CONCORDANCE BETWEEN MORPHOLOGICAL AND MOLECULAR MARKERS IN ASSESSING HYBRIDIZATION BETWEEN...

HYBRIDIZATION, DEFINED AS the interbreeding of recognized species, presents challenges to the reconstruction of phylogenies, formulation of species concepts, and biological conservation (Grant and Grant 1992). It is also pivotal in framing ideas about taxonomic relationships (Short 1963) and understanding

biological processes of evolution (Mayr 1963, Grant 1986). Genetic exchanges through hybridization have the potential to produce novel genotypes and phenotypes that may serve as starting points for new evolutionary trajectories (Lewontin and Birch 1966, Grant and Grant 1992, Arnold 1997), as well as expunge once distinct lineages (Gill 1997). Interest in avian hybridization has been rekindled in the past decade because ongoing transformations of natural landscapes have increased the likelihood of population expansions, contacts, and hybridization (Gill 1998).

During the breeding season, sharp-tailed sparrows (Ammodramus spp.) inhabit wet meadows, marshes, and salt marshes of central and eastern North America. The taxonomy, distribution, and evolutionary history of the group have been debated for more than a century (Dwight 1896, Montagna 1942, Beecher 1955, Greenlaw 1993, Rising and Avise 1993). In 1995, on the basis of morphological and genetic evidence, the American Ornithologists' Union (AOU) separated what had been considered a single species (Ammodramus caudacutus) with five known subspecies into two species: a northern species, Nelson's Sharp-tailed Sparrow (A. nelsoni, with three subspecies A. n. nelsoni, A. n. alterus, and A. n. subvirgatus) and a southern species, Saltmarsh Sharp-tailed Sparrow (A. caudacutus, with two subspecies A. c. caudacutus and A. c. diverus), limited to coastal wetlands (AOU 1995). Ammodramus nelsoni subvirgatus (hereafter "Nelson's sparrow") and A. c. caudacutus (hereafter "saltmarsh sparrow") are sympatric in coastal Maine, New Hampshire, and the northeast shore of Massachusetts (Hodgman et al. 2002) and have long been thought to hybridize (Montagna 1942, Rising and Avise 1993). Montagna (1940) collected a series of specimens from Popham Beach, Maine, that had intermediate characteristics between Nelson's and saltmarsh sparrows and considered that location an area of intergradation between those taxa. Furthermore, Rising and Avise (1993) found that 40% of assayed specimens from southern Maine were discordant between mitochondrial DNA (mtDNA) and morphology and suggested that there was some genetic exchange through hybridization.