marycatelli (marycatelli) wrote,
marycatelli
marycatelli

evolution and the sexes

Recently read a work in which an alien race had eight females to every male.  And solemnly explained it was the right ratio for their species for evolutionary reasons.

grumble

I forget the details, but my nose wrinkled at that.  Evolution doesn't care about the species -- evolution has never heard of the species.  In a species with sexes fixed at birth, and in which parthenogenesis did not occur, the ratio will be close to fifty-fifty for evolutionary reasons.

If you got that ratio by every female having seven daughters and a son, she would then have 112 grandchildrens:  each daughter would have eight, for 56, and because every female would need a father for her eight, the son would have to have children by seven females, for 56.  The thing is, if she had two sons, she would lose eight grandchildren from the missing daughter, and gain 56 from the son.  That much of a reproductive advantage would sweep through a population like wildfire.  (Assuming it wasn't attached to something extremely deleterious -- but then, if one gene that shifts it is deleterious, another mutation would happen along.)

Those are, of course, on average.  If a female has seven daughters and one son, and the sons fight to the death, so that only one out of ten lives, nine mothers will have only 56 grandchildren from their daughters, but one will have 560 from her surviving son.  Note that dying can result in skewed statistics that will not affect it in the same way.  Queen bees have as many sons as daughters.  However, their daughters kill the drones down to -- one eight of the children, IIRC.  (This appears to because while sister bees are more closely related to each other than they would be to their own children, they are less related to their brothers, because male bees are haploid and female ones diploid.)

To be sure, within a breeding population, there are sub-populations that would advantage one sex, but there are also sub-populations with the opposite advantage for the other.  Among some primate where the mother intervenes on behalf of her children, the daughters, who remain with the troop, gain more from having a high status mother, and so high status mothers have more.  By the same token, however, low status mothers have more sons, who can shake off their mother's influence in their new troop.
Tags: families: parent/child, world-building: non-human characters
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