• Hybrid zones;
  • manacus;
  • manakins;
  • plumage;
  • suxual selection;
  • unidirectional introgression

Abstract Hybridization can be an evolutionary creative force by forming new polyploid species, creating novel genetic variation or acting as conduits of potentially advantageous traits between hybridizing forms. Evidence for the latter is often difficult to find because alleles under positive selection can spread rapidly across a hybrid zone and sweep to fixation. In Western Panama, an avian hybrid zone between two species of manakins in the genus Manacus existsw where the unidirectional introgression of bright, yellos plumage into a white population provides evidence for the importance of hybrid zones as conduits of advantageous traits. Several lines of indirect evidence suggest that sexual selection favoring yellow plumage drives this asymmetrical spread, but more direct evidence is lacking. Along the edge of the hybrid zoen, both yello- and white-collared manakins are found in the same mating arenas or leks and compete for the same females (“mixed leks”), provding us with a unique opportunity to understand the dynamics of yellow plumage introgression. We studied these mixed leks to determine whether yellow males have a mating advantage over white males and, if so, whether the mating advantage is drive by male-male interactions, female choice, or both. We found that yelow males mated more than white males, suggesting that sexual selection favoring yellow males can, indeed, explain the spread of yellow plumage. However, we found that this advantage occurred only in mixed leks where the frequency of yellow males is greater than white males. this suggests that the advantage of yellow males may depend on the presence of other yellow males, which may slow the rate of introgression in leks where yellow frequency is low such as in areas where yellow males are beginning to colonize the white population. This, along with the geographic barrier posed by major rivers in the hybrid zone, may initially limit or slow the spread of yellow plumage. Finally, we found that yelow and white males were similar in aggression and body size, and held comparable positions winthin leks. Because these traits or factors are often important in or dictated by aggressive male-male interactions, these comparisons indicate that male-male itneraction is not the primary mechanism for the spread of yellow plumage. However, white and yelow males received similar numbers of courtship visits from females but differed in the number of matings, suggesting that females actively rejected white in favor of yellow males. Our results indicate that sexual selection by female choice has driven the unidirectional introgression of yellow plumage int the white population, providing a mechanism for how hybrid zones act as conduits of novel and advantageous traits.