lack of evolutionary pressures (fwd)

Jim Choate ravage at
Sat Aug 23 10:30:46 PDT 1997

Forwarded message:

> Date: Sat, 23 Aug 1997 10:18:44 -0700
> From: Tim May <tcmay at>
> Subject: Re: lack of evolutionary pressures

> Bluntly
> put, it ain't going _anywhere_, at least not very fast. Changes in the
> characteristics of a species, loosely speaking, "evolution," happen faster
> in small populations.

Actualy this premise is false. The results of evolution stand out more in
smaller populations because a given change can propogate through the
population with less chance of being swammped by other changes that are
also propogating through that population. There is also the fact that the
vast majority of changes are either harmless or else debilitating further
slowing the genetic drift.

Look at it this way. If we have 1000 mating pairs and one of those pairs
progeny has a genetic change it is much easier for that change to get
propogated because the odds of the offspring mating with the other 999 pairs
offspring are much higher than if there were say 10,000 pairs. The number of
generations required for that change to become ubiquitous in small
populations is much lower than in larger populations.

The genetic drift for a given species is usualy low because of this
dillution whereas if a punctuating event occurs it will lower the available
pool of mates and hence the relative changes increase until the new
population becomes stable, usualy because the mitigating effects of the
punctuating event have receded.

If you would like a concrete case to study, review the research about
genetic drift in monkey populations in the Great Rift Valley in Africa.

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