= Solution
\b[<Allopatric speciation>] begins when a geographic barrier divides a population and reduces <gene flow>. Independent <mutation>, <genetic drift> and divergent <natural selection> can then build <reproductive isolation>; if populations later meet, they remain distinct when isolation is sufficiently strong. Sister lineages of snapping shrimp in the genus <Alpheus> on opposite sides of the Isthmus of Panama illustrate the role of separated marine populations. Their divergences are not all synchronous with the final closure: genetic data indicate several separation histories, so one barrier date should not be assigned indiscriminately to every pair.<New dates and new rates for divergence across the Isthmus of Panama by Nancy Knowlton and others>[Abstract: comparisons of 15 trans-isthmian pairs and heterogeneous divergence]{ref}
\b[<Sympatric speciation>] develops without a geographic barrier separating the initial population. Divergent ecological selection combined with <assortative mating>, habitat-associated mating, different reproductive times or a chromosomal change can reduce <gene flow> within a shared area. The palms <Howea forsteriana> and <Howea belmoreana> on Lord Howe Island provide a documented candidate: different soil associations and substantially different flowering times contribute to <reproductive isolation>, and their sister relationship and timing support divergence on the island.<Sympatric speciation in palms on an oceanic island by Vincent Savolainen and others>[Abstract and Figures 2–3: flowering-time and soil associations]{ref}
The distinction concerns the geography of divergence, not whether present-day ranges overlap. Present coexistence alone cannot distinguish <sympatric speciation> from earlier <allopatric speciation> followed by range expansion. <Parapatric speciation> describes divergence between adjacent populations with limited exchange and provides an intermediate geographical category; the two main modes requested are the allopatric and sympatric endpoints.
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