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Dispersal Versus Gene Flow in Plants

Donald A. Levin
Annals of the Missouri Botanical Garden
Vol. 68, No. 2 (1981), pp. 233-253
DOI: 10.2307/2398797
Stable URL: http://www.jstor.org/stable/2398797
Page Count: 21
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Dispersal Versus Gene Flow in Plants
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Abstract

The notions that gene flow in plants is restricted and that neighborhood size and area are small are based upon data on pollination, and pollen and seed dispersal. However, neighborhood size and area estimates from these data incorporate several assumptions: (1) Pollinator flight distance is representative of pollen dispersal distance; (2) All pollen picked up by pollinators from one plant is deposited on the next one visited; (3) Pollinator flights between plants are random in direction; (4) Pollen-pistil compatibility is independent of the proximity of egg and pollen parents; (5) Seed viability is independent of the proximity of egg and pollen parents; (6) Seed production is independent of the proximity of seeds to the source; (7) The fitness of plants is independent of the distance between egg and pollen parent. These assumptions have been found to be unwarranted in many instances leading to the following conclusions: (1) Pollen carry-over may be considerable; (2) Pollinator flight sequences have a directional component; (3) Pollen-pistil compatibility may be lower among near-neighbors than among moderately spaced plants; (4) Seed abortion may be higher following self and near-neighbor crosses than following wide crosses; (5) Seed-set may be lower following crosses of neighboring plants than widely spaced plants; (6) Seed and seedling mortality may increase as the seed source is approached; (7) Fitness of offspring from distant crosses may be superior to that from self or near-neighbor crosses; (8) Gene flow over some distance may be higher in natural populations than in crops. There is abundant reason to believe that dispersal data underestimate gene flow. As a consequence, we may conclude that neighborhood sizes and areas are larger, there is less potential for random differentiation, there is less isolation by distance, there is less potential for geographical differentiation, and that stronger selection is needed to foster and maintain local differences than judged from dispersal data alone. Nevertheless, even if gene flow distances were twice as large as we now think, the spatial scale of gene dispersal: (1) is still small enough to allow substantial differentiation over short distances with moderate selective differentials, and (2) is too small to be a major cohesive force within a species.

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