Crow” terminologies (highly correlated with matrilineal descent) in Western, Eastern Eurasia, Southeast Asia and Australia and their relative salience in Sub-Saharan Africa and America (as well as in the Pacific). “Omaha” systems (highly correlated with patrilineal descent) are the only descent-skewed terminologies found in those areas.

Now that we have global molecular variance data for the haploid loci, we can compare them with the same statistics derived from autosomal loci (see below, from Tishkoff et al. 2009. “The Genetic Structure and History of Africans and African Americans,” Suppl.Mat., Table S3).

Anthropogenesis-MolecularVariance-Tishkoff2009Tishkoff et al.’s data is broken down by continent, by population and by economic system. Notably, African hunter-gatherers in general and Khoisans in particular show a different pattern from African pastoralists and farmers: they are closer to Amerindian and Oceanic populations in having a greater fraction of their variance come from inter-group values. This is consistent with the more specific data that we have on Hadza, linguistically the most divergent among Khoisan languages, that shows that Hadza has less intragroup variation than other Sub-Saharan Africans and is consequently closer to the Amerindian pattern of variance (see here and here). In turn, the Amerindian pattern of variance (more intergroup diversity, less intragroup diversity) is similar to the one found in Denisovans.

If molecular variance at the haploid loci points to the impact of residence and unilineal descent on the genetic structure of human populations, the sociodemographic reality behind the autosomal variance statistics is that older human populations associated with a more ancient subsistence pattern were smaller, more isolated from each other, more endogamous and affected by inbreeding and genetic drift to a larger degree than more recent populations associated with more recent subsistence patterns.

Cross-posted at www.anthropogenesis.kinshipstudies.org.

Kinship Systems Shaped by Vertical Transmission, not Environment

Tuesday, April 24th, 2012

One of the fundamental premises my book “The Genius of Kinship” is that human kinship systems and their linguistic expressions, kinship terminologies, are largely unacknowledged sources of insight into human prehistory and direct partners to historical linguistics and population genetics in the task of unraveling the enigma of modern human origins and dispersals. The following quote from an evolutionary anthropologist, Barry Hewlett, reinforces the same point:

“We are conducting further studies to evaluate the coevolution of genes, culture, and language in Africa and the Americas, and preliminary data suggest that kinship and family beliefs and practices tend to be conserved along with genes. In other words, aspects of kinship and family tend to be highly conserved, similar to genes, and their distribution across the landscape does not appear to be linked to adaptations to particular natural environments…The data imply that the current distribution of kinship and family patterns is due to demic diffusion and conservative cultural transmission. This is supported by a nonevolutionary study of kinship by Burton et al. (1996) where he uses a sophisticated analysis of kinship and family patterns to describe culture areas. His kinship culture areas fit very nicely with the world’s language and genetic distance trees (Jones 1999). He systematically generates two key dimensions of variability in family in kinship – a matricentric-patricentric continuum and a bilateral-unilineal continuum. For instance, Africa is strongly unilineal, but relatively egalitarian on the gender dimension, whereas the middle Old World (North Africa, the Middle East, South and Central Asia and most of China) is unilineal but patricentric. The distrubution of the various culture areas of kinship are linked to the movements and expansions of dominant peoples (e.g., demic diffusion and vertical transmission) throughout history (e.g., Bantu expansion)” (Hewlett, Barry S. “Neoevolutionary Perspectives on Human Kinship,” in New Directions in Anthropological Kinship, edited by Linda Stone. Lanham, 2001, p. 105).

It has taken evolutionary anthropologists 150 years to come around to the notion pioneered by the young Lewis H. Morgan that human kinship systems reflect what we might call these days “populational processes,” rather than abstract evolutionary stages. The Morgan of the “Systems of Consanguinity and Affinity of the Human Family” (1871) is very different from the Morgan of “Ancient Society” (1877) because of his new strong focus on those stages, rather than on geographically localized and demically conditioned types of kinship systems (the very name “Turano-Ganowanian” in the “Systems” was supposed to relate Tamil and Iroquois kinship systems into a tangible historical unity derived from the hypothesis of a migration of the ancestors of American Indians from Asia to the New World). While in those concrete specifics Morgan was wrong, the overall classification of kinship systems into “classificatory” and “descriptive” remains valid and finds parallels outside of kinship studies.