Disentangling the Speciation of Crowned-Sparrows (Zonotrichia atricapilla and Z. leucophrys)

Abstract

The question of how species arise is fundamental to evolutionary biology and our understanding of every known species’ origins. If speciation occurs too slowly to study in real time, we can look for traces of an organism’s evolutionary past in their DNA. Evolutionary trees can then be used to visualize relationships between species. However, the arrangement of these trees’ branches can be called into question when genes evolve along seemingly different trajectories. Although this can occur for any number of reasons, two processes are particularly challenging to tease apart; when different species mate to produce hybrid individuals, vs. when ancestral genes haven’t finished sorting into descendant lineages. At least one of these two processes is believed to be responsible for the close-relatedness of white-crowned (Zonotrichia leucophrys) and golden-crowned (Z. atricapilla) sparrows. Despite having distinct plumages, songs, and behaviours, they can often be nearly impossible to distinguish genetically. It’s even been suggested that their DNA sequences are so undifferentiated that they could easily be mistaken for a single species. Is this due to hybridization, or have their genes not had enough time to sort themselves appropriately? I concluded that both of these were important in shaping relationships within and between white- and golden-crowned sparrows. I also found strong support for their statuses are true species. Given the importance of Zonotrichia sparrows as model organisms, these results could be used to draw widely applicable conclusions about the history of other organisms, and contribute to a greater overall understanding of avian evolution.