Population divergence involves not just molecular changes to DNA composition but also physical rearrangements to genome architecture. Chromosome inversion, one class of chromosomal rearrangement, are powerful recombination modifiers often observed as fixed differences between species and polymorphisms segregating within species. Empirical studies have alternately attributed to chromosome inversions a role in sex chromosome evolution, supergene formation, local adaptation, and reproductive isolation. While birds have long been used to study the roles played by behavior and ecology in speciation, little attention has been given to the contribution of chromosome inversions - which are numerous - to avian diversification. My dissertation research is therefore focused upon better understanding the evolutionary forces driving chromosome inversion evolution in birds.
Chromosome inversion evolution in Passeriformes
Why do some taxa have more inversion differences than others? The answer to this question remains largely enigmatic despite nearly a century of study. Using karyotype records for 410 species from the most species-rich group of birds (order Passeriformes), I examine support for alternative theoretical models of inversion evolution that rest on contradictory predictions regarding the likelihood of inversion fixation and variance in the speciation history, demography, and ecology of species. I find that inversions are evolving likely because they are adaptive as the extent of inversion differentiation between passerine species is best predicted by a model in which inversions are selected for during speciation when gene flow occurs before reproductive isolation is complete.
Speciation genomics in the Australian grassfinches
The Australian grassfinches (family Estrildidae) exhibit one of the highest rates of pericentric inversion fixation observed in passerines (Hooper in prep). I am using whole genome sequence data from an entire clade of 8 grassfinch species in order to characterize the extent of structural variation between taxa and the evolutionary context in which these rearrangements have occurred.
Chromosome inversions and reproductive isolation in an avian hybrid zone
The long-tailed finch (Poephila acuticauda) is endemic to the northern tropics of Australia and comprises two subspecies that differ primarily in beak color: yellow in the west and red in the east. These subspecies meet, mate, and produce orange-beaked hybrids restricted to a narrow zone of admixture (<200km). I am using this hybrid zone to investigate the extent of genomic differentiation and the strength of reproductive isolation between subspecies, the genetic basis of beak color, and the contribution of inversion polymorphism on the Z chromosome to each of these processes.