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Actually, on second thoughts, it would be 3/4 non-choosy. Here's how it works out.

If a hermaphrodite produces a female egg, and mates with a male carrying the choosy gene, the egg has a 50% chance of being fertilized by a choosy sperm. But if the egg is male, it can only be fertilized by a non-choosy sperm. Hence only 1/4 of the offspring will carry the choosy gene. That's a big enough disadvantage to lead to it being outcompeted. Also, for the mechanism to work, the mutation has to be very tightly coupled with one of the sex selection genes - if it isn't, you'll get things like females who can only have female offspring, and can't be fertilized by males.


Pete Bleackley
The Fantastical Devices of Pete The Mad Scientist - http://fantasticaldevices.blogspot.com

-----Original Message-----
From: Alex Fink <[log in to unmask]>
To: [log in to unmask]
Sent: Fri, 12 Sep 2014 18:19
Subject: Re: [still OT] Re: Alien sexology (was: Native american altlangs)

On Thu, 11 Sep 2014 19:47:24 +0100, And Rosta <[log in to unmask]> wrote:

>Alex Fink, On 11/09/2014 13:11:
>> On Wed, 10 Sep 2014 19:12:21 +0100, Pete Bleackley <[log in to unmask]> wrote:
>>
>> they say (not in a good place to look up a better
>> citation now, sorry) that the reason mammalian births are half male
>> and half female is because that's the game-theoretic optimum -- at
>> least assuming the cost of bearing and raising a male and a female is
>> equal. Indeed, if the distribution were otherwise, say more than half
>> of births were male, then a mutation which brought about more female
>> births would be advantageous, as your children would be more likely
>> to have a larger pool of potential mates and thus spread their genes
>> broaderly; then in the long term this mutation would be selected for
>> and tip the balance back towards parity.
>
>Must the assumption be not only that the cost of bearing and raising a male and a female are equal but also that, independent of the numbers of each sex, males and females have an equal chance of attracting a mate? If not, then one can quite easily imagine a scenario in which the game-theoretic optimum is far from 50--50: for example, if one male can impregnate many females, and is happy to do so, then from the point at which there are enough males to impregnate all females, one is better off having female offspring, since females need not compete with one another as much as males, it being easier for a female to get impregnated than for a male to be an impregnator; the female--male ratio could, say, be 9--1, and with a harem-based bonding structure. (Or has my reasoning been betrayed by wishful thinking?)

To first order, the important thing is how many offspring you have, not simply whether you have any or none.  In your harem-type structure, with a tenth of the population being males, each new male can expect around nine descendants (per gestation period, say), each new female can expect around one, so males are still nine times as successful at getting their genes into the next generation, and so still nine times Darwinianly better an idea if you can choose your sex.  If competition means some males lose out -- e.g. male X comes and steals male Y's harem and gets eighteen kids this year while Y gets none -- it still doesn't matter to the averages.  That is, unless the competition is so severe that Y gets killed or castrated or otherwise suffers a permanent decrease in reproductive efficacy.  That's the sort of thing I meant to count as costs: it's twice as costly to raise one male to reproductive age, on average, if half of them get killed before then.  So a sufficiently bloodthirsty harem-based system could be stable.

Exercise for the reader: every once in a while, a survey hits the news finding something like "[straight] men have more sex partners than [straight] women".  Why must this reflect a bias in reporting or selection vel sim?

>I do like Pete's idea, I must say. For 54% of the population the pool of potential [reproductive] mates is 100% of the population and for the other 46% the pool of potential mates is 77% of the population, so on average the pool of one's potential mates is 89.5% of the population. Much better than the 50% that humans must endure. I wonder how heterosexuality would work in Peteworld: if hermaphrodites had mixed sexual traits then the heterosexual (MM, FF) would have to be attracted to the sexual ingredient they lack -- Fness/Mness -- but not repelled by the ingredient they have (for if repelled by hermaphrodites, the pool of potential mates would shrink by two thirds). If people in Peteworld aren't repelled by the sexual ingredient they have, and sexual congress is partly hedonic, then everybody would be at least Kinsey 1--5 (or 2--4, but certainly without the extremes of 0--6), which would mean that for everybody the pool of potential hedonic sexual partners would, paradisiacally, b
>e about 100% of the population.

Indeed, that's nicely utopian.  The hedonic part sits on top of an ordinary male/female XY sex determination system just as well as it does Pete's system, though; Roger cited it upthread as the way the Kash work.  


On 11 September 2014 18:13,  <[log in to unmask]> wrote:
>[Alex wrote:]
>> On Thu, 11 Sep 2014 14:57:24 +0100, Pete Bleackley
>> <[log in to unmask]> wrote:
>>
>>>Any mutation that increased the proportion of hermaphrodites would
>>>probably incur a cost in terms of overall reproductive fitness, and so not
>>>survive in the population.
>>
>> I don't follow.  If you're just stipulating this as a way of averting the
>> problem by authorial fiat, fine.  But if it was meant to be an argument from
>> the existing premises, you'll have to say more. 
>> I see no intrinsic reason why a gene that tips sperm-analogues to favour the
>> smell of egg-analogues with the opposite sex allele, or a gene that favours
>> the chromosomes being sorted out in such a way for a fused tetraploid cell
>> that then undergoes meiosis, or whatever you have, would have to impose
>> costs elsewhere.
>
> In fact, the simplest way I could think of for biassing the population in
> favour of more hermaphrodites was a mutation of one of the sex-selection
> genes that made single-sex offspring less viable. That would obviously
> impose a cost, and die out quickly.
>
> A second mechanism that I thought of was a gene that made single-sex
> individuals prefer single-sex partners to hermaphrodites. However, since it
> would reduce a carrier's mate choice by a factor of 3, that wouldn't really
> work either.

Agreed.

> As for the "choosy gametes" mechanism, the costs are more subtle, but they
> exist. When the mutation first arises, a carrier is only likely to have one
> copy of the choosy allele. That means that only half their gametes will
> carry the gene. The ones that don't will be more likely to fuse than the
> ones that do. If a single-sex carrier mates with a hermaphrodite
> non-carrier, 2/3 of the offspring will be non-carriers. 

That's not a cost at all!  That just means that this mutation will be slower to spread and to reach fixation, that it might need to arise several times before it avoids being snuffed out in the first few generations, etc.  But evolution is patient.

And shouldn't your 2/3 be 1/2?  I presume a child wouldn't ever take both its sets of chromosomes from the same parent.


On Fri, 12 Sep 2014 17:39:11 +0200, Leonardo Castro <[log in to unmask]> wrote:

>I've found some interesting papers about sex ratio and parental control
>over it (I've not read the whole of them though):
[snup]

Super, thanks!

Alex