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There is a tribal sentient species living in a forest (somewhat similar to Avatar one, with giant trees). I'm not gonna go into too much detail about them (if it will be required, I'll edit this question to add details). The peculiar thing about them is the distribution of probabilities of birth of individuals of certain genders genders:

  • 50% probability of birth of a fertile female (capable of giving birth);
  • 25% probability of birth of a sterile female (incapable of giving birth);
  • 25% probability of birth of a male.

And the distribution of genders in their society is approximately the same.

My question is, can such a civilization exist and survive (and maybe even develop itself), or is it biologically or even mathematically impossible?

EDIT1: Speaking of their reproduction rate: they are mammals with pregnancy period of about 8 Earth months, a female gets fertile at ~15y/o and generally has 1-3 children at once. As for lifespan, those who die from old age generally live up to 70 Earth years. The actual average lifespan is lower due to dangers like predators.

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    $\begingroup$ Yes this is possible. Just look at bee colonies. One fertile queen. Dones, that are fertile males steming from unfurtilized eggs. And workers, that are unfertile females. With this as a baseline it is obvious that you can make up any kind of distribution you like. $\endgroup$ – MichaelK Jun 21 '16 at 11:19
  • $\begingroup$ @MichaelKarnerfors Hmmm, come to think of it, I'll append the question concerning their reproduction rate. $\endgroup$ – Highstaker Jun 21 '16 at 11:35
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    $\begingroup$ Why? If it is your goal to have a setting with a gender distribution like that, then just state it as a fact. There is nothign improbable about it.... especially not since you are dividing them in neat quarters. $\endgroup$ – MichaelK Jun 21 '16 at 11:41
  • $\begingroup$ How would this have developed? - Sterile females cannot pass on the genes "for" sterility, so it must be something in the male or fertile female line. Is there a mechanism in place that influences the proportions? (As @MichaelKarnerfors hinted, sterile female worker bees are just nurtured differently but are genetically fertile females, just unable to develop properly. Fertile males on the other hand develop from unfertilized eggs, so they are only related to their respective mothers...) If you think it through, you might come up with a solution yourself! ;) $\endgroup$ – I'm with Monica Jun 21 '16 at 11:47
  • $\begingroup$ Are the infertile females distinguishable from the fertile by any means other than attempting to have children? $\endgroup$ – Murphy Jun 21 '16 at 12:18
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If you want it to be plausible you'll probably want to keep the fertile gender ratios fairly close or explain significant attrition in one gender or the other.

If the ratios are as you outline 25% male, 25%-unfertile female and 50% fertile female then any mother with a mutation that made it more likely to birth sons would gain a significant evolutionary advantage since her sons would be out-numbered 2 to 1 by fertile females. which would push the ratio of males born up.

If instead you made the ratio 25% male, 50% infertile, 25% fertile female then it would be fairly plausible and stable.

Evolutution-wise the sterile females just have to confer a strong enough advantage to their fertile relatives like with bees, ants or naked molerats so it would make sense for the sterile females to have a strong drive to protect/support closely related fertile individuals.

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  • $\begingroup$ Yeah, maybe it does make sense to switch numbers on the two female groups. Do you mean that sterile females have to be stronger? $\endgroup$ – Highstaker Jun 21 '16 at 12:41
  • $\begingroup$ It's not required: queen bees are bigger and stronger than worker bees. Ditto naked mole rat queens. But the infertile workers are willing to die to protect their sisters/nephews/nieces. $\endgroup$ – Murphy Jun 21 '16 at 13:06
  • $\begingroup$ The selection could easily go the other way - the infertile women, not distracted by having children, could become tinkerers, herbalists and knowledge keepers, passing on their wisdom to the next generation. $\endgroup$ – Peter S. Jun 21 '16 at 13:09
  • $\begingroup$ @PeterS. or they could become hunters, warriors and explorers. Since they cannot give offspring, it may sound more logical to give them more hazardous roles. $\endgroup$ – Highstaker Jun 21 '16 at 13:47
  • $\begingroup$ @Highstaker - certainly! The question is what kind of pressures exist and how are they resolved by this civilization (war/hunting/gathering/toolmaking/...). Plus it is always fun to defy expectations. :) $\endgroup$ – Peter S. Jun 21 '16 at 14:56
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Many cultures have had polygamous marriages where a man would have 3 or more wives.

