Past exam of the year 2 of the earth sciences (geology) course of the University of Oxford / 2022 / Paper 1 / 12 / a / Solution

In allopatric speciation, geographically separated biological populations experience sufficiently reduced gene flow for divergence to accumulate through natural selection, genetic drift and other evolutionary processes. A barrier can split a former biological population, or a colonizing biological population can become isolated. The eventual species distinction requires persistent reproductive isolation, not merely physical separation at one instant.

In sympatric speciation, reproductive divergence originates without the initial geographic separation required by the allopatric model, at the spatial scale relevant to mating and dispersal. Divergent ecological selection and assortative mating, or some forms of polyploidy, can reduce gene exchange despite geographic overlap. Present-day sympatry alone does not establish this history: previously isolated species can meet through secondary contact. The difference concerns the geography of the origin of isolation, not whether the resulting species currently occupy different places.