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).