Adam, Eve and Population Genetics: A Reply to Dr. Richard Buggs (Part 1)

[RichardBuggs]

Hi Dennis,
Sorry to take a while to respond. You have quite distracted me by bringing up the human-chimp similarity issue, which is very interesting. I had been wanting to re-visit that issue for some time. But back to bottlenecks…

@DennisVenema

Unless I’ve missed it - and I might have - I have not seen you propose any mechanism for a precipitous drop to 2 followed by exponential population growth. I don’t see this as plausible. @Swamidass gave it a go earlier, but that showed the difficulties.

This is an “argument from personal incredulity” (to borrow a phrase from Richard Dawkins). I am interested that this now appears to be your strongest argument against a bottleneck. I suspect that many people will find argument this less compelling than the “certain” case from population genetics that you appear to make in Adam and the Genome.

As far as I can see, a step-by-step evolutionary pathway for a lineage passing through a bottleneck is clear, and you just have doubts about the selective forces that could drive it. As arguments from personal incredulity go, that is not a very strong one.

@DennisVenema

In AatG I defend two claims - (1) that as our species, Homo sapiens (anatomically modern humans or AMHs), comes into being, we do so as a population. This is the “heliocentric” quote.

I also say that (2) the dip to ~10,000 seems to be the lowest our lineage experienced over the last 18 MY, based on the methods applied to date.

Both of these statements remain accurate.

I am sorry, but I don’t think this is a complete summary of your claims in Adam and the Genome. You made very specific claims about what the literature was saying, and these have not stood up to scrutiny.

In addition, I think that most readers of Adam and the Genome pp45-65 would conclude that your major claim was that a bottleneck of two in the human lineage is almost impossible on the basis of analyses of current human genetic diversity.

But we have discussed these issues before, and anyone can read your book and make up their own mind, so I won’t belabour the point.

@DennisVenema

(2) the dip to ~10,000 seems to be the lowest our lineage experienced over the last 18 MY, based on the methods applied to date.

Both of these statements remain accurate.

No, as we have discussed, methods applied to date suggest that effective population sizes have not dipped below several thousands, but they do not show that census population sizes have not dipped below that figure.

@DennisVenema

What we have established is that a very sudden bottleneck to 2 followed by exponential population growth might escape detection using current methods if it occurred before 700,000 years ago. That is interesting, but it does not change the points I made in the book.

As I say, I think that most readers of Adam and the Genome pp45-65 would conclude that your major point was that a bottleneck of two in the human lineage over the last 18 million years is almost impossible on the basis of analyses of current human genetic diversity. Your new view is a significant departure from this point.

@DennisVenema

I’ve learned a bunch from our discussions - and I’m grateful for it!

So have I! Thank you for coming to the table.

May we all be more and more like Chaucer’s Clerk: “And gladly wolde he lerne, and gladly teche.”

@DennisVenema

Now, if I was writing AatG today, I would include a discussion of this conversation, and what we’ve learned from it, to be sure. What I would also do, however, is discuss why I don’t find the “sudden catastrophic bottleneck to 2 followed by explosive exponential growth” hypothesis to be compelling.

Good to hear that. It would be a very different chapter, to be sure.

You would do your readers a service if you wrote a blog to tell them now, as far as you are able, that present day genomic diversity in humans does not preclude a bottleneck in the human lineage between approx 700K and 7myr ago. I think you owe this to them, and to everyone who has taken the time to participate in this discussion.