evolution and the sexes

[marycatelli]

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.

Comments

[lwe.livejournal.com]

I have a story in the early planning stages where there's an alien species with fifteen males for every female -- but they're VERY alien indeed, and... well, it makes sense in context. They're extremely sexually dimorphic, the females are about twenty times the size of the males and can be pregnant by several different males simultaneously, the males raise the offspring -- I don't think it's evolutionarily absurd.

But they're more like spiders than humans.

[ljlee.livejournal.com]

Wouldn't the genetic advantage of having female offspring persist, though, tending to take things toward 50/50? I googled sex ratios for spiders and found this post (http://www.epjournal.net/blog/2011/02/why-are-there-so-many-girl-spiders/) on possible explanations for sex imbalance in some species of spiders. One explanation is group selection, the other bacterial manipulation. Both look like interesting avenues to explore.

The gender imbalance aspect reminds me of Ursula K. Le Guin's The Matter of Segri, which is about a planet that has a male:female ratio of something like 1:17. That was not a result of evolution, however, but deliberate genetic tampering that the population didn't have time to recover from. The locals do try their damnedest, though:

The gender imbalance on Segri leads to men being treated as communal sexual property of the women, isolated in their own male society which is a sort of shared harem/brothel servicing women. The women mostly fall in love with and have sex with other women, but visit their local (or possibly a distant) brothel if they want to get pregnant or want some male companionship.


A story-within-the-story by a Segri writer depicts what happens when a man becomes known for impregnating his clients with live male births, an obviously rare quality. Women start flocking to him in droves hoping to have sons and he becomes like the hottest commodity ever. Culturally it's explained as male lives being precious to the Segri, but there's an obvious evolutionary advantage tucked in the longing for sons. Given enough time and chance this kind of selection would have led to a rising rate of male births.

Then again the evolutionary imperative is not an overriding one, since this precious son-producing man is later castrated when he threatens and rapes a client.

[marycatelli]

Bacterial manipulation, of course, creates a genetic void for those resistant to it to spread their genes through their sons.

[ljlee.livejournal.com]

Yeah, assuming there was enough time for bacteria-resistant phenomes to appear and spread. Remember, we're only challenging the idea that sex imbalance came about through evolution. That still leaves room for imbalances of more recent vintage (bacterial infection, genetic manipulation) that evolution hasn't had time to get around to. Group selection is interesting in that regard as a way for sex imbalance to evolve, though I'm fuzzy on the details of how it would happen.

[marycatelli]

Yeah, that's just the gap through which evolution will work; it's not instantaneous.

[marycatelli]

Do your spiders stay the same sex their entire lives? If they are male while young and female while old that would explain a great difference.

Or perhaps fourteen females die very young. After all, if they're that much bigger, they have to eat that much more.

Or perhaps there are others. But the female genetic advantage is not to be slighted.

[lwe.livejournal.com]

I worked out a system of genetics where there are far more than two chromosomes involved in determining sex -- remember, this species isn't from our evolutionary tree at all, it's from another planet. But I need to work some on why there isn't strong pressure toward 50/50, yeah. Or what overwhelms that pressure.

[marycatelli]

Ah, the thrilling life of a SF writer!

[izuko.livejournal.com]

Every author writing about evolution should be forced to read Richard Dawkins's "The Selfish Gene."

[marycatelli]

It may be too advanced for some of them.