Distinguishing genomic homogenization from parapatric speciation in an elevationally replacing pair of Ramphocelus...

Area of Science:

• Evolutionary Biology
• Genomics
• Ornithology

Background:

• Distinguishing between genomic homogenization and incipient speciation is challenging in geographically connected species with similar genomes.
• Weakly differentiated genomes can arise from either ongoing homogenization or early stages of speciation.

Purpose of the Study:

• To differentiate between genomic homogenization and speciation with gene flow in two closely related Ramphocelus tanager species.
• To investigate the evolutionary processes shaping genomic and phenotypic divergence in a hybrid zone.

Main Methods:

• Coalescent modeling was applied to a genome-wide single nucleotide polymorphism (SNP) dataset.
• Analysis focused on two phenotypically distinct but genomically similar Ramphocelus tanager species in the Andes.

Main Results:

• Overwhelming support for a model of genomic homogenization following secondary contact was found.
• Most of the genome rapidly homogenized over 254,000 years, reducing differentiation (FST) from 0.30 to 0.02.
• Despite homogenization, plumage remains distinct with a narrow cline, suggesting divergent selection on color loci.

Conclusions:

• The majority of the Ramphocelus tanager genome has undergone fusion after secondary contact.
• Divergent selection on specific genetic loci likely maintains the observed geographic color differences between these incipient species.