Text 1043, 88 rader
Skriven 2004-12-16 12:30:00 av William Morse (1:278/230)
Ärende: Re: The "fuel" of evoluti
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"Anon." <bob.ohara@NOSPAM.helsinki.fi> wrote in
news:cpde0u$24hr$1@darwin.ediacara.org: 

> William Morse wrote:

>> "EKurtz99" <EKurtz99@TwilightZone.com> wrote in
>> news:cp7f4i$7q9$1@darwin.ediacara.org: 

>>>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. 
 
> That depends on the relative magnitudes.  The number of offspring
> after the dieoff is the proportional to the product of the number
> before and the probability of surviving.  So, the s-type will be
> fitter if the ratio of s-survial to p-survival is larger than the
> ratio of the numbers of s-offspring to p-offspring.

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 :-)


Now let's look at organisms with few offspring, With say 8 offspring per 
individual, an s-stype has to have a survival rate of .25 to keep up with 
a p-type survival rate of .125. When we switch it around, the advantage 
of recombination has to be .875/.75, or almost 17%. 

Which leads to my previous statement:
 
> 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.

 
> A problem with parthenogenesis is that it is more difficult to produce
> variation (hence the Red Queen ideas about the evolution of sex).  I 
> have an idea that this is less of a problem for species that produce a
> lot of offspring, because producing more offspring means that you're 
> more likely to get a variant.  I'm not sure whether this works in 
> practice - one day I should look at this seriously.

Interesting - so even though parthenogenesis represents a short term 
benefit to K-selected organisms(to use a term that apparently is in 
disfavor), it is at a sufficient disadvantage so that it does not in fact 
persist for long enough to be evident in current populations. 

Or, more probably, my logic above is flawed. Perhaps (apart from random 
events) there is only in general a tiny difference between a surviving 
fawn and a dead fawn, and since a fawn represents a very large investment 
as compared to most fish eggs, a deer may not be able to afford the risk 
of having all its offspring die even for the potential reward of having 
twice as many offspring survive. 

And you have also given the reference that I snipped but will try to 
follow up , suggesting even more complex reasons. The more I learn, the 
more convinced I am of the statement  "evolution is cleverer than you 
are".

Yours,

Bill Morse
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