Abstract
I genotyped lek-mating Long-tailed Man- akins (Chiroxiphia linearis) at Monteverde and Santa Rosa, Costa Rica, 115 km apart. Cavalli-Sforza dis- tance was 0.04, DLR was 0.18, and RST and u were both 0.02. Bayesian clustering analysis indicated that both populations were part of a single cluster rather than from distinct clusters. I present a binomial test for probability of allelic absence as a function of sample size. Genotypic likelihood tests assigned 50% of Mon- teverde birds to Santa Rosa, versus 26% of Santa Rosa birds to Monteverde. Two lines of evidence supported the idea of asymmetric gene flow up the elevational gradient from Santa Rosa to Monteverde. Low differ- entiation at this spatial scale, despite intense sexual selection, suggests that sexual selection alone is un- likely to promote rapid divergence leading to specia- tion. Reduced gene flow, produced by geographic bar- riers or behavioral factors, may also be required.

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