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Abstract
Evolutionary and ecological factors influencing genetic and morphological variation in burrowing owl (Speotyto cunicularia) populations throughout their range in North, Central, and South America and the West Indies were examined. Genetic analyses included two areas of the mitochondrial genome: the control region and a fragment of the cytochrome b region. Results between the two regions were not concordant and control region sequences appear to be nuclear sequences of mitochondrial origin. These results emphasize the need to ensure the mitochondrial origin of sequences, and add to the growing body of literature documenting the occurrence of mitochondrial insertions into the nuclear genome. Cytochrome b sequences were authentic mitochondrial DNA sequences and revealed a strong split between burrowing owls from North and South America. This split dates back 2 million years to the connection of the two continents via the isthmian land bridge. Results suggest a North American origin for burrowing owls and subsequent dispersal to South America.
Morphological variation was extensive, particularly for owls from the South American continent. However, morphological data did not support the strong split between North and South American populations observed in the cytochrome b data. Adaptation to local conditions, resulting in latitudinal clines in body size, made the relative contributions of gene flow and natural selection to character variation difficult to evaluate. Latitudinal clines in body size were observed on either side of the equator but not within wide-ranging subspecies at the northern and southern extremes of the burrowing owl's range. High elevation populations in the Andes Mountains appeared quite distinct compared to more widespread lower elevational populations. Sexual dimorphism in size, recognized in North American populations of S. c. hypugaea, was not observed throughout the range of the owl. Genetic data, biogeographic history, the burrowing owl fossil record, and character variation indicate that subspecies differentiation within continents is of Pleistocene origin. The depth of the genetic split between burrowing owls from North and South America is consistent with species level distinction. Additional data from nuclear markers and/or ecological indicators such as behavior or vocalizations will be necessary to distinguish between these results.
