Maintenance of local adaptation despite gene flow in a coastal songbird. (1st July 2022)

• Record Type: Journal Article
• Authors: Clark, Jonathan D.; Benham, Phred M.; Maldonado, Jesus E.; Luther, David A.; Lim, Haw Chuan
• Is Part Of: Evolution. Volume 76:Number 7(2022)
• Page Start: 1481
• Page End: 1494
• Publication Date: 2022-07-01
• Subjects: Bill divergence -- contact zone -- divergent selection -- ecological speciation -- saltmarsh syndrome; Evolution -- Periodicals; Heredity -- Periodicals; Évolution (Biologie) -- Périodiques; Hérédité -- Périodiques; 338.47004094
• DOI: 10.1111/evo.14538
• Languages: English
• ISSNs: 0014-3820

Abstract

Abstract: Adaptation to local environments is common in widespread species and the basis of ecological speciation. The song sparrow ( Melospiza melodia ) is a widespread, polytypic passerine that occurs in shrubland habitats throughout North America. We examined the population structure of two parapatric subspecies that inhabit different environments: the Atlantic song sparrow ( M. m. atlantica ), a coastal specialist, and the eastern song sparrow ( M. m. melodia ), a shrubland generalist. These populations lacked clear mitochondrial population structure, yet coastal birds formed a distinct nuclear genetic cluster. We found weak overall genomic differentiation between these subspecies, suggesting either recent divergence, extensive gene flow, or a combination thereof. There was a steep genetic cline at the transition to coastal habitats, consistent with isolation by environment, not isolation by distance. A phenotype under divergent selection, bill size, varied with the amount of coastal ancestry in transitional areas, but larger bill size was maintained in coastal habitats regardless of ancestry, further supporting a role for selection in the maintenance of these subspecies. Demographic modeling suggested a divergence history of limited gene flow followed by secondary contact, which has emerged as a common theme in adaptive divergence across taxa.