This may sound like a dumb question, but I am trying to understand the difference between selection on standing variation and the concept of exaptation. They seem to mean the same thing? Am I missing something? Thanks for any help you can provide.
No problem. Exaptation almost always refers to a phenotypic trait, and specifically the case where it used to do one thing, and has changed because of natural selection for some other function.
Selection on standing variation is usually just a contrast with selection on a new mutation. A new mutation that comes under positive selection will rapidly increase in frequency and thereby generate lots of signs we can recognize, for example genetic hitchhiking.
Selection on an old mutation that has already existed in the population for a long time (and is therefore “standing” variation) also can cause the mutation to increase in frequency, but this will not necessarily cause hitchhiking or other easily recognizable patterns, because copies of the mutation that have existed in the population for a long time probably are not all linked to the same set of mutations at other loci.
Practical example: Lactase persistence. We know that lactase persistence in Europeans is selection on a new mutation. If people carrying the key lactase persistence mutation did not all share near-identical region of chromosome 2 around that mutation, we would suspect it was selection on standing variation (when we learned about lactase persistence more than 10 years ago, this was not resolved yet and many geneticists thought it would turn out to be standing variation). Lactase persistence is arguably an exaptation, because it uses the mechanism that evolved for one purpose (babies digesting mothers’ milk) and changed it under selection for another purpose (adults digesting cow milk).