Secular migration is diffusion that occurs at such a slow rate that the diffusing species experiences significant evolutionary change during the process. Over extended time periods, the species' distribution grows or alters (thousands or millions of years).
Natural selection works on descendent populations as the surroundings change. The llamas and vicunas of South America, for example, are derived from now-extinct North American members of the camel family who migrated through the Isthmus of Panama during the Pleistocene.
Local extinction works in tandem with spread processes to affect range alterations, allowing ranges to grow, decrease, or "creep" (expand in one direction while contracting in another).
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