Replicate avian hybrid zones reveal the progression of genetic and trait introgression through time

Replicate hybrid zones between the same pair of taxa provide a unique
opportunity to uncover repeatable outcomes of hybridization, which may
point to loci and traits under parallel selection and those contributing
to reproductive isolation. In addition, replicates allow for the
exploration of the factors causing shifts in hybrid zone structure over
time. Here we take advantage of a pair of avian taxa that form multiple
hybrid zones to assess the predictability of hybridization and explore the
progression of trait introgression through evolutionary time. The
Lemon-rumped (Ramphocelus flammigerus icteronotus) and Flame-rumped
Tanagers (R. f. flammigerus) inhabit the Pacific coast and mid-elevation
slopes of the Cauca Valley of Colombia,  hybridizing where they
encounter each other in low passes across the Andes. We sampled transects
along three such geographically separate passes and found that hybrid
zones along these transects were formed independently, show parallel
patterns of phenotypic divergence across ecological gradients, and have
similar demographic histories. We also found parallel patterns of
asymmetric introgression of neutral markers from the yellow icteronotus
subspecies into the hybrid zone across transects. However, the age of the
hybrid zones varied, as did the extent to which geographic and genomic
clines are displaced away from environmental transitions into the red
flammigerus range. The greatest displacement was in the oldest southern
transect, followed by moderate displacement in the middle transect and
little to no displacement in the youngest northern transect. Also, the
only shared introgression outliers across all three transects were in a
genomic region that predicts plumage color and clines for these loci were
consistently narrow independent of the age of the hybrid zone, suggesting
a role in reproductive isolation maintained over time. Altogether, our
analyses of replicate hybrid zones showed that 1) locus-specific
introgression is largely stochastic, but the magnitude and directionality
of neutral introgression can be predictable if factors that influence
major allele frequency dynamics—such as demography—are similar across
replicates, and 2) independent of time and local environmental conditions,
aspects of the hybrid zone dynamics can be predictable for traits likely
involved in reproductive isolation.