Abstract
Secondary contact, the reestablishment of geographic overlap between ranges of closely-related populations that were previously isolated, has important consequences for the evolution of phenotypic biodiversity, as well as for speciation. Yet, opportunities to study evolutionary processes in systems where secondary contact is historically recent are rare, leaving much unknown about this important stage of the speciation process. For my dissertation, I used an integrative approach to conduct the first study of the evolutionary consequences of secondary contact in one such system: the two species of nectivorous Myzomela honeyeaters, M. cardinalis and M. tristrami, which achieved secondary contact on the island of Makira in the Solomon Islands around the turn of the 20th-century. I determined that hybridization between Myzomela is ongoing on Makira, in contrast to previous characterizations of this system. Mitochondrial and nuclear DNA sequencing revealed that hybridization is highly asymmetric between these species, with this asymmetric reproductive isolation likely driven by strong cytonuclear incompatibilities. I further determined that hybridization following secondary contact has led to asymmetric introgression, including introgression of melanic plumage-related alleles from M. tristrami to M. cardinalis. Behavioral experiments to test whether difference in plumage color mediates interspecific interactions in sympatry revealed a potential adaptive role for this introgression. Sympatric species of both populations exhibited biased aggression toward red M. cardinalis mounts, suggesting that melanic plumage may be favored in smaller individuals with hybrid ancestry because it allows them to avoid harmful aggressive interactions. This biased aggression toward M. cardinalis was only exhibited by sympatric populations, indicating that it is a consequence of secondary contact. Similarly, secondary contact resulted in rapid asymmetric character displacement in body size between species, likely as a consequence of interference competition. Finally, I leveraged hybridization in this system to demonstrate that a previously-unknown neo-sex chromosome is strongly associated with speciation and plumage divergence in this system. This result provides important evidence in favor of long-standing, but poorly supported theories that genomic sex-linkage is important for the evolution of secondary sexual traits and speciation. Together, these findings provide novel and important insights into the processes by which phenotypic divergence evolves and is maintained in natural populations following secondary contact.