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Natural Selection on Beak and Body Size in the Song Sparrow
Dolph Schluter and James N. M. Smith
Vol. 40, No. 2 (Mar., 1986), pp. 221-231
Published by: Society for the Study of Evolution
Stable URL: http://www.jstor.org/stable/2408803
Page Count: 11
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We documented temporal patterns of natural selection on beak and body traits in a song sparrow population. We looked for evidence of selection in association with reproduction and overwinter survival in order to identify the conditions under which size in beak and body traits is adaptive. We also attempted to identify the specific traits most closely associated with fitness under these conditions. Selection was observed in association with both survival and reproduction. Patterns of selection differed between the sexes. Selection on males was weak and stabilizing in association with over-winter survival. Selection on females was strong, was both stabilizing and directional, and was associated with both survival and reproduction. In females, traits that enhanced juvenile survival also reduced reproductive success; i.e., there was a trade-off between survival and reproduction. Patterns of selection in the song sparrow parallel those reported for the Galapagos finch, Geospiza fortis. However, in song sparrows, selection occurred mainly on tarsus length and beak length, and not on beak depth or width as in G. fortis. This difference may occur because most North American sparrows partition food resources by habitat, while most Galapagos ground finches partition food by seed size.
Evolution © 1986 Society for the Study of Evolution