Male-driven reinforcement and cascade reinforcement in darters

Reinforcement can act in response to maladaptive hybridization by selecting for increased discrimination against heterospecifics mates in sympatry compared to allopatry (i.e., reproductive character displacement - RCD). Additionally, reinforcement can select for increased discrimination against heterospecifics in a fighting context in sympatry compared to allopatry (i.e., agonistic character displacement - ACD). Because it directly affects conspecific recognition traits (signals and/or preferences), reinforcement between two species in sympatry can incidentally cause behavioral isolation among populations within a species, termed cascade reinforcement. Here we demonstrate that a pattern consistent with male-driven RCD and ACD is present between two groups of darters (orangethroat darter clade Ceasia and rainbow darter Etheostoma caeruleum). Increased male discrimination against heterospecific females as mates and increased male discrimination against heterospecific males as competitors is present in sympatry. Furthermore, there is a pattern consistent with male-driven cascade RCD and cascade ACD within Ceasia. We found low levels of discrimination between two species of Ceasia that occur in allopatry from one another and in allopatry with E. caeruleum. This result contrasts that of a recent study which observed high levels of behavioral isolation between Ceasia species that occurred in sympatry with E. caeruleum. We suggest reinforcement between Ceasia and E. caeruleum in sympatry has led to the evolution of behavioral isolation between lineages within Ceasia that occur in sympatry with E. caeruleum but in allopatry with respect to one another (i.e., cascade reinforcement). This study demonstrates the ability of male behavior to simultaneously drive sympatric and allopatric speciation via reinforcement.