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I had an idea for a humanoid species that has a significant number more female births than male births, how would this trait have provided enough of an advantage to allow the species to survive to develop civilization?

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closed as too broad by pluckedkiwi, L.Dutch, Giter, Aify, Mołot Jun 12 '18 at 18:12

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ This is an astoundingly broad question. The possibilities are truly endless. $\endgroup$ – pluckedkiwi Jun 12 '18 at 17:19
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    $\begingroup$ Not only is it broad, it's also well documented. Try this for breeding ratios, it's a bit dry. ncbi.nlm.nih.gov/pmc/articles/PMC2833377 $\endgroup$ – Naryna Jun 12 '18 at 17:25
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    $\begingroup$ @Aify, POB is way, way, way overused. What, exactly, do you think it means (especially when both of Giter's suggestions would have led to definable and judgeable answers)? $\endgroup$ – JBH Jun 12 '18 at 18:28
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    $\begingroup$ @Giter My problem with that is, "yes, because X" versus "No, because Y" could both be equally valid - which leaves it POB up to OP to decide. $\endgroup$ – Aify Jun 12 '18 at 18:58
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    $\begingroup$ @Aify: The 'opinion-based' close vote is used if the question causes answers based primarily on opinion. The 'too-broad' close vote is used if the question causes more than one answer to be valid. If several answers are equally valid, then even if an opinion is needed to choose one, as long as the answers themselves aren't opinions then the question is too broad but not opinion-based. $\endgroup$ – Giter Jun 12 '18 at 19:09
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The evolution of sexual reproduction comes at a significant cost to a species because males cannot produce offspring. Species that are all hermaphrodites are capable of producing twice as many progeny each generation as species with an equal sex ratio. This is called the two-fold cost of sex. Sexual reproduction evolved and persists however because species that routinely recombine their genes are, to put it simply, better at evolution.

So following this line of thought, you need at least some males in order to have a sexually reproducing species. But why does the cost have to be “two-fold”? Why should half the species’ population be male when a much smaller proportion of males could serve the needs of the species. In fact, a lower ratio of males would be evolutionarily optimal from the perspective of the species but it won’t occur because evolutionary game theory occurs at the individual level. It turns out that a 50-50 male/female ratio is an evolutionary stable strategy according to Fisher’s principle. Essentially, if a population has a skewed sex-ratio, then it becomes evolutionarily advantageous for individuals to produce offspring of the minor sex as they will have better reproductive opportunities. Thus the system is always self-correcting towards the evolutionarily stable strategy of 50-50 male/female.

So, having more females than males actually is an evolutionarily favorable strategy as it increases the total potential progeny of the species, but it can never persist stably because evolution is always pushing against it. If you want a population like this you need to break Fisher’s principle in some way. Check out the parasitic/symbiotic bacteria Wolbachia that change the sex of infected insects and the sex of their progeny to suit their own needs.

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  • $\begingroup$ I cant remember where, but there are suggestions that there are a few more males born than females. But eventually more females are left to breed. This is fairly obvious if you look at mortality rates between males and females by age. At puberty theres a huge spike in male mortality, this reduces the total amount of men to women ratio, which is actually very healthy as there are less men necessary for a healthy genepool while enough women are necessary because the time and dangers of pregnancy and nurturing. $\endgroup$ – Demigan Jun 12 '18 at 21:17
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    $\begingroup$ @Demigan: For humans, the sex ratio at birth is about 106 boys to 100 girls (between 105 and 107, depending on sources). Boys are somewhat more fragile, so the sex ratio becomes very close to 1:1 in the general population. $\endgroup$ – AlexP Jun 12 '18 at 22:01

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