Patrilineal segmentary systems provide a peaceful explanation for the post-Neolithic Y-chromosome bottleneck

TL;DR

The paper uses forward-time simulations to test whether patrilineal segmentary systems, without violent competition, can explain the post-Neolithic Y-chromosome bottleneck. It finds that variance in reproductive success between patrilineal groups combined with lineal fission can substantially reduce male effective population size, producing female-to-male effective population size ratios up to around 19, comparable to the ratio of 17 reported by Karmin et al.

Key findings

• In π-based estimates, adding variance in reproductive success between patrilineal descent groups (σ² = 0.1) and lineal fission reduced male effective population size by over 20 after 100 generations (scenarios 2g and 2h), regardless of whether violence was included.
• Coalescent-based estimates showed the same relative pattern but smaller reduction factors: scenario 2g had a male effective population size reduction factor of 2.77 and scenario 2h of 3.09, compared with 0.82 for basic patrilineal descent with random fission and no variance (scenario 2a).
• Lineal fission plus variance in reproductive success was more efficient at reducing male effective population size than violence alone; without variance in reproductive success, lineal and random fission had little difference.
• A Kipsigis-like polygyny scenario reduced male effective population size much less than patrilineal lineal fission with violence and/or variance, with π-based and coalescent-based reduction factors around 4.

Methods

• The authors built a socio-demographic model in SLiM v4.1: 1,500 individuals simulated for 20,000 generations of panmixia, then divided into five villages with patrilocal residence, bilateral or patrilineal descent, exponential growth of 1% per generation, and 100 generations of the kinship rule of interest; 200 replicates were run per scenario.
• Males carried a 1 Mb Y chromosome and females a 10 kb mtDNA; mutations were generated at 2.5 × 10⁻⁸ mutations/nucleotide/generation for the Y chromosome and 5.5 × 10⁻⁷ for mtDNA. Scenarios varied fission type (random vs. lineal), variance in reproductive success between descent groups (σ² = 0, 0.05, 0.1, 0.2), violent competition (15% of males killed per generation), male migration (0 or 2%), post-fission migration, and polygyny.
• Male and female effective population sizes were estimated using π-based nucleotide diversity over time and coalescent-based Bayesian skyline plots generated in BEAST v1.10.5.

Limitations

• The paper states that past rates of violence remain uncertain, pre-Neolithic residence patterns are poorly known, and the model does not formally model long-range migrations such as the Steppe migration.
• The paper states that the exact nature of the transition from segmentary patrilineal systems to kinship systems less efficient at reducing Y-chromosome diversity requires future study, and that coalescent-based maximum female-to-male effective population size ratios are underestimates because peaks do not occur simultaneously across replicates.
• Evident from the text: the study is a simulation modelling exercise rather than an empirical test, with parameters calibrated from ethnographic and literature-based estimates; the source of violence in the model is not specified.