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After a global apocalypse, a remote, large island with fertile areas and large amount of livestock and pre-apocalypse technology is populated with 8 humans.

Description of 8 humans are as follows:

Boy 1: 17 years old

Boy 2: 16 years old

Girl 1: 17 years old

Girl 2: 18 years old

Girl 3: 16 years old

Girl 4: 7 years old

Woman 1: 27 years old. She is doctor.

Woman 2: 39 years old. She is the mother of Boy 1.

  • The average reproductive(allowed to undergo pregnancy) age of females is 16-44 years.
  • There should be minimum gap of 6 months between two consecutive pregnancies.
  • Anyone of suitable age can mate with anyone. Incestuous mating s discouraged but not impossible.
  • Not more than 3 females can become pregnant at the same time to provide adequate workforce (This criteria was added later.)

The group decides to repopulate the Island.

What should be their reproductive strategy for next 25 years so as to get the maximum number of offspring with moderate genetic diversity?

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    $\begingroup$ they probably might not survive in the long run , due to the fact that inbreding would become a serious problem after only two generations , and all manner of genetic disorders would appear. $\endgroup$
    – user15036
    Mar 6, 2016 at 17:19
  • $\begingroup$ There are, however , two solutions that can be produced depending on when the apocalypse happened , and the field of expertise of the survives. If it was near modern times , then the only real solution , ( although very dangerous and dark ) would be to irradiate their genitalia / semen with x-ray radiation , this might solve the issue of lack of genetic variation although there is a very high possibility for genetic defects $\endgroup$
    – user15036
    Mar 6, 2016 at 17:26
  • $\begingroup$ The second option is to manually change the dna of the egg /sperm cells , although this would rely on a high amount of luck , since one of the 8 would have to be a geneticist , there would have to be a modern ( relative to the time period ) hospital , and the apocalypse would have to take place in the future. $\endgroup$
    – user15036
    Mar 6, 2016 at 17:30
  • $\begingroup$ You should change your changed criterion of "Not more than 3 females" to reflect the fact that, starting at year 16, the female children will begin to become fertile. At that point, the labor of the subadult children will have become significant, and the older first-gen children can start bearing. $\endgroup$
    – WhatRoughBeast
    Mar 6, 2016 at 19:02
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    $\begingroup$ Note that there is another question on here that places the minimum starting human population that can survive over the long term at 160: worldbuilding.stackexchange.com/questions/3/… One immediate survival problem they will face as part of this question is infant and pregnancy mortality. Not all fetuses, babies, and toddlers will survive, but even worse, not all mothers will survive childbirth. $\endgroup$
    – Todd Wilcox
    Mar 6, 2016 at 23:18

4 Answers 4

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This is clearly a polygamous situation as there just aren't enough males so that each female gets a unique male.

We start off with making four initial bonds:

Boy 1 => Girl 2 (after 1 year when she is 19 years of age)
Boy 1 => Woman 1

Boy 2 => Girl 1 (after 2 years when she is 19 years of age) Boy 2 => Girl 3 (after 3 years when she is 19 years of age)

Each bond is aimed at making 6 babies so as to allow some genetic variation later on.

Assuming that boy-girl ration in offsprings is 1:1, there would be 12 new boys and 12 new girls in 7 years.

After about 12 years girl 4 would reach 19 years and would be ready to participate in the process (if she chooses). Boy 2 would be 28 years old then so it would be a slightly odd pair, but within current social norms anyway.

After 19 years the population of the island would be nearly 8 (initial) + 24 (first generation) + 6 (first generation, delayed) = 38 individuals, 29 of which (2 initial boys + 3 initial girls + 24 of first generation) are still in procreation age.

(12 x 12) + (2 x 3) = 150 different mating combinations are now possible (considering no incest). My idea would be to reproduce slowly, taking into factors such as problematic genes (such as disease vulnerability and IQ). In this way you can allow all members to procreate while also applying some eugenics.

EDIT TO ADD

Here is a chart for the long term process. Males are presented as letters (a and b for Boy1 and Boy2 respectively) while females are represented as numbers (1, 2, 3, 4, 5, 6 respectively girls 1-4 and women1-2)

There can be types of repopulating plans. First one centers on self respect and dignity (no incest or cross pairing) while the other centers on genetic diversity and an option for later implementation of eugenics. The first chart is shown below. The x-axis represents time (in years) as alphabets (year 0 is now while year A means after 1 year, B means after 2 years and so on).

enter image description here

Correction: Girl 4 is missing from the chart. she joins in at year L (12 from the start) when she is 19 years old and is paired with b.

I will try and make the second chart sometime.

Here is the second chart. The second generation children pairing can go the same as in chart 1. The main difference between chart1 and chart2 is how people of generation 1 are paired. Chart 2 includes girls at ages 18 (instead of 19 as in chart 1) and includes cross pairing for increased genetic diversity.

