Hybrid zone movement
For each transect-year we fit hybrid zone equilibrium cline models using
population ancestry proportions to estimate the shape and center of the
hybrid zone. First, for each transect, the distance (in km) of each trap
from the respective westernmost trap for that transect (trap T00
[42.6714° N, 73.0145° W] for the Massachusetts transect, and trap
CT01 [41.0798° N, 73.7054° W] for the Connecticut transect), was
calculated using the Latitude/Longitude Distance Calculator available at
(https://www.nhc.noaa.gov/gccalc.shtml). Then, for each
transect-year, the population coefficient of assignment (Q ) to
Bruce spanworm at each trap was estimated using the software program
Structure v.2.3.2 (Falush, Stephens, & Pritchard 2003;
Pritchard, Stephens, & Donnelly, 2000) based on two population
(K= 2) analyses using the admixture model, correlated allele
frequencies, and default settings, with random starting values, runtimes
of 200,000 generations, and burn-in periods of 20,000 generations, for
each year-transect combination. Finally, using the R package ‘hzar’ v
0.2-5 (Derryberry, Derryberry, Maley & Brumfield, 2014), a null model,
plus three different hybrid-zone models: 1) minimum and maximum
frequencies fixed to 0 and 1, and with no exponential decay tails, 2)
minimum and maximum frequencies as free parameters, with no exponential
decay tails, and 3) minimum and maximum frequencies as free parameters,
with both tails as independent parameters, were used to estimate the
center of the hybrid zone based on three independent analyses with chain
lengths of 100,000 generations, burn-in periods of 10,000 generations,
using random starting variables for each analysis. The maximum
likelihood values for each run were then compared to determine which
model provided the best fit to the observed dataset, from which the
center of the hybrid zone and the hybrid cline was estimated for each
transect-year combination.