Dear Phylobabble community. I'm working with epidemiological surveillance data for a bacterial pathogen (N. meningitidis) and am trying to figure out how to describe the changes in the population of disease-causing isolates over time. Conventionally, this has been done by placing the bacterial isolates into discrete categories (e.g. by MLST) and then reporting the abundance of the various categories, and changes in abundance. I'm trying to obtain similar information using phylogenies inferred from whole genome sequence data. Can you suggest some statistics that would help us to quantify how quickly the population is changing and good ways to identify clades that are expanding or contracting exceptionally fast? If so, I'd greatly appreciate hearing about them.