 Solution

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.^[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.^[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.