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.