Relaxed Observance of Traditional Marriage Rules Allows Social Connectivity without Loss of Genetic Diversity

Abstract

Marriage rules, the community prescriptions that dictate who an individual can or cannot marry, are extremely diverse and universally present in traditional societies. A major focus of research in the early decades of modern anthropology, marriage rules impose social and economic forces that help structure societies and forge connections between them. However, in those early anthropological studies, the biological benefits or disadvantages of marriage rules could not be determined. We revisit this question by applying a novel simulation framework and genome-wide data to explore the effects of Asymmetric Prescriptive Alliance, an elaborate set of marriage rules that has been a focus of research for many anthropologists. Simulations show that strict adherence to these marriage rules reduces genetic diversity on the autosomes, X chromosome and mitochondrial DNA, but relaxed compliance produces genetic diversity similar to random mating. Genome-wide data from the Indonesian community of Rindi, one of the early study populations for Asymmetric Prescriptive Alliance, are more consistent with relaxed compliance than strict adherence. We therefore suggest that, in practice, marriage rules are treated with sufficient flexibility to allow social connectivity without significant degradation of biological diversity.

Keywords: Approximate Bayesian Computation; Asymmetric Prescriptive Alliance; Indonesia; genetic diversity; mating systems.

Fig. 1.

Kinship under APA. Each box represents a clan; each row represents a generation. The red clan acts as wife-giver to the black clan, which in turn acts as wife-giver to the blue clan. Dashed arrows represent marriage alliances with women moving from their natal clan to the community of their husband.

Fig. 2.

Genetic diversity (${\theta }_{\pi }$) across a grid of random values for π_MBD and π_mig under APA for (A,E) autosomes, (B,F) X chromosome, (C,G) Y chromosome, and (D,H) mtDNA over 50,000 simulations (3,000 individuals, 20 demes). (A)–(D) present fitted values (sheet with black to red color range for better visualization in 3D) from the GAMs, together with simulated values (dots). (E)–(H) are projections of simulated mean diversity values across the grid of parameters.

Fig. 3.

Actual rate of MBD marriage observed in simulations as a function of π_MBD (x axis) and π_mig (color scale) (25,000 observations during 50 simulations for each data point). The dashed line represents the rate of MBD marriage observed by Forth (1981) in Rindi (10%); the observed migration rate to the prescribed clan is 26%.

Fig. 4.

ABC for Rindi, a population practicing APA on Sumba in eastern Indonesia. Posterior distributions are shown for N, π_MBD, and π_mig.