Speciation and gene flow across an elevational gradient in New Guinea kingfishers

Area of Science:

• Evolutionary Biology
• Ornithology
• Speciation Research

Background:

• Parapatric elevational ranges in closely related species are common in tropical mountains.
• The gradient speciation hypothesis posits in situ ecological speciation driven by divergent selection across elevation.
• Previous studies on cichlids, sticklebacks, and Timema insects support ecological speciation driven by other selective pressures via parallel speciation.

Purpose of the Study:

• To test for parallel gradient speciation in replicated populations of Syma kingfishers on the island of New Guinea.
• To investigate the role of divergent selection and gene flow in the formation of elevational species series.

Main Methods:

• Analyzing gene trees and nuclear DNA clusters of Syma kingfishers across elevation and mountain ranges.
• Employing demographic modeling to assess gene flow and selection pressures.
• Compiling and reviewing published literature on gradient speciation in birds and other organisms.

Main Results:

• Syma kingfisher species showed reciprocally monophyletic gene trees and distinct nuclear DNA clusters, rejecting the gradient speciation hypothesis.
• Demographic modeling indicated that selection likely maintained species boundaries despite gene flow after secondary contact.
• In situ gradient speciation appears rare in birds, but divergent selection and post-speciation gene flow may be significant in generating elevational series and tropical biodiversity.

Conclusions:

• The gradient speciation hypothesis is not supported by the Syma kingfisher data.
• Divergent selection and gene flow play a crucial role in maintaining species boundaries and shaping biodiversity in tropical mountains.
• Further research is needed to understand the underappreciated forces driving tropical beta diversity along mountain slopes.