 In short, the P. dominula studies turned on a slightly different but related proposed unique outcome for drift, that of fluctuations over time rather than variation at a time as for C. nemoralis, in both cases comparing small populations to large (as well as considering the capability of populations to produce large fluctuations), but with P. dominula, drawing definitive conclusions ran into difficulty from lack of data concerning the key variable of population size. There is also the recognition that fluctuating selection can produce the same outcome of fluctuations of gene frequencies across generations as drift can, making it difficult to resolve the empirical case. Finally, note that biologists have continued to study both C. nemoralis and P. dominula, with the result that additional processes have been identified that further complicate the empirical analyses (Millstein 2008; Skipper 2009).