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In my pre-bronze age city, there are roughly 20,000 individuals. 9,000 sexually mature males, 9,000 sexually mature females, and 2,000 children. For a variety of unimportant reasons, they have been cut off from the rest of mankind. They need a strong work force as soon as possible, so they resort to population boosting.

Question: assuming that no one has sexually transmitted diseases, what is the most effective way to increase the population while keeping the negative effects of inbreeding at bay?

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  • $\begingroup$ highly related (possibly duplicate?): worldbuilding.stackexchange.com/q/26908/12207 $\endgroup$ – nitsua60 Mar 10 '16 at 1:18
  • $\begingroup$ @nitsua60 not duplicate , I'm asking how to increase population while doing that , and I've never seen that question $\endgroup$ – user15036 Mar 10 '16 at 1:22
  • $\begingroup$ Right--I didn't feel I knew enough of what you were intending to VtC as duplicate, so that's why I just linked it. If you've never seen that one, I hope some of the answers there help you think about your problem. $\endgroup$ – nitsua60 Mar 10 '16 at 1:25
  • $\begingroup$ Avoiding inbreeding is trivial - forbid marriages/sex between relatives. If you want to maximize the genetic diversity in the long term, you would need to encourage/force birthing children outside wedlock at some high rate to spread the genes out as soon as possible. $\endgroup$ – Oldcat Mar 11 '16 at 0:16
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This was going to be a comment, but it ended up too long.

Your population may not run into a huge number of problems with inbreeding if your starting population has a fairly large spread of human genetic variation. (If you have a population with limited human genetic variation, you can ignore the rest of this answer as it won't apply).

It's very likely the entire human race has been reduced to that number at least once. Humans don't have as much genetic variation as one would expect. It is sufficiently limited, in fact, to be seen as evidence of at least one population bottleneck. To get more directly to the point, "roughly 20,000 individuals" is right in the mid-range for human population estimates in the Toba catastrophe theory, which is one possible example of a human bottleneck. (There are other theories, at least one of which suggests a bottleneck of 5000.) That would seem to suggest that 20,000 may sufficient to prevent catastrophic inbreeding problems.

Something you may want to look into is "Minimum viable population," which is a calculation of the probable minimum population of a species to give it at least a 90% chance of survival over the next thousand years. When inbreeding effects are included, the median MVP is a little under 4,200.

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  • $\begingroup$ Don't forget, the diversity among 20k people would have been much higher before the bottleneck. So reducing the ancient population to 20k would be much less confining than reducing a modern population to the same count. $\endgroup$ – JDługosz Mar 20 '16 at 3:57
  • $\begingroup$ Minimum viable population isn't applicable here. It measures the likelihood of a population going extinct from "natural disasters or demographic, environmental, or genetic stochasticity." As such it is primarily concerned with the population's resilience in the face of natural disasters such as disease and famine. It is not a measure of the minimum number of individuals required to prevent inbreeding effects. $\endgroup$ – Mike Nichols Mar 20 '16 at 5:51
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What you need is the same thing actual ancient societies needed to survive: functional government enforcing basic societal institutions, as well as the absence of external threats.

Given your population size, inbreeding would be nowhere near the top of the threats to your community.

External threats would probably be the biggest worry: invading forces, disease, crop failure due to weather, and so on. Some of these are eliminated by your scenario (since you aren't in contact with other humans) but others remain. There isn't much you can do about them, though (assuming a pre-bronze-age understanding of things like medicine).

What your society would need to address are internal threats, from individual offenses such as murder, theft, to major incidents like civil war. These are the things that will keep your society from flourishing.

What you need is a stable society where property rights are enforced, the strong aren't able to come and just steal everything the weak produce, crimes are punished, and so on. This will create the conditions for growth.

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The primary danger with inbreeding is the resulting genetics becoming weaker due to the possibility of non-optimal genes or recessive genes being able to reproduce and become dominant in a smaller gene pool.

To counter this with your smaller population, just make it a law that you can't have more than one child with a person. This ensures that large amounts of combinations are presented for optimal growth.

Although with a population the size of 20000, inbreeding isn't too large of a problem. It would take a few generations for any negative effects to come forward, but non-segregated breeding should prevent that. Your population may see a shift just due to being separated, but overall it won't have that much of an affect.

Oh and you want to maximize the population right? Just pop them out as fast as you can without over expanding your resources. Have fun ya filthy animals.

EDIT:

Just some points of clarification based upon comments below.

First, segregation was in reference to classes or families. So long as the population doesn't divide up by class with a "pureblood" type breedin mentality, the genepool should remain diverse. If that already doesn't exist, then awesome.