While this arrangement was necessarily limited to a small fraction of the male population, if the gender ratios were 3:1, there is no reason to believe that this could not be quite common.

Few men could support 3 wives (multiple wives definite sign of wealth), but there would be no reason that this had to be true if the polygamous marriage was common.

Re: the biological aspects, many species have a disparity between males and female birth. Hive insects are the best known examples.

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  • $\begingroup$ Yeah, I thought that their society would probably not be monogamous. Also, this would probably affect their social roles, like, sterile females will be more encouraged to take on dangerous specialties, like hunters or warriors. $\endgroup$ – Highstaker Jun 21 '16 at 11:03
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Wood lemmings (Myopus schisticolor) have a sex ratio at birth of 1 male to 3 females. This is because there is a mutant gene on the X chomosome which "switches off" the "become a male" genes on the Y and/or other chomosomes.

if X is the normal X chromosome and Xm is the mutant one, then: XX and XmX and XmY = female and XY = male

In lemmings, the XmY females can only give birth to daughters, and there are more of them in the population than would be expected (45% of females, instead of exactly a third), possibly because they mature faster than XmX and XX females. On your planet, you could make them sterile instead.

Because of "a mechanism of double non-disjunction in the fetal ovary, Xm-carrying eggs only are produced" by the XmY females. Source: RV Short & E Balaban (eds.) The Difference Between the Sexes, p423-4, Cambridge University Press.

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  • $\begingroup$ Ah, great! This is one thing I was looking for as well, because I had a hard time explaining it using my knowledge of chromosomes (which doesn't really go beyond high-school biology course) $\endgroup$ – Highstaker Jun 22 '16 at 17:55
  • $\begingroup$ Problem with this is there is no way for the Xm mutation to be passed on if the Xm females are infertile and the Xm males don't exist. $\endgroup$ – Marshall Tigerus Jun 22 '16 at 19:20
  • $\begingroup$ Sorry not clear - I intended XmY females to be infertile. XmX females are perfectly fertile, and can have both daughters (XX, XmX, XmY) and sons (XY). The main thing needing explained is why the Xm mutation is not outcompeted by normal X, since a quarter of their offspring are infertile. Disease resistance, maybe? Like the way sickle cell anaemia gene is an advantage in malaria regions. $\endgroup$ – DrBob Jun 22 '16 at 19:28
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The simple way to make this happen is to imagine that this species has three chromosomes per cell.

  • XXY = "Full" female, fertile.
  • XYY = Female, infertile
  • YYY = Male

There is also a slight peculiarity in that if the left or middle chromosome is an X, then the other of those two will be forced to an X as well. The right chromosome is always Y.

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  • $\begingroup$ Or, more likely, if the far right chromosome is an X, then the offspring terminates itself as being incompatible with life. $\endgroup$ – Marshall Tigerus Jun 22 '16 at 19:21
  • $\begingroup$ @MarshallTigerus Yeah... you can do it that way too: some combos self-terminate... they are not viable. Point being: the questioner do not need to lock themselves into a thinking of only two chromosomes per cell in the organism. Heck even plain ol' humans are viable with three chromosomes per cell. And if we are talking exotic alien life on another planet.. then HighStaker can just tailor this as he likes. $\endgroup$ – MichaelK Jun 22 '16 at 19:45
  • $\begingroup$ You could even give your protagonist a rare condition where they only have 2 chromosomes and call it Ups Syndrome.....and then be the pariah of the world for making a joke at the expense of a genetic condition. $\endgroup$ – Marshall Tigerus Jun 22 '16 at 19:51

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