Geographic population structure and subspecific boundaries in a tidal marsh sparrow

Abstract

Subspecific genetic diversity is a source for ongoing evolutionary processes, can be predictive of a population’s ability to respond to conservation challenges, and may represent the raw material for incipient speciation. As such, diagnosable subspecies are increasingly recognized as an important unit for conservation. Differentiating among subspecies can be particularly difficult in ecosystems characterized by recent phenotypic divergence, such as the tidal marshes of North America. These systems require approaches that can capture selective changes which occurred over only a few millennia as species adapted to new habitats following the Holocene glacial retreat. Here we test for genetic differentiation in morphologically distinct tidal-marsh-endemic subspecies of the swamp sparrow (Melospiza georgiana) using nuclear microsatellites. This case study serves as a test approach for the quantification of conservation units for tidal marsh ecosystems. Though prior surveys of mtDNA variation showed no detectable phylogeographic structure, we found evidence of genetic differentiation in seven microsatellite loci between two M. georgiana subspecies. The most likely model of population structure suggested two clusters in western Maryland/Pennsylvania and Delaware/Chesapeake Bays, with a zone of uncertain population assignment in New Jersey. The microsatellite intergrade zone is broader than the known area of morphological intergrades. We show that microsatellites can be used to support a subspecific status for tidal-marsh taxa such as the swamp sparrow, where changing post-glacial environments likely selected for locally adapted traits while neutral genetic structure is weak. This approach thus allows for the identification and conservation of hot spots that foster ongoing evolutionary change.