For vicariant events, such as Avise’s (2000) category
I phylogeographic pattern, haplotypes are shared between
daughter populations immediately following the splitting.
During the ensuing coalescent process, mutations that
result in new haplotypes are restricted to one or the other
of the daughter populations; these may increase in
frequency and eventually the shared ancestral haplotypes
become rare. If the two vicariant populations differ in
size, then the smaller is expected to become monophyletic
(coalesce) first, leaving the other population paraphyletic
due to incomplete lineage sorting (a potential example
is Spizella breweri and S. taverneri; Klicka et al. 1999).
Eventually the two populations become reciprocally
monophyletic