It is easy to imagine the presence of some sort of <PERSON> gene that results in relatively slow variation of an individual's genetic code in some of their cells - You could add a second <PERSON> process that copies 'good' mutations across to other cells, or alternately allow the body to become chimeric, with improved kidney genes only found in the kidneys and good muscle genes found only in some or all muscles.
But the problem is in determining what factors provide the 'feedback' signal to determine if a gene is beneficial or detrimental.
In a population and in normal evolutionary development, the feedback is purely and simply based on success at passing genes on to the next generations - more or less by definition a 'better-adapted' individual is one who has more reproductive success. In addition it needs a large number of 'trials' via numerous individual's success/failure to filter out the random noise that more significantly effects the reproductive success of individual animals.
In your scenario you have to choose some other mechanism to determine adaptive success, and there lies the problem. Your genetic mechanism could make tiny random changes in genetic code and 'watch' for feedback in terms of physical health, or stress levels, or (at a stretch) happiness - but there is an awful lot of noise in the signal - are you (un)healthier because of the change in some gene related to digestion or because you ate lots of vegetables or got food poisoning? As a result of the very low signal to noise ratio there would probably be more random genetic drift than ther would be beneficial changes in any single individual - even over thousands of years.
In addition, unless the feedback system is very 'clever' then, even if it does succeed you may not get what you want. Even 'natural' survival-of-the-fittest evolution shouldn't be considered as a path to higher quality. Lets say the <PERSON> relies entirely on bodily health - you may end up evolving you brain function so you are adapted to be ultra-paranoid about any external dangers. 1008
You live like <PERSON> in insane isolation but keep safe from germs. Or say you use happyness as your trigger - you may 'evolve' over time to be happy but really stupid.
About the only way I can see this working would be if the genetic changes were consciously chosen. So either the individual can look through a catalogue (maybe from GenesRUs Inc. or iGenes Ltd.) and 'purchase' improvements in heart function or memory retention - if that is what they want. Or there is some sort of embedded AI process that does the same thing for you. 335