walkitout (walkitout) wrote,

Notes for an analysis of (a) Rohde paper, part (1) LONG

I'm at my "sampling" stage, where I flip around in a text and read snippets, to get an overall view, before going back to the beginning and reading until I reach the end and then stop. I find that sampling counters the inevitable effects of narrative/rhetorical structure imposed by the author. Or at least exposes it.

Few agree with me, but I do it anyway.

"The first simulation, referred to as A1, used fairly liberal
parameters, as shown in the top row of Figure 3.
The maximum population was 25 million, reached in the
year 4000 BP. The ChangeTownProb was 20%, meaning
that 80% of the sims marry within their birth town. The
ChangeCountryProb was set to 0.1%, so about 1 in 1000
sims leave their home country, which may seem somewhat
high. But to put this in perspective, with a population of
25 million, there are about 48,000 people born in each
country per generation. So, on average, a ChangeCountryProb
of 0.1% will result in 48 people leaving a country
every 30 years, which certainly does not seem excessive."

(1) ChangeTownProb is _way too fucking high_.
(2) ChangeCountryProb is _way way way way_ too fucking high.
(3) 48 people leaving a country every 30 years over all human history is _insanely_ _mindbogglingly_ excessive. (Let's not even get into definition of "country", okay?)

"More liberal is the fact that, in simulation A1, there are
no locality constraints on inter-country migration or the
use of ports. Migrants have an equal chance of traveling to
any country within the continent and can use a port from
anywhere within the continent. The PortRate was set to 10
migrants per generation, in each direction, which is about
1 migrant every three years."

Tell that to the massive waves of migration that came into what is now New York Harbor and took many, many generations to get past New Jersey. In an era with _train travel_. This isn't "more liberal". _Insects_ don't behave this way.

I'm temporarily ignoring the population size and size change issues, because I think the migration assumptions are more damaging. I'm also ignoring statements like this: "Simulation A3 lowered the PortRate from 10 sims per generation to just one per generation." because I'm not entirely certain I understand what the author(s) mean(s).

Next is some discussion of the impact of changing individual parameters, followed by what happens when a bunch of parameters change simultaneously.

"If these five parameter changes have independent effects,
we might expect the net effect of combining all of
them to be either the sum of their independent additive effects
or the product of their multiplicative effects."

I cannot imagine why anyone would expect the parameters above to have independent effects. They aren't independent. At all.

"Thus, the effects of the parameters
appear to be greater than their independent additive
or multiplicative combination and we might conclude
that there is interaction between them."

Gee. Ya think?

"the NonLocalCountryProb, the
ChangeCountryProb, and to some extent the NonLocal-
PortProb have greater effects in simulations A7-A12.
This indicates that these parameters are interacting, probably
with one another."

Like, maybe _technology_ is involved?

"Lineage can spread fairly rapidly if either
the ChangeCountryProb is high, meaning there are more
inter-country migrants or the NonLocalCountryProb is
high, meaning that there may be only a few migrants
but they are more likely to travel long distances. When
there are both few migrants and they tend to move short
distances, there is a much greater resulting effect on the
MRCA date."

This matches my historical and human-behavior intuition quite well, and is actually a quite succinct way of describing my issues with a recent MRCA. My sense of migration patterns is simple:

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

Makes it hard for lineages to spread fast. Really, the twin inventions of automobiles and high schools are probably the only reason meaningful mixing has occurred at all.

That model did not have geographic limitations like oceans.

Here are snippets from Model B:

"However, because there are so
few ports and they are intended to resemble fairly major
intercontinental passages, the ports were given migration
rates 10 times higher than those in the first model."

I'm not sure what planet that would make sense on. Less connectivity leads to _more_ traffic? But they had to do that because:

"Following the increase in migration rate, simulations
B1-6 result in very similar MRCA dates as the corresponding
A1-6 simulations, averaging less than a 2% increase
in MRCA time."

Who wants a boring old MRCA long, long ago, after all? That's not going to get you a headline anywhere.

The parameter interaction problem has, as I would have expected, gotten stronger in Model B, including all parameters. I'm continuing to ignore population size and size change issues except for this:

"Therefore, although we
were not able to simulate a full-size world population, it
seems that the use of a reduced population has made these
models more, rather than less, conservative, resulting in
MRCA and ACA dates that are somewhat older than they
should be."

It may seem that way, but it's overly optimistic to believe that. I'm about a third through; I'm going to post this so I don't lose it.
Tags: genealogy

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