Text 1077, 84 rader
Skriven 2004-12-18 06:15:00 av William Morse (1:278/230)
Ärende: Re: The "fuel" of evoluti
=================================


wirtatmar@aol.com (Wirt Atmar) wrote in
news:cpvp7u$1m83$1@darwin.ediacara.org: 

> 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. (In fact, it's the problem with all
> armchair/mathematical biology. One aspect, often because it is more
> tractable to mathematical analysis than others, tends to be emphasized
> to the exclusion of all other attributes, and thus leads to some truly
> odd conclusions that bear little resemblance to reality.) 
> 
> Although Alexy Kondrashov has compiled 27 different advantages
> attributed to sexual reproduction in the literature, the big three
> remain: 
> 
>      o rapid phyletic adaptation to novel conditions
>      o the escape from parasites due to genetic diversity
>      o a mechanism that combats the aging of the germline
> 
> Which of these is true? All of them are. It's not a "tastes
> better/more filling" kind of argument, but without the last advantage,
> a phyletic lineage would soon decay into nothingness, in the same way
> that xerox copy of a xerox copy soon decays into a page of illegible
> black and white dots after just a few dozen replications, and this is
> the principle reason that apomictic populations, especially complex
> organisms, tend to only exist for very short periods of time.

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.

Even if my argument is correct, that still leaves the question of why 
parthenogenesis does not arise more frequently in organisms with few 
offspring. Your "big three" are fine, but they all require some time for 
their effects, so I might expect that any one point in time we would see a 
significant number of parthenogenetic species, even though we would expect 
them all to be short-lived. Regarding this your last point regarding 
complex populations may be the most germane. The organisms that practice 
extensive parental care of a few offspring are mostly quite complex, so 
even though parthenogenesis has more of a potential benefit in these 
organisms, the rapid interference of germline decay with an extended 
developmental sequence makes parthenogenesis a very short lived phenomenon.

But it does appear that facultative parthenogenesis is not that rare (based 
on a quick search on Google Scholar), so it might be interesting to pursue 
the conditions for facultative parthenogens which favor asexual vs. sexual 
reproduction. A greater degree of heterozygosity is apparently one 
condition, which agrees with your big three. Another (among Timema -walking 
sticks- in California) was being at the northern extent of the range. Is 
this based on reproductive advantage overcoming parasite resistance at the 
limit of the range, where parasite resistance is not that important, or is 
it due to some temperature effect on development?  



Yours,

Bill Morse
---
ū RIMEGate(tm)/RGXPost V1.14 at BBSWORLD * Info@bbsworld.com

---
 * RIMEGate(tm)V10.2á˙* RelayNet(tm) NNTP Gateway * MoonDog BBS
 * RgateImp.MoonDog.BBS at 12/18/04 6:15:00 AM
 * Origin: MoonDog BBS, Brooklyn,NY, 718 692-2498, 1:278/230 (1:278/230)