Liza,
AFAIK, you'll have to write your own
Statistics object to do this. It's not too hard—an explanation of how to do so is found in
Tutorial 3.
Personally, the first thing I often do when creating an ECJ application is to write a Statistics object that records the genomes and fitnesses of the entire population for every generation. That creates a ton of data, but it makes it easy to ask whatever questions I want after a run completes.
Siggy