Halting NS experiment
From: William L Hunt (wlhunt_at_earthlink.net)
Date: 02/10/05
- Next message: Robert Karl Stonjek: "Paper: Red leaves, insects and coevolution: a red herring?"
- Previous message: Jim McGinn: "Re: "Kooks" dominating sbe"
- Messages sorted by: [ date ] [ thread ]
Date: Thu, 10 Feb 2005 17:51:37 -0500 (EST)
"Only if the total number of fertile forms reproduced into one
population can be artificially maintained the same for all members of
one population over many generations can natural selection be halted.
Try anything else. Try life spans or perhaps the number of eggs laid,
anything you like.
None of them can halt natural selection within one population. Only
the proposition I have provided for Total Darwinian Fitness (TDF)
works. It is that simple."
John Edser
----------
I have seen this proposed experiment mentioned in several threads, so
I thought it might be useful to comment on other "halting"
experiments.
In 1928 Muller first proposed an experiment to measure mutation
accumulation in a chromosome of Drosophila by "halting" natural
selection. Terumi Mukai began doing such an experiment in 1964 and
continued through the 70's, sometimes maintaining these populations
for 100's of generations. These experiments might be considered the
classics. Since then many other such experiments have been done.
Common species used include Drosophila, E. coli and C. elegans but
other have been used.
Usually experiments that "halt" natural selection have one or two
reasons but often there is some twist involved. One reason is to take
a direct measurement of the rate of spontaneous mutation. Indirect
measurements that compare DNA sequence data "see" only neutral or
effectively neutral mutations and positively selected mutations.
Deleterious mutations are invisible using sequence data since they are
removed via natural selection. Often a second reason is to measure the
effect a level of spontaneous mutations (including deleterious ones)
has on fitness.
From the beginning a question existed as to what is the proper
control. I think today the preference (when possible) is to separate
the initial population into 2 or more parts, then put 1 (or more) in
stasis by freezing, run the experiment for some period of time and
then remove the control from stasis and do the measurements including
changes in fitness. Fitness is measured in different ways.
Experiments always begin with a mostly homozygous population (you
don't want any deleterious recessives). Ideally the population would
derive from a single homozygous individual.
Natural selection cannot be completely "halted". Even with clonal (or
selfing) organisms, propagating each line through a single individual
each generation, a lethal mutation cannot be propagated. Though even
highly deleterious mutations can be since conditions are maintained as
ideal as possible. For a sexual species, the minimum population size
is 2. Maintaining this "effective" size means that any mutation with a
selection coefficent less than .50 is effectively neutral. So with
sexuals only the most very deleterious mutations see "natural
selection".
In sexuals (Drosophila) often a marked balancer chromosome (that
doesn't recombine) is used. Mutations accumulate on this single
autosome that is propagated through a single heterozygous male in each
line. Then final fitness measurements will be concerned with how
having this marked chromosome effects an individual's fitness.
Sometimes worded as measuring a fitness change in this chromosome.
William L Hunt
- Next message: Robert Karl Stonjek: "Paper: Red leaves, insects and coevolution: a red herring?"
- Previous message: Jim McGinn: "Re: "Kooks" dominating sbe"
- Messages sorted by: [ date ] [ thread ]
Relevant Pages
|
