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Drought, Diastrophism, and Quantum Evolution

Daniel I. Axelrod
Evolution
Vol. 21, No. 2 (Jun., 1967), pp. 201-209
DOI: 10.2307/2406669
Stable URL: http://www.jstor.org/stable/2406669
Page Count: 9
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Drought, Diastrophism, and Quantum Evolution
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Abstract

Restricted areas of greater drought probably were sites for quantum evolution of Cretaceous angiosperms as they radiated polewards from ever-wet rain forests. With only moderate tectonism and volcanism, local dry sites would appear on lee slopes and in intra-range valleys almost "instantaneously" (i.e. 0.5-2.0 million years). From an evolutionary standpoint they would be analogous to a new archipelago, or a string of isolated lakes. They would persist for fully 10 to 30 million years (or longer) before similar conditions spread out over the lowlands. New adaptive types could evolve there without leaving a record in accumulating deposits. Dry areas would have a diversity of potential habitats, be essentially free of competition, encourage the fragmentation and diversification of small to medium sized populations, cause geographic isolation, permit the establishment of founder populations with accompanying catastrophic selection, and enable new adaptive types to evolve at a rapid rate, leading to quantum evolution. Inasmuch as plants at the margins of rain forest were preadapted to some seasonal (monsoon climate) drought, strong selection for those capable of withstanding greater periods of drought would propel populations into these local drier areas which were, in essence, the emerging new vegetation zones that later shifted into the lowlands as dry climate expanded there. Plants adapted to longer periods of drought probably continued to evolve in local dry sites, sending out waves of new taxa that were more highly adapted to the increasingly drier climates characterized by greater temperature extremes that spread out over the lowlands during the Cenozoic.

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