Chapter 4
by 
Actiaeon
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Effects of a small population
Spoilers, for the first couple of chapters of the story; do not read if you do not want it spoiled.
(I'm not a scientist I'm merely sharing this information, and trying to make it easy to digest; obviously if your more interested please research the topic for yourself.)
So in this story, the main character finds himself the only male in a small group of females. He now needs to repopulate with them; however, there is a problem.
What is the minimum population needed to do that?
One study puts the number at 160, even allowing a number as low as 80; however, after 20 generations they would need to interbreed with people back from the earth; well the story takes place light-years away; so that is not going to happen.
That's a problem.
Well, according to one study, from 2002; it takes 10,000, with 40,000 being far better.
This chart looks at the simulated genetic variation in a population; genetic variation is essential to prevent potential health complications. Populations with low genetic variation have increased rates of genetic conditions, i.e., Cystitis Fibrosis, Sickle Cell, and Tay Sachs.
Take, for example, Ashkenazi Jews. They are believed to have started with a group of ~330 individuals (their mitochondrial DNA coming from 4 individuals). However, they have many ~20 recessive hereditary disorders; Tay Sachs mentioned above being just one of them and one that only affects them.
However, also take the Sentinelese; they live on an island in the Andaman chain, they have lived in isolation on their island for 60,000 years, with only rare visitors which they usually attack. They also refuse contact with the outside world. With the possible population being from 39-400 individuals; they also seem to be quite healthy. (With them attacking anyone who visits, it's a bit hard to find out exactly how healthy they are.)
Well it seems the population could be anywhere in there, lets say 40-40,000; that is a huge range.
But such a small population would certainly invite disaster, even a minor event could wipe them out.
The small starting populations tend to decrease over time, with smaller populations being likely to be wiped out. The smaller populations in the graph were allowed to have 2-3 offspring. However, we don't want negative or no growth. So the birthrate must be higher than that, or the starting population must be higher than at least 2,000.
OK, but that is if we pack them all into one ship. What if we don't pack all the proverbial eggs in one basket.
Well, surprise, they fair better. With the population spread out, disasters are less likely to wipe the whole population out, the populations in the graph even being restricted in the number of children they could have.
So obviously the saying has its merit.
So you should spread out a starting population, start with as many people as you can, and finally get the most genetically diverse people you can get. However, what is the minimum? What if you did screening to eliminate potential recessive genes?
I'm going to say, 4,000; with it being the absolute, bare, minimum.
That may seem like many people, but humans tend to group into social groups of no more than 150 individuals; after that, you don't know everyone. It is even the number of people tribes tend to reach before splitting off from each other.
So with that information that would be ~27 groups of people; spread around an area that is larger than Europe (as for Ross 128 b, you can't inhabit much of the night or day sides).
That is low, but genetically it could be feasible; assuming GREAT genetic pre-screening.
However, please remember that this is a fictional story. I may be basing it in science; it is still fiction. So I can bend the rules, so to say, to make a more exciting narrative like not killing everyone in a disaster, or not giving the main character a debilitating disease that makes them paralyzed.
[https://www.newscientist.com/article/dn1936-magic-number-for-space-pioneers-calculated/]