Originally Posted by Ben_Hur I've always been under the impression that - at least with humans, no mutation is good or beneficial.
Positive mutations do exist in the human genome and the natural selection in favour of these beneficial mutations leaves a genetic signature behind:
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Strong directional selection for a favorable new allele can cause a ‘‘selective sweep.’’ As the new mutant rises in frequency, adjacent chromosomal regions are also swept to fixation in a process sometimes called genetic hitchhiking. These events can be recognized as regions of low nucleotide diversity."
A recent study has looked for this signature of natural selection (1). Human genes that have been found to be subject to strong directional selection include: 18 genes involved in neural development, 15 in general skeletal development and 56 genes of miscellaneous function, there are also 470 hypothetical genes of unknown function.
The major difference between humans and the other apes is brain function so it is unsurprising that the largest category of genes recently subject to Darwinian selection are brain related. Natural selection on brain related genes has also been detected in other reports such as that on FOXP2 a gene involved in speech/language (2). The other major difference is body morphology the second major category of genes. Of particular interest is
PAPA-1 which is particularly involved in hand morphology so within the past few million years natural selection seems to have favoured changes in hand morphology in humans. Another major difference is the human subcutaneous fat layer which the other apes lack and sure enough the fat metabolism/body weight/fat distribution control genes AGRP, OSBPL9 and APOL5/6 have been subject to recent natural selection.
This research indicates that not only are there beneficial mutations that are fixed in the population by Darwininian selection but in humans these genes are functionally related to the major differences between human and the other apes. Therefore it is logical to conclude that many human specific features of are the result of beneficial mutations fixed by natural selection.
1) Mol Biol Evol. 2002 Dec;19(12):2342-5. Selective sweeps in the human genome: a starting point for identifying genetic differences between modern humans and chimpanzees. Diller KC, Gilbert WA, Kocher TD. 2) Genetics. 2002 Dec;162(4):1825-35.Accelerated protein evolution and origins of human-specific features: Foxp2 as an example. Zhang J, Webb DM, Podlaha O. 
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