Beneficial Mutations and Evolution

[KINGOFKNGS]

I don't think you're making much sense. If because of a mutation, a given individual lives longer than they would without the mutation (and in that time reproduces more), in what sense is it not beneficial?

Agreed...

Suppose I find a way to mutate humans so that they avoid all medical conditions until they are 200, but at age 200 they die of complications from the mutation. Is this still a "partially" negative mutation on your view?

I guess you could say that it's negative if it results in the ending of a life. A "better" mutation would allow for the person to keep all the advantages and live to 201, from a strictly evolutionary point of view in that there is an extended period of time for more reproduction. The point of judgement of a mutation is whether or not it allows for a greater reproduction of a species' genes. This is talking on a population level.

I get the impression that some people here feel that the quality of life of an individual within a species determines whether or not the mutation is beneficial. Whether or not it causes pain to an individual is irrelevant in an evolutionary sense--if that pain, however, causes a decreased ability to reproduce, then it is not a beneficial mutation to the species.

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My understanding of SCA is that in times of stress, the cells sickle. When the Plasmodium enters the cell, it sickles, and it's viewed as "bad" by the immune system and is therefore destroyed before Plasmodium can reproduce. Apparently, heterozygotes have this ability as well. If the Plasmodium cannot reproduce, then there is resistance to the disease. Someone correct me if I'm wrong. I'm trying to remember details of a biochemistry lecture from a few years back, and I acknowledge that the details may be a little sketchy. Just wanted to share the mechanism by which SCA can provide resistance to malaria.

 

[KINGOFKNGS]

Sickel cell anemia is a crippling painful disease. It will kill an affected individual by their 40th year. Milaria is a fatal infectious disease that kills at all ages. individuals that have SCA can live to their 30's in relatively good health in areas where milaria is endemic. They are immune to the rapid killing infection but have a genetic disorder that will kill them in middle age. SCA is a benificial mutation if one lives in milaria country.

This is the gist of the argument. Those affected by SCA (and carriers) have resistance to malaria. I think that perhaps a better way of saying it is SCA is a beneficial mutation if someone is afflicted with malaria! Perhaps that's looking at an individual level as opposed to across a population, in which case the last line of the above paragraph would be correct.

As an addendum to my last post, the benefit of a mutation can only be judged relative to the "normal" or perhaps "wild" type. Therefore, a mutation that will allow someone to live to 200 with no ill effect, yet will kill the person at 200 will be beneficial to the species, when compared to the normal phenotype. However, suppose that there were a 201 year mutation, theoretically, that would be the more beneficial mutation, assuming that it allows greater penetrance of the gene due to greater reproductive success (even if it's just one more year of reproduction).

 

[Sealeaf]

The concept of a "benificial" mutation is off base to begin with. Mutation just happens. It is a continuous random process. Think of it as "imperfect reproduction". It is a biological example of Murphy's law. If something can go wrong it will. Many opportunities for things to go wrong exist in biological reproduction.

"Benificial" is a value judgement placed in hindsight. A thousand years ago some viking in what is now Norway had a mutation. It was not benificial, it just made he or she a bit different. Nobody noticed. The difference had no particular survival importance. The difference was passed down to the offspring of those vikings. With the advent of AIDS we now see this mutation as "benificial" because it confirs resistence to HIV infection. Something like 20% of the population of Europe has the gene.