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dynesius - Evolutionary consequences of changes in species...

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Evolutionary consequences of changes in species’ geographical distributions driven by Milankovitch climate oscillations Mats Dynesius* and Roland Jansson* ² Landscape Ecology Group, Department of Ecology and Environmental Science, Umeå University, SE-901 87 Umeå, Sweden Edited by Alfred G. Fischer, University of Southern California, Los Angeles, CA, and approved June 2, 2000 (received for review February 22, 2000) We suggest Milankovitch climate oscillations as a common cause for geographical patterns in species diversity, species’ range sizes, polyploidy, and the degree of specialization and dispersability of organisms. Periodical changes in the orbit of the Earth cause climatic changes termed Milankovitch oscillations, leading to large changes in the size and location of species’ geographical distribu- tions. We name these recurrent changes ‘‘orbitally forced species’ range dynamics’’ (ORD). The magnitude of ORD varies in space and time. ORD decreases gradual speciation (attained by gradual changes over many generations), increases range sizes and the proportions of species formed by polyploidy and other ‘‘abrupt’’ mechanisms, selects against specialization, and favor dispersabil- ity. Large ORD produces species prone neither to extinction nor gradual speciation. ORD increases with latitude. This produces latitudinal patterns, among them the gradient in species diversity and species’ range sizes (Rapoport’s rule). Differential ORD and its evolutionary consequences call for new conservation strategies on the regional to global scale. C limate has fluctuated widely during the history of the Earth. Beyond seasonal variations, climatic variability increases in amplitude toward longer time scales, but have a marked peak on the time scale of 10–100 thousand years (kyr) caused by Mi- lankovitch oscillations (1). Such are found during the entire Phanerozoic (2), and have varied in amplitude among geograph- ical areas and geological epochs. The mean duration of a species in the fossil record varies among taxa from about 1 to 30 million years (3), implying that they possess properties that allow them to survive many Milankovitch oscillations. Milankovitch oscillations lead to large changes in the size and location of species’ geographical distributions (4, 5). These orbitally forced species’ range dynamics (ORD) can be seen as an overarching tier constraining evolutionary processes acting on shorter time scales (5). Adaptive changes may accumulate during the relatively short periods in between climatic shifts, but much of this is lost as populations go extinct or selection pressures are changed in conjunction with the next upheaval (5). Here, we systematically explore evolutionary consequences of ORD. We conclude that ORD decreases gradual speciation rates (as opposed to conventional wisdom), increases species’ geograph- ical ranges and the proportion of species formed by abrupt speciation, selects against specialization, and favors dispersabil- ity (Fig. 1). Large ORD leads to species prone neither to extinction nor gradual speciation. We suggest differential mag-
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