Re: limit of selection???

From: A.C.H. (br.hessels_at_planet.nl)
Date: 08/31/04 

Date: Tue, 31 Aug 2004 04:34:40 +0000 (UTC)

William Morse <wdmorse@twcny.rr.com> wrote in message news:<cgvefb$2sgo$1@darwin.ediacara.org>...
> br.hessels@planet.nl (A.C.H.) wrote in
> news:cgeh3j$u28$1@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.

There will be a complex coevolutionary dynamic, traits changing,
changing the selection pressures on other traits. Many things, higher
energy use, bigger heads, wider hips, will evolve together.

>
> 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.

The argument i was trying to make was more specific then that. I was
trying to argue that the selection pressure DECREASES when a
population gets more adapted.
And because natural selection decreases (though it doesn't stop), a
state of perfect adaptation (ALL individuals survive a certain
selection pressure) can not be reached.

In short:
- adaptive change decreases natural selection.
- decreased natural selection decreases adaptive change.

Therefore the outcome of natural selection will be a state of
imperfect adaptation, while natural selection is still working on it.

 

> Yours,
>
> Bill Morse

Relevant Pages

  • Re: How Our Brains Ignore Unpleasant Facts was: Re: The Reasonable
    ... isn't that the evo term for rejecting the words of ... Natural Selection, in evolution, the process by which environmental ... Natural selection thus tends to promote adaptation by ... maintaining favorable adaptations in a constant environment ...
    (talk.origins)
  • Re: How Our Brains Ignore Unpleasant Facts was: Re: The Reasonable
    ... isn't that the evo term for rejecting the words of ... Natural Selection, in evolution, the process by which environmental ... Natural selection thus tends to promote adaptation by ... maintaining favorable adaptations in a constant environment ...
    (talk.origins)
  • Re: How Our Brains Ignore Unpleasant Facts was: Re: The Reasonable
    ... Natural Selection, in evolution, the process by which environmental ... effects lead to varying degrees of reproductive success among ... Natural selection thus tends to promote adaptation by ... maintaining favorable adaptations in a constant environment ...
    (talk.origins)
  • Re: "Random evolution"
    ... that the main mechanism of evolution is random genetic drift. ... visible phenotypes that are neutral with respect to adaptation. ... just-so stories in order to attribute those to natural selection. ...
    (talk.origins)
  • Re: limit of selection???
    ... > selection pressure) can not be reached. ... > - adaptive change decreases natural selection. ... > - decreased natural selection decreases adaptive change. ...
    (sci.bio.evolution)