Directional sexual selection in polyandrous species is an important evolutionary force, but dissecting the comparative importance of pre- and post-insemination processes of sperm selection has proved difficult. In the January 23 Nature, Jonathan P. Evans and colleagues at the University of Padova, Italy, show that directional sexual selection can proceed exclusively through postcopulatory processes at a physiological level (Nature, 421:360-363, January 23, 2003).

Evans et al. used artificial insemination in guppies to eliminate the role of pre insemination sexual selection. They observed that when equal numbers of sperm from two males competed for fertilization, more colorful males achieved greater parentage rates than their less ornamented counterparts. In addition, they found that relatively small individuals were advantaged in sperm competition, suggesting a possible trade-off between sperm competitive ability and body growth.

"An important direction for future research is to identify the specific sperm traits that confer reproductive...

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