Second, the law wasn't restricting two people to producing only one child. It was saying that each distinct couple can only have one child. So I could have 5 children so long as it is with 5 different women and vice versa. It ensures variance. So yes, promiscuity

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    $\begingroup$ A law restricting each couple to have only one child would be counter-productive, probably even suicidal for the society. In primitive societies, stable life-long family units were essential for providing the resources necessary to raise children. You are basically prohibiting this structure from producing enough offspring to survive. Plus, given the infant mortality rates in pre-bronze-age society, you would be ensuring that a large percentage of couples had no children at all. $\endgroup$ – user16107 Mar 10 '16 at 13:38
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    $\begingroup$ @dan1111 I agree . each family should be obligated to raise a minimum of 3 children to adolescence. And sexual promiscuity should be encouraged $\endgroup$ – user15036 Mar 10 '16 at 14:11
  • $\begingroup$ I'll clarify in an edit as well, but the first two comments show my explanation to be sub-par. First, segregation was in reference to classes or families. So long as the population doesn't divide up by class with a "pureblood" type breedin mentality, the genepool should remain diverse. If that already doesn't exist, then awesome. Second, the law wasn't restricting two people to producing only one child. It was saying that each distinct couple can only have one child. So I could have 5 children so long as it is with 5 different women and vice versa. It ensures variance. So yes, promiscuity. $\endgroup$ – ChronoD Mar 10 '16 at 18:06
  • $\begingroup$ @ChrisG whoops , I meant to say there , not they $\endgroup$ – user15036 Mar 10 '16 at 21:25
  • $\begingroup$ @ChrisG, I did understand your proposal, but I still think it would greatly restrict the number of children. Even ignoring human tendencies toward life partnership, the one child per couple rule greatly reduces the opportunity to have children in ancient society where travel of even a few miles is difficult and providing enough food to raise children requires the dedicated work of the whole family unit. $\endgroup$ – user16107 Mar 14 '16 at 11:31
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Any population expansion will likely take several generations.

A range of things would be necessary to ensure population growth. Here are a few.

A large, diverse, consistent food supply. Many societies crumbled when the food supply faltered. Not only did mass starvation occur on occasions, but it was a primary driver of civil war. Also good nutrition prevented a range of diseases that weakened many societies that relied on narrow food choices. Seafood is particularly valuable due to its supply of both protein and omega three fats.

Social cohesion. This is a sense of something that binds the people together from generation to generation. The Egyptians did this well through a common work (pyramids) and the Romans through citizenship laws. While a strong common focus exists, divisions that lead to war can be countered.

Laws or cultural emphasis on sanitation. Especially with cities, sanitation usually set a upper limit for population. Children are particularly vulnerable, with typical mortality rates of upwards of 50% before modern medicine. This is perhaps the major cause of population growth in the last century. Hebrews and Romans flourished in part due to their strong focus on sanitation. Invention followed the expectations.

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Inbreeding is a problem when it results in a doubled recessive gene. This can have the following possible effects:

  1. The gene is lethal in utero. The fetus miscarries.

  2. The gene produces an obvious birth defect. Midwife makes sure it is stillborn.

  3. The child is physically or mentally handicapped. This one is tough: Sacrifice to the gods? Survival games at age 12 that you must pass? Let your imagination run.


Social institutions can help in other ways:

3 failed pregnancies is grounds for no-fault divorce. Just a bad combo of genes. Let both parties try with other people. In practice there will be some stigma attached to this. Divorced couples will likely only be able to find new partners among similarly stigmatized

Men who have something more on the ball (heroes, leaders...) are encouraged to father extra kids outside of their marriage. In essence, make extra copies of known good genes.

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There's actually a small tribe in the Amazon that utilizes a unique strategy called linguistic exogamy to encourage gene flow in and out of their otherwise very small society. Members are expected to marry someone that speaks a DIFFERENT language than them, and then learn their spouse's language and teach them their own through conversation and living together. Thus they're encouraged to seek out people beyond their immediate surroundings, but also are able to spread their culture. Obviously this particular approach doesn't really fit your scenario, but it's an interesting display of how cultural practices can be shaped to fit biological needs.

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There will be no inbreeding depression in a population of 20,000 individuals as long as your individuals don't intentionally marry their cousins. One of the most basic principles of genetics is called Hardy-Weinberg Equilibrium and it's fairly straightforward. The basic idea is that in a population of individuals the frequency of an allele will tend not to change. There are lots of things that can change allele frequencies, like natural selection or migration, but for our purposes we can assume that allele frequencies won't tend to change. What that means is that as your population increases in size the frequencies of negative alleles won't change. This means that whatever your starting population's frequency of genetic disorders will be the same as a later, expanded population. Increasing in population size won't magically increase the frequency of negative alleles in your population, and why would it? The negative and positive alleles are treated equally (discounting mutation and natural selection of course) by the genetic lottery and so the ratios of both will remain unchanged even in an expanded population.

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Since this is a bronze-age city, there is no reliable contraception, so anything which encourages marriage will also encourage population growth.

Assuming you have leet social engineering skills, I'd try

1) Elimination of dowries. Couples can marry young.

2) Discourage marriages within clans. This is actually something of a problem, since bronze-age societies were quite violent, and social groups were quite close-knit - at least, the ones which survived.

3) Riffing on 2), make outmarriages the accepted mechanism for preventing inter-clan conflict. In effect, the widespread intermarrying will tend to break down clan barriers. Don't expect miracles, though, especially in a bronze-age society where long-range communication and transportation (and I mean more than a few blocks) is difficult. Communication is essentially entirely face to face, since literacy is almost non-existent and cheap printing and paper are unknown. Figure on a combination of political and religious pressure. This is decidedly non-PC, since the historical uses of intermarriage seem to have favored shipping off the girls, not the boys (who are needed at home as warriors).

Furthermore, fast population expansion is a problem, since this implies fast resource expansion. With agricultural efficiency low, large populations imply large farming areas surrounding the city. As these areas expand they will tend to start splintering into local political units, with all the problems of political struggle and war which that implies.

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