The map I was just referring to, map B in Figure 4, as displayed in the previous post, is a bit confusing and I must admit, after reading a comment by Maju, that I am now uncertain as to whether or not to take it seriously as evidence for a discontinuous early migration. Maju writes:
Check the rectangular plots in that same figure for true alleles and not just catch-all simplifications. There is no "blue allele" but an array of non-red alleles. The only thing that all "blue" carriers share is a negative one: they don't share the red allele, which is what the maps are focused on.I'm not sure what to make of Maju's observation because frankly I don't understand exactly what the rectangular plots on the right refer to. It does seem clear that there is a positive-negative relation between the red and blue pie charts in the maps, because, as it seems, all humans carry some form of the gene, either ancestral or derived. So anyone not carrying red is carrying blue. And the caption for the figure identifies the blue alleles as "ancestral" and the red as "derived." In that case, I'm not sure whether it matters whether we are looking at the blue dots or the red dots, since both can be understood as representing migrations, as far as I can see. One is just the negative image of the other.
If I'm wrong about this, then Dediu and Ladd are also mistaken in attempting to correlate the distribution of the ancestral forms of ASPM and Microcephalin with the distribution of tone language, in the paper of theirs to which I've already referred: Linguistic tone is related to the population frequency of the adaptive haplogroups of two brain size genes, ASPM and Microcephalin. The data they are using appears to be drawn from the same source as the data used in the Coop et al. paper, since the layout of their maps is very similar. According to Dediu and Ladd, "Those areas of the world where the new alleles are relatively rare also tend to be the areas where tone languages are common." So clearly, as far as they are concerned, the distribution of the rare occurences is as important as the distribution of the frequent ones. And I see no problem with that aspect of their paper (though I do have a serious problem when they attempt to assign a causal relation between the derived allele and the absence of tone language).
Nevertheless, I have to admit I'm stumped by what Maju has to say about the haplotype data displayed in the rectangular plots, which means that I don't fully understand what the research is all about. So for now, I'm going to withhold judgement on this "evidence" until I understand it better.
Another problem with the same evidence, also noted by Maju, is the absence of any data from the southern or eastern portions of South Asia. South Asia is referred to in the text, but on the map most of it is blank. So even on that score, these results are problemmatic.
The map of tone languages presented in the WALS website seems much more straightforward and the apparent correlation between the distribution of tone languages, in Africa, SE Asia and Melanesia, and the distribution of P/B along the southern route looks impressive, especially when we can point to essentially the same gap in both distributions. But there are many questions regarding this evidence as well, since it's not clear how much of the very dense distribution of tone languages in SE Asia is due to the influence of tone languages from the north, especially China. I see evidence of what might be called "primordial" tone languages in this area as well, but the picture is not clear, so again I would prefer to withhold judgement on this matter until I have a chance to research it more fully.
All in all, and on balance, I do believe I see a pattern of evidence that looks very promising with respect to the hypothesis I've developed. I do see strong evidence of a gap in the same place Oppenheimer sees it, only not necessarily due to Toba. But I also see problems with some of the supporting evidence I've been presenting, so for now I'll simply repeat that all remains: hypothetical.