Many species, from mammals to microorganisms, release sex pheromones to attract a potential partner of the opposite sex. The combination of a pheromone and its corresponding receptor determines the species-specific ability of males and females to recognize each other, and therefore causes reproductive isolation. This barrier, which has arisen to restrict gene flow between mating pairs, might facilitate reproductive isolation leading to incipient speciation, but how do new combinations of pheromone and receptor evolve? Our recent study demonstrated an "asymmetric" pheromone recognition system in the fission yeast  Schizosaccharomyces pombe : among the two pheromone/receptor pairs in this yeast, recognition between one pair is stringent, while that between the other pair is rather relaxed. We speculate that the asymmetric properties of these pheromone recognition systems are beneficial for gradual evolution resulting in reproductive isolation in yeasts.