Location: Sydney, NSW, Australia
Mel that's a really good point you make.
There is a GA setting to 'enable internal validation' - I switch it on all the time now to reduce the risk of curve fit, that's a must if employing the concept proposed.
But with your thoughts in mind, perhaps there is an additional stronger safeguard to avoid curve fit with the concept proposed:
That is for the GA to keep 2 sets of results - one set using the concept as suggested over 'n' iterations, and a second set over 'n' iterations using ALL the sample data.
Then compare and see which of the 2 sets, or both, holds up in the forward test.