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\b[<Allopatric speciation> is the predominant classical model and is generally regarded as especially important in animals.] Geographic separation directly reduces <gene flow>, making it easier for divergent adaptation and incompatibilities to accumulate. The exact <frequency> across all organisms and historical events is not directly observable; a universal numerical fraction is not established by this statement.
Evidence combines the geographic arrangement of close relatives, <phylogenetic trees>, genetic divergence and independently reconstructed barrier histories. Sister lineages on opposite sides of barriers, island radiations founded by colonization, and differentiated <biological populations> that remain reproductively isolated after contact are mutually reinforcing observations. They are stronger when separation is independently dated and the origin of reproductive barriers is distinguished from later range changes.
A concrete primary example is the comparison of 15 trans-isthmian snapping-shrimp pairs using allozymes and mitochondrial COI. Concordant genetic differences support separation of lineages by the developing Isthmus of Panama, but divergence varied substantially between pairs. This cautions against assigning every pair to one simultaneous final closure event.<New dates and new rates for divergence across the Isthmus of Panama by Nancy Knowlton and others>[Abstract: 15 Alpheus pairs, concordant markers, variable divergence and different cessation times of gene flow]{ref}
The inference is not exclusive. The island-palm study of <Howea forsteriana> and <Howea belmoreana> used sister relationships, island history, soil association and flowering-time differences to support a sympatric origin.<Sympatric speciation in palms on an oceanic island by Vincent Savolainen and others>[Abstract: endemic sister taxa, dated island history and flowering-time disjunction correlated with soil preference]{ref} Such cases show that <sympatric speciation> is possible. Conversely, sister-species allopatry by itself cannot prove historical allopatry, because <extinction> and range shifts can change their present distributions. The case for the common mode rests on combined historical evidence and the widespread effect of barriers on <gene flow>.
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