Geological data show that high Andean habitats have been available for plant colonization only since the end of the Tertiary. The manner in which plant species moved into these habitats, the times during which, and the methods by which they differentiated during the Pleistocene varied altitudinally and latitudinally along the tropical Andes. The process of speciation in all areas, however, was the same as that in temperate environments, namely, geographic isolation and subsequent divergence. Except on the Altiplano, most plant species expanded their ranges during glacial periods when vegetation zones were lowered. In the northern paramos at elevations above treeline, colonization was greatest during glacial periods but has always occurred in a manner similar to that of oceanic islands. At lower elevations in the northern Andes, and along the Eastern Cordillera, direct migration was possible in glacial times because of increased contiguity of upper montane forest habitats. On the upper slopes of the west coast of Perú, glacial-age plant migrations were fostered more by changes in precipitation than by the lowering of vegetation belts. In all of these areas, interglacial periods were, and are, times of isolation and differentiation. Across the Altiplano in contrast, glacial periods were times of population fragmentation accompanied by differentiation and/or speciation.
Late paleocene flora of the northern Alaska Peninsula: the role of transberingian plant migrations and climatic change
