How are tension zones maintained in nature and what are their evolutionary outcomes?

In studying non-model birds, “tension zones” are a repeated phenomenon cropping up across the globe. Tension zones are narrow areas (narrower than expected under neutral diffusion) of secondary contact and hybridization between lineages. The geographically restricted nature of these zones suggests that they are held in place by unseen evolutionary forces (i.e., held in tension). Parsing the specific forces holding any single tension zone in place is a challenging endeavor, but diligent and careful study across tension zones continues to reveal new insights into the broadly repeatable evolutionary mechanisms that generate this repeatedly observed phenomenon.

Revealing the dynamics of a classic avian tension zone

• open access link to the manuscript: Hybrid zone or hybrid lineage: a genomic reevaluation of Sibley’s classic species conundrum in Pipilo towhees. One avian tension zone that I have studied in depth is between two towhee species (genus Pipilo) that come into contact across the transvolcanic belt in central Mexico.

I was part of a paper where we sequenced genomic loci from across this transect, to assess the dynamics of secondary contact between these two divergent lineages. Across the transect (sampling localities labeled 1-8 from west to east), we found very limited genetic divergence, and a relatively continuous distribution of ancestry.

We ultimately revealed that although there are apparently few intrinsic barriers to reproduction between these phenotypically divergent lineages, geography appears to play an important role in maintaining this tension zone, with limited gene flow across geographic barriers of unsuitable lowland habitat