br.hessels{at}planet.nl (A.C.H.) wrote in
news:cgeh3j$u28$1{at}darwin.ediacara.org: 


 
> Compare these two situations: 
> 
> 1) Childbirth is a problem, females die during birth. Evolution by
> natural selection is happening, which is the cause of adaptation
> (easier childbirth).
> 
> 2) Women are perfectly adapted to giving birth, childbirth is no
> problem at all, no women die giving birth.
> In this case, the natural selection, which caused the adaptation in
> the first place, falls away.
> 
> Therefore the state of perfect adaptation (all women give birth easy)
> cannot be reached, because, paradoxically, the cause of the
> adaptation: natural selection, falls away when you approach this
> state.
> 
> So i predict: child birth will be less hazardous, but will never be
> easy.


The general question of the extent to which natural selection can refine 
a trait has been well discussed on this thread. And the thought that hip 
width vs. head size is a crucial limit in fetal head size seems to be 
taken for granted. I would just like to point out that head size vs. hip 
width may not in fact be the limiting factor as  has often been stated. 
Energy use by the fetal brain may in fact be the limit on fetal brain 
size. 

I do agree with the general nature of your argument - that there will be 
no continuing evolution by natural selection (OK Larry, drift may still 
play its part) in the absence of continuing selection pressure.


Yours,

Bill Morse
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