Re: Good Mutations.

R. Joel Duff (
Tue, 2 Dec 1997 15:52:40 -0600 (CST)

>At 07:43 AM 11/27/97 -0600, R. Joel Duff wrote:
>>I often get the same question. I answer with a question: What do you mean
>>by mutation? I find that most of the time the person is either overtly or
>>subconciously defining a mutation as something detrimental to the organism
>>thus cannot conceive of a mutation doing "good." Good and bad mutation are
>>highly subjective terms depending on what we think is progress. Point out
>>that there are many types of mutations: point mutation, chromosomal
>>mutations etc... In each catagory there are mutation that are detrimental,
>>helpful, or effectively neutral to the survival (fitness) of the organism.
>>Off the top of my head I would give a bacterial example:
>>If you plate some bacteria on a media with high salt all will die but a
>>couple. These potentially had mutation that allowed them to survive. Take
>>a single bacteria and grow it up and plate it on a media with double the
>>salt concentration and most of them will die but a few may survive. Most
>>of them were genetically the same as the original bacteria and couldn't
>>handle the higher salt concentration and so the few that survive you may
>>surmise had some sort of mutation. Is this a "good" mutation? It is if
>>you are interested in bacteria that live in high salt environments. It
>>isn't if you didnt' desire the bacteria to live.
>I am not sure I understand. Is it possible that the bacteria have a sort of
>bell-shaped curve of characteristics that determine what the bacteria is and
>by means of environmental changes we can select the parts of the bell-curve
>which we desire. Sort as animal breeding. Do we indeed have to talk of
>mutations? Can we tell at the molecular level? What makes us say that we are
>dealing then with the same bacteria?


I think there is much less room for phenotypic variation in this case.
Usually in such an experiment one plates bacteria onto a media and picks
bacteria from a single colony. It seems safe to assume that all the
bacteria in that colony came from a single bacteria. One then grows
bacteria from that single colony up in some liquid media and then plates
out several billion bacteria onto a different media (in this case higher
salt concentration). Only a few of the millions/billions of bacteria may
manage to survive. Since these bacteria all came from a single bacteria at
one point we are definitely dealing with the "same bacteria." Rather than
selection on a bell shape curve of characteristics due to inherent
variation, it is more likely that the ability to survive on the salt media
is due to a point mutation of some type that enables the bacteria to
acquire an enhanced ability to deal with a high salt environment. Hope
that helps.


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