Comments on effective population sizes

Those are nice tricks, but there are some limitations. The biggest is that $N_e^{(v)} \ne N_e^{(f)}$ if the population size is changing from one generation to the next.²⁰ So you have to decide which of these two measures is more appropriate for the question you're studying.

• $N_e^{(f)}$ is naturally related to the number of individuals in the parental populations. It tells you something about how the probability of identity by descent within a single population will change over time.

• $N_e^{(v)}$ is naturally related to the number of individuals in the offspring generation. It tells you something about how much allele frequencies in isolated populations will diverge from one another.