What is the process by which humanity gains its prescience-immunity in the Dune series?

Question

I've always had this mental block about the timelines of the Scattering and the prescience-immunity conferred by the Siona genes.

If the Scattering happens at the end of Leto's rule and the Honored Matres come back in 1500 years, how is it that humanity is now more or less immune to prescience?

It is implied that Siona is, apparently, the only person with naturally immune genes by the time of Leto's death, and that through breeding her humanity gains immunity to prescience.

However, in order for that to make sense, Siona's genes would have had to spread throughout the Scattering.

Since it happened so quickly after Leto died and Siona is ultimately bound by how quickly a human can give birth, what is the process by which humanity gains its prescience-immunity?

Answer 1

Immunity from prescience is granted by association with someone with immunity from prescience, hence the need for a Guild Navigator to be involved in the plot against Muad Dib in Dune Messiah. You don't need the entire population to have a Siona gene for their movements to be invisible to those with prescience, just their leaders and people of influence.

“We don’t understand this Power of second sight,” Irulan said. “There’s a point. Edric says my husband cannot see, know or predict what happens within the sphere of a Navigator’s influence. But how far does that influence extend?”

“There are people and things in our universe which I know only by their effects,” Edric said, his fish mouth held in a thin line. “I know they have been here … there … somewhere. As water creatures stir up the currents in their passage, so the prescient stir up Time. I have seen where your husband has been; never have I seen him nor the people who truly share his aims and loyalties. This is the concealment which an adept gives to those who are his.”

Scatter (pun intended) a few hundred thousand prescience-immune people into a population of billions and you're going to have gaps everywhere. With each passing generation it becomes more and more difficult to see anything of use at all.

Answer 2

Siona had to have had many sons and grandsons who are not as limited in how many babies they can have in one lifetime.

Additionally I think it is safe to say that the Scattering wasn't instantaneous- if the first wave of immigrants happens over 200 years that gives her descendants time to be part of it.

Also if she has just one descendant on each small colony ship that gene can overtake the colony through a founder effect (and some luck). This is how a beneficial mutation can get established - which is easier to become high frequency in a small population, like a spaceship colony, than a large one - a planet with 10 billion people. Once that spaceship colony grows to a billion people everyone will be descended from the founder related to Siona (again this takes some luck but I think we can grant that in the Dune universe).

https://en.m.wikipedia.org/wiki/Founder_effect "In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population."

Answer 3

It is not as implausible as all that. From your numbers, 1500 years have elapsed since there was one person with the genes. Let us suppose that a generation takes 20 years. So there have been 75 generations since there was one person with the special genes.

There is a concept in genetics called Identical Ancestors Point https://en.wikipedia.org/wiki/Identical_ancestors_point which states that if we go back in time we eventually reach a point where every person in the past either has no descendants alive at all, or is an ancestor of every person who is alive today.

According to the article the we can suppose (for a well-mixed population) that we need to go 1.77 * log_2{N} generations for a population of size N to reach this point. The biggest N for which this would work is 2^(75 / 1.77) = 6 * 10^12.

The population of Earth is around 7 * 10^9. So allowing for a 1000 worlds each with an Earth sized population (I expect there would be more worlds, but with fewer people per world), it is likely that the Identical Ancestor Point has been reached.

Of course I am assuming that geography permits intermarriage between worlds fairly frequently, and I have no idea how many worlds there are supposed to be in scattering. But descriptions I read of the scattering talk of "trillions of people" (see https://en.wikipedia.org/wiki/Heretics_of_Dune) (i.e. 10^12 is the right order).

So it is not a huge reach to suppose that one person's genes are now spread to most of humanity.

Answer 4

That the Siona was immune to prescience was a fact know to most, if not all, actors of political scene at the time.

Bene Gesserit, Bene Tleilax, Ixians, New Tleilaxu, Fish Speakers, among countless others, took steps to ensure that genetic trait be introduced to all their important lines.

Which means that propagation of that genetic trait in the general population, be it in Old Empire or in the Scattering, was by design in large part. Which means that in just 1500 years majority of the population would be immune to prescience, which was, of course, the point.

But it didn't have to be. After all, key part of The Golden Path as implemented by Leto II was to ensure humanity survives by having that option available. This, in turn - again, by the definition as in The Golden Path - was not synonymous with EVERYONE surviving. Just enough.

So, to answer your question: it doesn't matter how this happened. It only needed to happen just enough times for just enough groups to have that genetic trait in just enough people, though preferably in groups choosing to Scatter. As Valorum nicely noted in his answer : Scatter (pun intended) a few hundred thousand prescience-immune people into a population of billions and you're going to have gaps everywhere. With each passing generation it becomes more and more difficult to see anything of use at all.

Also, science can tell us things, too... For example: today, on Earth, it is estimated that for 2 two random people to find a common ancestor, they need to go back mere 20 generations. go back 35 generations more and those two people arrive at EXACTLY SAME set of ancestors (of course, via different genetic routes). FYI - 20 generations for us is barely 500 years.