Sexual selection promotes hybridization between Pecos pupfish, Cyprinodon pecosensis and sheepshead minnow, C. variegatus J. A. ROSENFIELD & A. KODRIC-BROWN Biology Department, Castetter Hall University of New Mexico, Albuquerque, NM, USA Keywords: female choice; fish; genetic introgression; hybrid zone; male competition; mating system; Pecos River; reproductive isolation; sexual selection; speciation. Abstract Rapid and extensive genetic introgression has occurred between Pecos pupfish (Cyprinodon pecosensis) and sheepshead minnow (Cyprinodon variegatus) in the wild. We studied both female mate choice and male–male competition for mates among C. pecosensis, C. variegatus, and their F1 hybrids to determine what role these behaviours played in the formation of the hybrid swarm. Female C. pecosensis preferred male C. variegatus to conspecific males, C. variegatus females displayed no significant preference when given a choice between purebred males, and neither C. pecosensis nor C. variegatus females discriminated against F1 hybrid males. We found no evidence for female olfactory recognition of mates. Male F1 hybrids and C. variegatus were more aggressive than C. pecosensis males, achieving greater reproductive success under two different experimentally-induced mating systems. Hybrids were superior to C. variegatus when only two males competed (dominance interactions), but the two types were competitively equivalent in a territorial mating system. Our results indicate that active inter- and intra-sexual selection contributed to the accelerated hybridization between these two species. By including the possi- bility that some aspects of a hybridization and introgression event may be under positive selection, researchers may better understand the dynamics that lead to hybrid zone stability or the spread of introgressed genetic material. Introduction When allopatric populations are brought into secondary contact, the mechanisms that retard gene flow between them are of interest because they provide insights into the processes of speciation (Hewitt, 1988; Arnold, 1997; Boake, 2000). Divergent sexual selection (e.g. West- Eberhard, 1983; Boake, 2000; Uy & Borgia, 2000), natural selection (e.g. Schluter, 1998), and random genetic divergence (Dobzhansky, 1940; Muller, 1942; Orr, 1995) are each expected to reduce reproductive compatibility between geographically isolated popula- tions (but see Rice & Hostert, 1993). The reproductive isolating mechanisms produced by these processes can be assigned to two broad categories (1) premating isolating mechanisms, which limit the number of matings be- tween heterospecifics, and (2) post-mating isolating mechanisms, which limit the survival and reproduction of hybrid offspring. In contrast to the two well-studied outcomes of hybridization (random genetic mixing and slower-than- random gene flow between hybridizing populations), the mechanisms that drive faster-than-random fusion of genomes have received little study. Sexual selection could promote gene flow between populations if hybrids benefit from female mate choice preferences or asymme- tries in male–male competition for mates. Both processes, acting alone or in combination, would promote hetero- specific matings and could result in faster-than-random fusion of genomes. Pre- and post-mating processes that promote greater inter-population cohesion than intra- population cohesion (sensu Templeton, 1989) have been documented. Pre-existing sensory bias (e.g. Kaneshiro, Correspondence: Jonathan A. Rosenfield, Wildlife, Fish, and Conservation Biology Department, One Shields Avenue, University of California – Davis, Davis, CA 95616-8751, USA. Tel.: +1 530 752 8409; fax: +1 530 752 4154; e-mail: jarosenfield@ucdavis.edu 1 J. EVOL. BIOL. 16 (2003) 595–606 ª 2003 BLACKWELL PUBLISHING LTD 595