Abstract
Sexual selection on a phenotypic trait arises when that trait covaries with mating success among individuals of the same sex, usually males. As such sexual selection is thought to occur most readily and strongly in polygamous species where variance in mating success is high, but it can occur also in monogamous species where there is competition among males for early breeding females (Darwin, 1871 Ch. 8; O'Donald, 1980). As Darwin originally recognized, sexual selection in monogamous systems will be intensified when the sex ratio is skewed in favor of males. This paper reports on sexual selection on male body size, plumage and territory size characteristics in a small population of Darwin's Medium Ground Finches (Geospiza fortis) on I. Daphne Major, Galapagos, Ecuador. The species is monogamous, but during this study (from 1979 to 198 1) males have outnumbered females by a factor of three to two.
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