RE: Punctuated Equilibrium: reviving the dead man?

Cummins (
Tue, 20 Apr 1999 12:49:15 -0500

> []On Behalf Of Ami Chopine

> I am not suggesting these mutations are suppressed. I am suggesting two
> things: since under the pressures common to that population, that
> will not be selected for. Under some circumstances, given enough time, it
> may gradually take over the entire population, much like diffusion. If
> there is a direction, however..a stressor event, or the effective
> of part of the population in a slightly different enviroment then you will
> see those mutations being selected for. What was once a slow, almost
> still process, will become almost instantaneous in the geologic record.

Mutations are constant, selection is constant, therefore evolution is
constant. That's a simple and reasonable argument. But, supporting PE by
appealing stressor events such as a change in the environment or part of the
population being isolated isn't so clear. I still don't see a mechanism.
It sounds like you're suggesting that mutations are accumulated and kept in
reserve until needed. I believe we already addressed the argument that
environment doesn't affect the rate of evolution, only the group of selected
mutations, so what difference would a different environment make? A monkey
on the way to become a human might instead, if the environment changes, be
on the way to become a bat, but it's still going to change at the same rate.
An isolated population might lose some of the genetic variation of the
parent population, but it's not going to evolve faster, it's just going to
be limited to the genetic variation of the parents of that new isolated

> Without specific pressures selecting for those mutations? A long time.

This is your problem. Pressure is constant (except when the population
changes in size). There isn't going to be stasis while the population waits
for specific pressures. There's always mutation and selection -- ALWAYS.

> When there is a basic equilibrium between the pressure and the species
> original traits. The very great majority of mutations make no difference
> whatsoever. They occur in non-protein coding DNA.

An "equilibrium between the pressure and the species' original traits" never
In fact, I'm having a very difficult time finding any meaning it. Is that
some form of the argument "species that are perfectly fit aren't going to
change"? If they're so fit, how come the population isn't growing
exponentially? If you mean "the population size doesn't change"
that seems to better fit your wording, but I doubt it's what you were trying
to say.

As for mutations that are eliminated or that have no effect, we can
disregard them. Unless you have a mechanism that effects which part of the
DNA mutations are most likely to occur, or a mechanism which controls what
amount of mutations will be "good." (this last point is what you seem to be
fishing for)