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Context
I know enough about genetics to know I need to ask others. I'm writing an alien humanoid species that revers the number 6 as part of their culture and religion. Their marriage equivalent forms polyamorous groups of 3 males and 3 females (lets call them units for now) where members can reproduce in any combination they see fit. Children are then raised by all unit members.

Question
I am intending the species to stay small and to grow very slowly if I can avoid bottleneck issues, even into a space faring civilization. How many children should a unit need each year/lifetime? How would these units improve or worsen their genetic diversity? How small an age window can we have possible unite mates? What might I have overlooked?

What I hope will provide enough information to answer how careful they need to be, ask if you need more.

  1. For the sake of this question they are similar enough genetically or mimic the same processes we do for reproduction so we can use our genetics to determine answers.
  2. The culture is not developed enough to understand genetics but will recognize that visible traits are inherited, they will likely account for a history of sickliness or deformities when choosing a mate.
  3. Concerning possible mates they do not allow children of a unit to mate with any member of said unit even if they are not blood related.
  4. Most will form units with others of their same generation at the age of consent. It is very rare to not form a unit with people out of their same generation (within a few years).
  5. They have a very narrow period they can conceive yearly that cannot by realistically adjusted. However their chance to conceive is very high so it is unlikely they will not have children if they try. This means there is a regular influx of new members of the species all within roughly a months time each year.
  6. A unit is under no directive to breed if they do not want, but they can have children for a roughly 20 year period.
  7. While I will be tweaking numbers we can use and assume normal human lifespans and calendar for discussion where I have not specified otherwise.
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    $\begingroup$ The six-mate-units are utterly irrelevant. To keep the population stable, on the average each woman needs to be replaced by a daughter. Since boys are also produced with the same probability as girls, each woman needs to produce 2 children who survive to reproduce. Since some children die before reaching adulthood, you need a safety factor which depends on the specific conditions. (It's about 2.05 children per woman for modern developed countries.) To find out how many children each woman needs to produce per year on the average, divide by the average lifespan of the women. $\endgroup$
    – AlexP
    Apr 27, 2022 at 0:50
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    $\begingroup$ "Most will form units with others of their same generation at the age of consent." Just a side note, since this alien culture is supposed to be "less developed": on Earth there was no concept of "age of consent" in any pre-modern society (say, 18th/19th century and earlier). Marriageable age was reached with sexual maturity. Of course, aliens do not have to be that way. $\endgroup$
    – Avun Jahei
    Apr 27, 2022 at 1:05
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    $\begingroup$ We humans have 2*23 chromosomes. Potatoes can have anything between 3*12 chromosomes and 5*12 chromosomes - all can interbreed (the potatoes with potatoes, not with humans, probably). It strikes me that you can pretty-much make-up your own rules about group size, reproduction and mutation so it fits your story, then design the genetics to fit that set of requirements. You could ask a question about that - but it could get complicated. $\endgroup$
    – Angry Muppet
    Apr 27, 2022 at 2:24

1 Answer 1

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150/1,500

The minimum viable population on Earth uses the 50/500, 50 for the minimum of genetic diversity, 500 for genetic drift needed to combat environmental changes, but the problem is that your system allows for the possibility of 'dead ends' in regards to the gene pool. If you have three potential fathers for a group of three females, there's a very strong chance that one father may have any children with any of the given females, creating 'dead ends'. In other words, this is a bad reproduction system. Thus, the 50/500 number must be tripled to compensate for the 'dead ends'.

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  • $\begingroup$ Thank you for your response. I do not quite follow the "dead end" part of this. Do you mean that in a given cycle its possible one male will mate with two or three thus keeping the other males genetic code out of the gene pool? $\endgroup$
    – Sith Philosopher
    May 4, 2022 at 13:58
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    $\begingroup$ Yes, that is what I mean when I refer to as 'dead end' - a male in a relationship attempting to have offspring, but not producing any. This is selected against from an evolutionary standpoint, by the way. $\endgroup$
    – Halfthawed
    May 6, 2022 at 1:40

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