Dear Charity,
How many mutational screens have you performed which have led you to conclude that are no beneficial mutations? What sort of positive mutation can you envisage which would be as obvious and clear cut in drosophila as an embryonic lethal. It is highly unlikely that the majority of beneficial mutations take the form of radical reorganisations of organ or limb morphology. The most severe beneficial morphological mutation I can think of is the loss of wisdom teeth. Certainly not as obvious to the eye as someone with a genetic defect leading to malformed/ missing limbs.
As far as a beneficial mutations go, your inability to see any suggests either a reluctance to read the primary literature or else a obstinate intransigence stopping you from believing what you read. Perhaps your criteria are unusual, what would you consider to be the hallmarks of a beneficial mutation, from an evolutionary perspective it would be one that would impart a reproductive advantage to the posessor of that mutation, or if you like selfish gene theories then to the mutant gene itself. Clearly such a beneficial mutation (Still thinking about flies here), unless it imparts a truly exceptional benefit, is going to take many many generations to become apparent through the numbers and also an awful lot of wor to identify both by classical genetic methods and molecular genetics.
I'm not going to go into an exhaustive list of references for beneficial mutations here, there are already enough of those on the many previous beneficial mutations threads.
Just bear in mind that it is almost always easier to break a complex system than it is to improve it, and living beings and their development are highly complex systems, that doesn't mean that they are incapable of improvement.
TTFN,
WK
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