Re: Good Mutations.

R. Joel Duff (
Thu, 27 Nov 1997 07:43:12 -0600 (CST)

>A question to those more genetically inclined:
>I got into a discussion the other day about whether there ever are any
>"good" mutations, the kind that evolution requires in order for changes to
>occur. The only example I could think of had to do with sickle cell
>anemia. Are there any examples of mutations that aren't harmful (aside
>from the mutations to regions of DNA that don't do much)? Any help would
>be appreciated.


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.

To be fair though the original questioner is really thinking of obvious
morphological anomylies such as new limbs, eyes, etc.. when they ask the
question. You have to work from the ground up though to help the person
overcome some misperceptions about mutation, their frequency, how they
occur, etc.. before directly addressing the particular concern. One thing
I like about the bacteria example is that it demonstrates several
1) They don't happen very often (1 in billions of divisions)
2) They mutation may only be beneficial in a particular environment. The
same mutation for salt tolerance may occur in a bacteria not in a salt
environment and so would not confer an advantage to it over other other
bacteria as it did on the plates. Nice example of selection in different
media. In fact the mutation in the wild would likely be lost because the
mutation would not increase the fitness of the individual bacteria and may
even possibly lower it in some cases
3) Shows that populations evolve (micro- anyway) not individuals. It is a
mutation in one member of a population that is selected for not one
indidual that mutates in repsponse to its environment - i.e. the mutation
isn't necessarily being caused by the NEED to mutate but is simply selected
out of a pool of individuals that contains that mutation. Note: there is
some recent and controversial evidence of some Lamarkian-type mutation
ability in some bacteria (i.e. mutations are caused when needed rather than
the organism just selecting random mutations so in this case it is the
indidual that is "evolving"). Sorry don't have the ref on the top of my
brain, maybe someone else knows more about this and can correct any
misconception I might have on this.

Just some quick thoughts on this thanksgiving morn.

"Give thanks to the Lord for all he has done"

Joel and Dawn Duff / | ' \ Spell Check?
Carbondale IL 62901 ( ) 0
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