3.6 | Admixture histories of the African American ASW and Barbadian ACB
Figure 5 visually synthesized the estimated posterior parameters of the complex admixture scenarios reconstructed with theMetHis – ABC framework, and associated 95% CI (Table 2 ).
We found a virtual complete replacement of the ACB and ASW populations at generation 1, thus consistent with our inability to accurately estimate the founding proportions from the African and European sources at generation 0. Furthermore, we found an increasingly precise posterior estimation of introgression rates forward-in-time. This is also consistent with the nature of recurrent admixture processes, where older information may be lost or replaced when more recent admixture events occur.
Interestingly, we found that the recurring introgression from the European gene pool rapidly decreases after generation 1, for both the ACB and ASW, albeit with substantial differences (Figure 5 ). Indeed, we found that, for the ACB, European introgression falls below 10% at generation 9 until no more than 1% in the present. Comparatively, the European contribution diminished substantially less rapidly for the ASW, going below 10% only after generation 12 until roughly 2% in the present. Therefore, it seems that neither sustained European migrations, nor the relaxation of social and legal constraints on admixture between descendant communities subsequent to the abolition of slavery and the end of segregation, have translated into increased European genetic contribution to the gene-pool of admixed populations in the Americas.
Finally, we found substantial recurring contributions from the African source for both admixed populations (Figure 5 ). For the ACB, we found a progressive decrease of the African recurring introgression until a virtually constant recurring admixture close to 28% from generation 10 and onward. For the ASW, our results showed a sharper decrease of the African contribution after founding until a virtually constant recurring admixture process close to 24% from generation 5 until present. Nevertheless, the ACB occupy an ambiguous region of the parameter-space, and results should be considered cautiously, as another complex admixture model might more accurately explain this data. Altogether, the signal of substantial ongoing admixture from Africa could stem from the known importance of African recurring forced migrations during the TAST into the Americas; further prompts the influence of African slave descendants migrations within the Americas before and after the end of slavery (Baharian et al., 2016).