(A) To speciate, two lineage have to diverge enough to substantially reduce cross-reactivity, that is needs to be comparable to . Inset: Illustration of the definition of time to most recent common ancestor and the time interval by which advances. (B) If such speciation happens, the host capacity - the average number of infected individuals increases two-fold. (C) The probability of such deep divergences decreases exponentially with the ratio , where effective antigenic diversity is . In the presence of deleterious mutations, the relevant is not necessarily the total advance of the pioneer strains, but only the antigenic contribution. This antigenic advance can be computed as with antigenic variance , where is fitness variance due to deleterious mutations. With this correction, speciation times agree with the predicted dependence (colored lines).