= Solution
In \b[<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 \b[<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.
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