Studies in Avian Biology No. 32:54-75 EVOLUTION AND CONSERVATION OF TIDAL-MARSH VERTEBRATES: MOLECULAR APPROACHES

The tidal marshes of North America are home to a diverse collection of morphologically differentiated reptiles, birds, and mammals. We reviewed the existing molecular studies on endemic tidal-marsh vertebrates, including turtles, snakes, sparrows, rails, shrews, and rodents. We found both deep and shallow divergences from their nearest upland relatives in all geographic regions. In the Northeast, the Saltmarsh Sharp-tailed Sparrow (Ammodramus caudacutus) has probably been isolated from the inland forms of the Nelson's Sharp-tailed Sparrow (A. nelsoni), for >600,000 yr, while the salt-marsh form of the Swamp Sparrow (Melospiza georgiana) evolved from upland relatives <40,000 yr ago. On the West Coast, saltmarsh forms of the Song Sparrow (M. melodía) and the ornate shrew (Sorex ornatus) show low levels of genetic differentiation from neighboring upland forms, while the salt marsh harvest mouse {Reithrodontomys raviventris) living in the same marshes shows deep genetic divergences from upland forms dated to nearly 4,000,000 yr ago (MYA). On the Gulf Coast, saltmarsh Clapper Rails (Rallus longirostris) show either a very recent split from, or high levels of gene flow with freshwater King Rails (R. elegans), but Seaside Sparrows (A. maritimus) probably diverged from an upland ancestor 1.5-2 MYA and diamondback terrapins (Malaclemys terrapin) 7-11 MYA. The timing of those divergences ranges from late Miocene to late Holocene, and suggest a complex history of multiple invasions and differentiations in saltmarshes. Molecular approaches have increased our understanding of the evolutionary origin of these unique forms, revealed the complex patterns of genetic structure within them, and furthered conservation efforts.