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. 2020 Feb 4;117(5):2378-2384.
doi: 10.1073/pnas.1917716117. Epub 2020 Jan 21.

Evolution of kinship structures driven by marriage tie and competition

Affiliations

Affiliations

  • 1 Department of Basic Science, Graduate School of Arts and Sciences, University of Tokyo, Meguro-ku, Tokyo 153-8902, Japan.
  • 2 Department of Basic Science, Graduate School of Arts and Sciences, University of Tokyo, Meguro-ku, Tokyo 153-8902, Japan; kaneko@complex.c.u-tokyo.ac.jp.
  • 3 Center for Complex Systems Biology, Universal Biology Institute, University of Tokyo, Tokyo 113-0033, Japan.

Evolution of kinship structures driven by marriage tie and competition

Kenji Itao et al. Proc Natl Acad Sci U S A. .
. 2020 Feb 4;117(5):2378-2384.
doi: 10.1073/pnas.1917716117. Epub 2020 Jan 21.

Affiliations

  • 1 Department of Basic Science, Graduate School of Arts and Sciences, University of Tokyo, Meguro-ku, Tokyo 153-8902, Japan.
  • 2 Department of Basic Science, Graduate School of Arts and Sciences, University of Tokyo, Meguro-ku, Tokyo 153-8902, Japan; kaneko@complex.c.u-tokyo.ac.jp.
  • 3 Center for Complex Systems Biology, Universal Biology Institute, University of Tokyo, Tokyo 113-0033, Japan.

Abstract

The family unit and kinship structures form the basis of social relationships in indigenous societies. Families constitute a cultural group, a so-called clan, within which marriage is prohibited by the incest taboo. The clan attribution governs the mating preference and descent relationships by certain rules. Such rules form various kinship structures, including generalized exchange, an indirect exchange of brides among more than two clans, and restricted exchange, a direct exchange of brides with the flow of children to different clans. These structures are distributed in different areas and show different cultural consequences. However, it is still unknown how they emerge or what conditions determine different structures. Here, we build a model of communities consisting of lineages and family groups and introduce social cooperation among kin and mates and conflict over mating. Each lineage has parameters characterizing the trait and mate preference, which determines the possibility of marriage and the degree of cooperation and conflict among lineages. Lineages can cooperate with those having similar traits to their own or mates', whereas lineages with similar preferences compete for brides. In addition, we introduce community-level selection by eliminating communities with smaller fitness and follow the so-called hierarchical Moran process. We numerically demonstrate that lineages are clustered in the space of traits and preferences, resulting in the emergence of clans with the incest taboo. Generalized exchange emerges when cooperation is strongly needed, whereas restricted exchange emerges when the mating conflict is strict. This may explain the geographical distribution of kinship structures in indigenous societies.

Keywords: incest taboo; kinship structure; multilevel selection; social physics.

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Conflict of interest statement

The authors declare no competing interest.

Figures

Fig. 1.

Fig. 1.

Examples of kinship structures. Each…

Fig. 1.

Examples of kinship structures. Each symbol A, , ,…

Fig. 1.
Examples of kinship structures. Each symbol A, , , B, is a clan. When clans are identified by a single trait, we denote them by characters as A, B, , whereas when they are represented by two traits, they are denoted both by characters and indices as , , . The double arrow from clan X to Y () shows the marriage rule indicating that women in clan X marry men in clan Y. The arrow from clan X to Y () shows the descent rule indicating that children belong to clan Y when their fathers belong to clan X. Hereafter, we refer to the length of cycles of and needed to return to the original point, as marriage cycle and descent cycle . Structures are classified according to those cycles. (A) for incest structure. (B) for dual organization. (C) for restricted exchange. (D) for generalized exchange.
Fig. 2.

Fig. 2.

Schematic of the model. (

Fig. 2.

Schematic of the model. ( A ) Life cycle in the model. Communities…

Fig. 2.
Schematic of the model. (A) Life cycle in the model. Communities (green) consist of lineages (blue), whose population (black) can grow. Generation is separated into children (dark gray) and adults (light gray). As children grow up, some die because of a lack of cooperation or mating conflicts. Adults mate, give birth to children, and die. When the population of a lineage (community) goes beyond a given threshold, the lineage (community) splits. When a community splits, another community is removed from the system at random to keep the number of communities fixed—i.e., we adopt the hierarchical Moran process. (B) Lineages cooperate (solid line) and conflict (dashed line), depending on their traits and mate preferences . Kin and mates cooperate, whereas mating rivals conflict. Lineages i and j are kin (blue) when is sufficiently small, mating rivals (red) when is sufficiently small, and mates (orange) when or is sufficiently small. Only the relationships with the upper left lineage are shown.
Fig. 3.

Fig. 3.

Example of clan separation. (

Fig. 3.

Example of clan separation. ( A C )

Fig. 3.
Example of clan separation. (AC) values for lineages in a community after 500 calculation steps. Clusters of lineages are generated, which are clans. The arrow shows the preferential relationship that corresponds to in Fig. 1. (DF) Time series of separation of lineages. Temporal evolution of the values of traits and preferences of lineages in a community are represented in blue and red, respectively. Parameters are in A and D; in B and E; and in C and F; for all cases.
Fig. 4.

Fig. 4.

Time series of …

Fig. 4.

Time series of . ( A ) Time series of

Fig. 4.
Time series of . (A) Time series of in a single community, i.e., under lineage-level selection without community-level selection. (B) Time series of the average over 100 communities both under lineage- and community-level selection. . Blue and orange lines show the results under parameter values of and , respectively.
Fig. 5.

Fig. 5.

Phase diagrams on the emergence…

Fig. 5.

Phase diagrams on the emergence of the incest taboo against the parameters

Fig. 5.
Phase diagrams on the emergence of the incest taboo against the parameters and . At the parameter values with red points, the frequency of incest taboo is more than 0.9, whereas with blue points, it is less than 0.9. (A) . (B) .
Fig. 6.

Fig. 6.

Examples of emergent structures.

Fig. 6.

Examples of emergent structures. values for lineages in a community…

Fig. 6.
Examples of emergent structures. values for lineages in a community after 500 steps of simulation. Images show the map, map, map, and the corresponding structure from left to right. Kinship structures emerge as the marriage and descent relationships of clusters. (A) Generalized exchange. Green, red, and blue clusters correspond to clans A, B, and C, respectively, in Fig. 1. (B) Restricted exchange. Red, green, blue, and purple clusters correspond to clans A1, A2, B1 and B2 respectively, in Fig. 1.
Fig. 7.

Fig. 7.

Phase diagrams of kinship structures…

Fig. 7.

Phase diagrams of kinship structures against the parameters and …

Fig. 7.
Phase diagrams of kinship structures against the parameters and . Images show the classes of structures that appear most frequently under several conditions, according to the classification in Fig. 1. Incest structure is plotted in orange, dual organization in green, generalized exchange in red, and restricted exchange in purple, whereas the conditions with which all communities are extinct are plotted in blue. (A) . (B) .

Comment in

References

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    1. Lévi-Strauss C., Les Structures Élémentaires de la Parenté (Presses Universitaires de France, Paris, France, 1949).
    1. Maddock K., Alliance and entailment in Australian marriage. Aust. J. Anthropol. 7, 19–26 (1969).
    1. Malinowski B., Sex, Culture, and Myth (R. Hart-Davis, London, UK, 1963).
    1. Murdock G. P., Social Structure (Macmillan, New York, NY, 1949).

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