Parallel Divergence by Allochrony and Cryptic Speciation in two Highly Pelagic Seabird Species Complexes (Hydrobates spp.)
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Sympatric speciation, the evolution of reproductive isolation between populations without physical barriers to dispersal, is less likely than allopatric speciation due to the need to overcome potentially homogenizing gene flow. Cryptic species - two or more species mistakenly classified as one usually due to morphological similarity - can form in allopatry or sympatry. One potential mechanism of cryptic speciation in sympatry is allochronic speciation, i.e. divergence driven by differences in breeding time. In this thesis, I first collated examples where allochrony caused divergence between populations to glean new insights into drivers of shifts in breeding time across taxonomic groups and the genetic underpinnings involved, and to create a framework for future investigations. Using both genetic and genome-wide sequencing techniques, I then investigated drivers of cryptic divergence and the evolution of allochronic populations across the global breeding ranges of two cryptic seabird species complexes, Leach’s and band-rumped storm-petrels (Hydrobates spp.). In Leach’s storm-petrels, I found non-physical barriers to be stronger drivers of divergence than physical ones, with some genetic differentiation between ocean basins but higher differentiation among colonies in the Pacific Ocean. Phylogenetic reconstruction revealed that Guadalupe seasonal populations likely speciated allochronically. In band-rumped storm-petrels, colonies in different ocean basins were genetically differentiated, however phylogenomic reconstruction placed South Atlantic colonies as sister to Pacific colonies, and revealed strong genetic structuring within ocean basins, again suggesting non-physical barriers as drivers of divergence. Band-rumped storm-petrels consist of seven reciprocally monophyletic groups, and thus likely represent up to seven different species. I then investigated whether allochronic populations of band-rumped storm-petrel formed due to a mutation shifting breeding time in a founder event, standing genetic variation at the population level, or plasticity in breeding time. My findings indicate breeding season changes are unlikely to have involved a mutation and founder event, however further work is needed to tease apart the potential roles of standing variation and plasticity. Altogether, this thesis reveals the importance of non-physical barriers to gene flow in Leach’s and band-rumped storm-petrels, uncovers cryptic species needing taxonomic and conservation consideration, and furthers our knowledge of allochronic divergence, an underappreciated mechanism of cryptic diversification.
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