walkitout (walkitout) wrote,

Notes for an analysis of (a) Rohde paper, part (2) LONG

"We found that, even with different architectures and
quite widely varying parameters, the date of the MRCA is
relatively stable, roughly falling between 2000 and 6000

Well, because you manipulated the fuck out of your migration rate to _get_ that effect, sure.

"Model B was possibly overly
conservative in that it allowed only four intercontinental
ports with between 10 and 100 migrants per generation
across them. Certainly the interchange between Africa
and Eurasia has been higher than that, and there are potentially
other routes of passage between the continents,
such as migration from Borneo to Madagascar and potential
contact between Greenland Inuit and Vikings and
between South America and Polynesia."

If Rohde et al were arguing for a Eurasia MRCA in the time frames under discussion. We all _agree_ that interchange between Africa and Eurasia was higher than the rate you picked, but that doesn't really explain why you picked a rate that was so unbelievably high for exchange rates between the Americas and Eurasia (never mind all the Pacific Islands and everywhere else).

I'm continuing to ignore population issues.

Model C is supposed to fix some of these problems, but will in fact make some of them worse (for example, I disagree with his pick for likely start of habitation in the Americas). I'm more than a little disturbed by comparing the rates of migration between some of his ports:

Africa and Eurasia, there are ports between modern-day
Morocco and Spain (100 sims/generation), Tunisia and
Italy (100 s/g), Egypt and Israel (500 s/g), and between
Ethiopia and Yemen (50 s/g), providing several points of
contact. Other static ports include a pair between Thailand
and Malaysia (100 s/g), and from the tip of Indonesia
(Timor) to Arnhem Land and from New Guinea to Cape
York, both with a rate of just 5 s/g."

Really? Just 20X going between Morocco and Spain vs. New Guinea and Cape York? Yeah, I don't think so.

"The migration rates used in this model are not based on
firm historical data, because such information is, for the
most part, unknown (Jorde, 1980)."

Yes, Rohde et al used a _1980_ citation in support of this assertion. He has no problem at all deciding the Americas had no one in them before 20K BP, either. This isn't a matter of whether historians or archaeologists or whoever have one or many opinions. Rohde is basically creating a MRCA and ACA calculating SIM based on a Risk board. This thing bears exactly the same relationship to population genetics that Risk bears to military strategy, as well. All right, what's Jorde?

Jorde, L. B. (1980). The genetic structure of subdivided populations:
A review. In J. H. Mielke & M. H. Crawford (Eds.),
Current developments in anthropological genetics: Vol. 1
(pp. 135-208). New York: Plenum Press.

Believe me, there are more recent reviews of this issue than Jorde.

"Without a firm basis in fact, an attempt was
made to err on the side of conservatism."

I can't tell whether they really mean this, or if they just feel they need to say it. I'm suspicious they actually mean it.

"However, experience suggests that its results
are quite stable and insensitive to all but the most
significant changes."

That's grad student speak for, "I really want this to be true, and have worked hard to find a range of inputs that produces stable results. When you come up with more believable inputs that produce wildly different results, I'm going to be shocked, shocked, I tell you!"

"In the previous models, immigrants using a port could
settle in any random town in the destination country. As
a result, immigrants were immediately assimilated into
the host community. It is more often the case in modern
times, and presumably throughout history, that immigrants
will gravitate towards a sub-community of fellow
immigrants who share the same cultural or linguistic
background. The result is a delay in the exchange of
lineages between the immigrants and hosts. This is simulated
in the model by having new immigrants initially
choose from one of five towns, out of up to 46, in the destination
country. This set of towns is dependent on the
source country from which the migrant came. As a result,
immigrants will tend to cluster, though they will not be
entirely segregated."

I won't lie. I actually found this paragraph very early in the sampling process and have saved it until now, because you really needed to understand just _how amazingly awful_ the previous models were that the authors could truly believe this was an improvement.

This is not how immigration works. Once again, from the previous entry, I'll start calling them walkitout's rules of immigration if it'll make you happier:

(1) People don't move.
(2) If they do move, then they stay put.
(3) They go with people they know.

Immigrants don't come to 1 out of 5 out of up to 46 (2-10% of the available choices of where to settle) based on their source country. They come to the same few blocks of _one_ town -- where that town is well under 1% of their choices of where to settle. And they do that even with significant, cheap transport options to do otherwise.

"A basic assumption of the model is that sims act independently
in their migration decisions."

You know, I can't do this any more.

The whole MRCA question is completely open, as far as I'm concerned. We're not going to get any kind of answer out of simulations until their modeling improves. This isn't a believable approximation, and there isn't any obvious excuse for this. Spherical chickens of uniform density are required for FFT calculations; if you're running SimLineage, you can put in any assumptions you like.

Hopefully, they'll be better assumptions than these.
Tags: genealogy

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