  (where \(\phi(\omega ,t)\) is a function that gives the size of a daughter population at some future time \(t\)).  Of course if every possible daughter lineage becomes extinct eventually then in the limit \(F = 0\), and if some populations fluctuate wildly, \(F\) may not converge to a stable value in the limit either. But there are conditions under which the limit as the number of generations goes to infinity is guaranteed to exist and under which \(F\) is well-defined (see Pence and Ramsey 2013, 862).