>> "EKurtz" (NoJunk{at}ForgetIt.com) writes:
>>> Consider the case of a sexual species into which a parthenogenic
>>> female is introduced by mutation. Assuming that she and her immediate
>>> offspring survive, and that the population size is constant, her
>>> offspring will effectively displace the sexual type in a few dozen
>>> generations.

Catherine Woodgold wrote
>> Even if the parthenogenetic individuals average only 1.5
>> fertile offspring each, they will tend to take over the whole
>> population.  The sexual ones could be declining while
>> averaging 1.8 offspring each, while the parthenogenetic ones
>> grow exponentially at 1.5 offspring each.  So number of
>> fertile offspring is not a very useful definition of "fitness".
>
"William Morse"  wrote
> But what if (as is normally the case for most organisms) the average 
> number
> of offspring (before predation and disease) is on the order of hundreds to
> thousands? In this case the parthenogenetic individuals have very little
> advantage in terms of having more offspring - they invest less energy per
> offspring but that investment is not particularly large - while their
> offspring suffer higher rates of predation and especially disease.

Why is this? Assuming populaton size to be constant, a single parthenogentic 
type will, after predation etc have taken their toll, produce 2 p-type 
offspring, whereas 2 sexual types, M&F, are required to generate 2 offspring 
s-types. As Catherine Woodgold states, this will lead to an exponential 
increase of the population size of p-types, with corresponding decline and 
ultimately elimination of the s-type.

I would guess that this effect will take place, though more slowly, if only 
a fraction of the eggs in the "p-type" female are clones, the
rest requiring 
normal fertilization. However, to prove the point would involve some 
mathematical modelling, which would upset some of the sbe regulars no end.

> Under
> these circumstances (which again are the norm for most organisms)
> parthenogenesis does not have any advantage.
>
>
> Yours,
>
> Bill Morse
>
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