Non-Poissonian variance in the number of variants per plate can occur when mutants arise at a time before application of selection pressure. Spontaneous mutation can also generate variance, not only because the mutants arise early, but because they arise at a variety of early times, all preceding application of selection. Feedback and interaction effects limit the generalizability of these statements.
Useful slides from Matthias Heymann, Bioinformatics Learning and Study Session (NYU 2003 Oct 15) http://www.bioinformatics.nyu.edu/BLISS/HeymannSlides.ppt
