"EKurtz99"  wrote in
news:cp7f4i$7q9$1{at}darwin.ediacara.org: 

>>> "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. 

This is quite true if there is no difference in fitness between p-types 
and s-types. My point was that if there is a huge die-off of offspring in 
any case, a very small edge in survival of s-types will eliminate the p-
type advantage of more offspring. Of course what one should expect based 
on this logic is that parthenogenesis would be more common in organisms 
that have relatively few offspring, and AFAIK this is not the case.I 
suspect this may have something to do with parental investment, but I 
would appreciate any insights.


 
> 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. 

I don't know if we need to actually resort to such desperate measures as 
mathematical modelling :-) Facultative parthenogenesis is apparently 
fairly widespread (based on a quick google search), and a look at the 
conditions under which this occurs should shed some light on the 
tradeoffs between sexual reproduction and parthenogenesis. I might even 
pursue the subject further when I get a significant chunk of spare time - 
which based on my current workload should be in 2006 :-(

 
Yours,

Bill Morse
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