On what grounds is the statement "mutations are not beneficial" made?

dna mutation

by Helen Fryman

QUESTION: On what is the belief in the statement "mutations are not beneficial" grounded when there is clear and convincing evidence to the contrary?

RESPONSE: Personally, I don't agree that there are not beneficial mutations, but I have to qualify that statement somewhat severely. Mutations that can be termed beneficial are beneficial in restricted environments. Antibiotic resistance, which is commonly referred to, does not promote stronger bacteria, only a special kind of bacteria which flourishes in environments where other bacteria are not there to compete. When the early, more "generic" bacteria are in competition with the antibiotic resistant bacteria, the former prove the stronger in a non-restricted environment.

Sickle cell anemia is another example that is often brought up. And yet it is only beneficial in the heterozygous state and only when malaria is a threat. Without malaria in the environment, the S allele does not prove advantageous and is lethal in the homozygous state.

What is seen is a vast preponderance of negative mutations compared to any mutations which might be considered positive by any standards at all (neutral mutations are not being considered here). So while I disagree technically with the statement "mutations are not beneficial," I understand what is being said--all mutations are by definition changes in the genome. Even by evolutionary standards the genomes present at the family level--if not lower in the taxonomic order--are extraordinarily stable. Changes, then, from this stability would yield an instability, which--although evolution theoretically depends on it--does not bode well for any population that is known today.

In other words, when a breeder or parent is told "this one has a mutation," one does not hear, "Wow! Is it a good one?" The assumption across the board, and rightly so, is that expressed mutations are deforming and/or deleterious at the least and lethal at the most.