Abstract
BackgroundDespite intensive efforts devoted to collecting human polymorphism data, little is known about the role of gene flow in the ancestry of human populations. This is partly because most analyses have applied one of two simple models of population structure, the island model or the splitting model, which make unrealistic biological assumptions.ResultsHere, we analyze 98-kb of DNA sequence from 20 independently evolving intergenic regions on the X chromosome in a sample of 90 humans from six globally diverse populations. We employ an isolation-with-migration (IM) model, which assumes that populations split and subsequently exchange migrants, to independently estimate effective population sizes and migration rates. While the maximum effective size of modern humans is estimated at ~10,000, individual populations vary substantially in size, with African populations tending to be larger (2,300–9,000) than non-African populations (300–3,300). We estimate mean rates of bidirectional gene flow at 4.8 × 10-4/generation. Bidirectional migration rates are ~5-fold higher among non-African populations (1.5 × 10-3) than among African populations (2.7 × 10-4). Interestingly, because effective sizes and migration rates are inversely related in African and non-African populations, population migration rates are similar within Africa and Eurasia (e.g., global mean Nm = 2.4).ConclusionWe conclude that gene flow has played an important role in structuring global human populations and that migration rates should be incorporated as critical parameters in models of human demography.
Highlights
Despite intensive efforts devoted to collecting human polymorphism data, little is known about the role of gene flow in the ancestry of human populations
Two commonly used models include the island model, which assumes that populations have no shared ancestry and are related only through gene flow (Figure 1A), and the phylogenetic branching or splitting model, which assumes that populations diverged at some time in the past and have remained completely isolated ever since (Figure 1B)
Individual estimates of N have some uncertainty, mean effective sizes are statistically larger for African populations (2,300–9,000) than non-African (300–3,300) populations (t20 = 6.9, P
Summary
Despite intensive efforts devoted to collecting human polymorphism data, little is known about the role of gene flow in the ancestry of human populations. This is partly because most analyses have applied one of two simple models of population structure, the island model or the splitting model, which make unrealistic biological assumptions. Models of population structure reflecting the (A) island, (B) splitting and (C) isolation-with-migration (IM). The divergence model describes an ancestral population, which splits at time t into two daughter populations that do not exchange genes in subsequent generations. The isolation-with-migration model describes a constant-sized ancestral population that splits into two daughter populations that can exchange genes and change in size. There are seven parameters in the isolation-withmigration model: effective population size of the ancestral deme (NA), effective population sizes of the two descendent demes (N1 and N2), unidirectional migration between the descendent populations (m1 and m2), proportion of the ancestral population founding deme 1 (S), and population divergence time (t)
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