From: Chris Colby
To: All Msg #68, Nov-23-92 05:53PM
Subject: "good"/"bad" mutation ratio - answer to Rice
Organization: animal -- coelomate -- deuterostome
From: email@example.com (Chris Colby)
In article David.Rice@ofa123.fidonet.org writes:
>What is the probability of a "random" mutation being benificial
>to an organism?
This is a great question. Unfortunately, I don't think it can be
answered easily. The probability of a mutation being "good" (i.e.
increasing relative fitness) as opposed to being "bad" (lowering
relative fitness) must be very low. However, it would depend
on many factors and (IMHO) it would be hard to arrive at a
meaningful answer to this question.
For a well adapted species in a stable environment,
the ratio of "good" to "bad" mutations may be extremely low. For
a generalist species invading virgin, unoccupied territory the ratio
may be relatively high. Any measure of a "good"/"bad" ratio is likely
to be species and context dependent. Hence any average figure given
is likely to be utterly meaningless (sort of like the "average"
human is a little over half female, a mosaic of races, etc.).
This would be further confused by the fact that some bacteria
and yeast -- and probably many other unicellular organisms --
can produce "directed" or "adaptive mutations" under very
stressful environmental settings (like starvation). In these cases,
the "good"/"bad" ratio would depend on how badly the organism
needed the mutation. In fact, the "good"/"bad" ratio becomes
nearly infinity -- in one of Barry Hall's papers, the _only_
mutations he detected (*) were beneficial ones! [(*)In the bacteria
that reverted to "wild type".)]
I would bet almost anything that this has been measured for
laboratory _Drosophila_ living in culture tubes. (There was
a recent study which measured the average fitness consequence
of a mutation in fruit flies recently in Nature.) But, I don't
know how much that would really tell us about evolution.
>Can any evolutionary scientists out there (or Creationists,
>for that matter) even map out what is required to produce
>a valid probability, let alone calcutate it? Make it simple:
>what is the probability of a rose mutating beneficially?
ARGH, how about something with a short generation time? A simple
experiment would be to take two strains of bacteria that were
identical in every way except for one neutral marker. Grow them up
together in a flask and wait for the frequency of the neutral marker to
start changing quickly. Normally the freq. should fluctuate slightly
due to drift. But, once a beneficial mutation occurs it will
sweep through the population and carry the neutral marker along
(since bacteria have a single chromosome.) Deleterious mutations
should be happening constantly to both strains, but this won't
change the frequency of the neutral marker if the mutation rate is
the same in both strains.
Now, you could measure
the time until a beneficial mutation occurred (run the experiment
a few hundred times to get a good distribution) and the population
size of the bacteria. The mutation rate of bacteria (if you use _E.
Coli_) is well known. From there you could calculate the total
number of mutations that occured in the population up until the
"good" one did. And there you have it -- the "good"/"bad" mutation
rate of a species of bacteria under the given growth conditions.
Like I said, I'm sure this has been done (probably in bacteria,
yeast and fruit flies at least). But, I'm not sure how beneficial
this knowledge would be. (Don't get me wrong -- you have to start
answering the big questions by starting out small. I'm not saying
a study like this would be useless, just limited.)
>just mutate without harm, but BENEFIT itself and its species
>if the mutation breeds true. ^^^^^^^^^^^^^^^
This last part is irrelevent, if the mutation is beneficial to
the individual it will sweep through the population. If not, it
won't. Altruistic mutations (i.e. mutations that benefit a species/
population) will only spread under a limited set of circumstances.
(Populus, the simulation -- not the game, has a simulation of this.)
Natural selection does not favor traits that benefit a population
unless they also benefit the individual. Selection favors selfishness.
Chris Colby --- email: firstname.lastname@example.org ---
"'My boy,' he said, 'you are descended from a long line of determined,
resourceful, microscopic tadpoles--champions every one.'"
--Kurt Vonnegut from "Galapagos"