enter image description here

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    $\begingroup$ Is there a reason you're insisting on waiting for them to turn 19? 18 is legal in most countries, but since we're post-apocalypse, you don't even need to care about legality - all but the 7 year old are biologically old enough to bear children already. (Speaking strictly pragmatically, of course, not condoning that sort of thing under normal circumstances.) $\endgroup$
    – Darrel Hoffman
    Mar 6, 2016 at 15:41
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    $\begingroup$ First and foremost is the consideration of maturity. Not the age of legality, but that of maturity. When the person (specially a female, as she has to bear the hard part) is mature enough and understands the implications of their decision. Second is the consideration of biology. A girl aged 13-16 would have severe discomfort with even a normal coitus process, let alone pregnancy and childbirth. 19 appears to be an age when girls of all cultures are physically and emotionally strong enough for motherhood. Women are not baby-making machines. They are as human and honorable as everyone else. $\endgroup$
    – Youstay Igo
    Mar 6, 2016 at 16:54
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    $\begingroup$ Sorry, didn't mean to touch on a controversial topic like that, but given the premise of the question, we're already way past those kinds of concerns in terms of cultural taboos. Given that we're allowing polygamy, mandatory arranged pairings and incest, all of which are considered taboos of varying degrees in most of the world, removing age limits seems fairly minor by comparison. Also, the percentage of girls who are voluntarily sexually active between the ages of 13-16 or even earlier is much higher than you might expect. Again, not saying it's right or wrong, just pointing out facts. $\endgroup$
    – Darrel Hoffman
    Mar 6, 2016 at 18:09
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    $\begingroup$ Yes, you are correct about that. What I wrote is what I would have done, if I was in charge of the situation. Other people could have other priorities of course. Besides the issues of dignity and respect (first and foremost for me) there are also serious health concerns with pregnancy and childbirth in early ages. When you only have 5 women to populate the place with, you wouldn't want to lose any of them :p So even if there was a brute with no regard for human rights or whatever, he would still probably refrain from including too young girls for the risk of losing a child making machine (jk). $\endgroup$
    – Youstay Igo
    Mar 6, 2016 at 19:06
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    $\begingroup$ OK sure. I will add content to my answer and post a comprehensive pair-time chart about it to make things easily understandable. $\endgroup$
    – Youstay Igo
    Mar 7, 2016 at 9:04
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All females stay pregnant, except for the 6 month gap you've stipulated.

The father of each pregnancy alternates between the two males.

Woman 2 might be able to have 4 children in the time before she hits 44. She should have 2 from each male. I believe the dangers of inbreeding are offset by the wider genetic pool. The true dangers of inbreeding come from repeated generations rather than a single generation. This population will likely suffer from this to some extent in the future, but widening the pool as much as possible should minimize this.

Girl 4 has 9 years before she can start reproducing.

These alternate pregnancies will continue for 16 years (with Woman 1 and Woman 2 dropping out during this time). Woman 1 has been training everyone with as much general medical knowledge as she can, with several getting more intensive "apprenticeship" training.

Once the next generation is ready to reproduce, the females can start the alternate pregnancies, skipping the father and any full brothers. Boy 1 and Boy 2 may need to be given some priority here, to minimize the amount of DNA that will be lost to this "bottleneck".

EDIT: I suggest that pregnancies be begun at the end of autumn so that the early months are during the less productive winter months. I also suggest that a year between pregnancies for better recovery, and more care given to each infant.

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  • $\begingroup$ I have edited the question to address the problem of inadequate workforce.Please check it out,if you wish. $\endgroup$
    – user18100
    Mar 6, 2016 at 16:12
  • $\begingroup$ You might also come up with requirements like "try to have five babies in the first year, while there is still canned food left" or "wait until Girl 4 is old enough to look after babies, and until you have a good idea what your sustainable food production looks like." Plenty of drama for your story. $\endgroup$
    – o.m.
    Mar 6, 2016 at 17:03
  • $\begingroup$ I guess I can put up any requirments I wish.What is the big deal? $\endgroup$
    – user18100
    Mar 7, 2016 at 5:10
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The survivors in your story will have two areas of concern, getting the right mix of children and having them survive. Michael answered regarding combinations, so let me remind you of the need for surviving offspring. Possible tactics go from breeding as many babies as possible, in the hope that a few will survive, to a more deliberate pace.

  • Pre-apocalypse supplies will run out, go bad, etc. Pretty soon the doctor will have only "expired" medications to work with. The likelihood of infant or maternal death will go up sharply, and being pregnant that often won't improve their health.
  • Gasoline will go bad in a couple of years. No more mechanized agriculture.
  • Are the buildings prepared for wood cooking and heating?
  • If lifestock is released into the wild, it might unbalance the ecology. If not, who keep it under control? It might become necessary to slaughter a lot of them rather than risk uncontrolled rabbit or goat infestations.

Your survivors are seven adults or near-adults. Looking after dozens of toddlers will tax their endurance.

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  • $\begingroup$ I have edited the question to address the problem of inadequate workforce.Please check it out,if you wish. $\endgroup$
    – user18100
    Mar 6, 2016 at 16:13
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You are way below the Minimum viable population which will allow your humans to build a viable, stable long-term population. This means your new population of humans would suffer severe effects of inbreeding.

Current estimates for human MVP

  • You need at least around 80 individuals for a stable population. This is if you have hand-picked your starting individuals for high and very favorable genetic diversity. Also you need a strict breeding plan, which is based purely on genetics.
  • You need around 160-200 individuals if you don't have the strict parameters mentioned above.

Source: “Magic number” for space pioneers calculated

Many of the human MVP estimations also only cover relatively short periods, e.g. population ages of 200 years. For infinitely stable poplulations the MVP limit is likely to be even lower, some numbers estimating it at 5000 individuals.

Source: A Magic Number?

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  • $\begingroup$ These estimates are based on lack of evolutional pressure to eliminate genetical illness. $\endgroup$
    – Ginden
    Mar 8, 2016 at 16:36

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