William Morse  wrote or quoted:
> wirtatmar{at}aol.com (Wirt Atmar) wrote in
> > Bill writes:

> >>Very true. Let's put in some numbers. For an organism having 1000 
> >>offspring per individual, an s-type has to have a survival rate of
> >>.002 to keep up with a p-type with a survival rate of only .001,
> >>assuming that the p-type can have twice as many offspring. But looked
> >>at the other way, if only .998 of the s-type's offspring die in
> >>comparison to .999 of the p-type's offspring, the s-type will keep up.
> >>This is a sexual advantage of only .999/.998, or slightly more than 1%
> >>.Figures don't lie, but liars figure :-)
> > 
> > What you guys are missing in all of your armwaving is that the primary
> > fault with parthenogenesis lies in its implicit accumulation of
> > defects, generation to generation. [...]
> 
> Thanks for your input, Wirt, but you may have misunderstood my argument. 
> The original poster (and I have now forgotten who that was) noted the often 
> quoted 2:1 advantage of parthenogenesis over sexual reproduction. This has 
> been brought up frequently as a conundrum, since most of us know that in 
> fact strict parthenogenesis is extraordinarily rare in nature. The point of 
> my "handwaving" argument was that in organisms with large numbers of 
> offspring, the supposed twofold advantage of parthenogenesis is 
> nonexistent, and that in fact only a 1% advantage of sexual reproduction is 
> sufficient to overcome the additional offspring of parthenogens.

It seems like a valid example to me: what seem like small advantages can 
be magnified into big ones if there's a large population - by forcing the 
organisms to compete with one another - and then selecting only the  
winners.

It could happen with something like disease resistance - where a 1%
greater chance of immunity in a sexual population results in a wipeout
of the competing asexual strains in each generation - overcoming the
two-fold cost of sex with relatively little effort.

The explanation /only/ works among organisms which have many offspring -
of course - and they are certainly not the only sexual organisms.

IIRC "Sex and Evolution, George Christopher Williams, 1975" had a
whole chapter devoted to the effect of large infant mortality in
each generation on the evolution of sex.  The chapter title has
"elms and strawberries" in it I think.  It's a while since I read
that - and I don't have a copy to hand.  I don't remember it making
the point above - but it could well have done.
-- 
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