Population-level predictors of phylogeographic diversity
To explore how microevolutionary processes (population size, dispersal
restriction and population stability within lineages) relate to the
geographic scale and depth of phylogeographic structuring within species
(complexes), we estimated mean observed heterozygosity (Ho ) and
Tajima’s D (Tajima, 1989), using the dartR package (Gruber,
Unmack, Berry, & Georges, 2018), as well as the strength (i.e. slope)
of IBD (as above). These measures provide us complementary information
about persistence, where higher mean Ho is related to larger
local effective population sizes (Ne), IBD slope to
lower dispersal and/or smaller local Ne, and more
negative Tajima’s D values to stronger deviations from mutation-drift
equilibrium, as could arise from population fluctuations. We note that
our quasi-random sampling of individuals across space is expected to
reduce confounding effects of spatial structuring on these summary
statistics, assuming modest to large local effective population sizes as
seems reasonable for these highly abundant lizards (Battey, Ralph, &
Kern, 2020). To test whether lower dispersal (or local
Ne) and more stable populations at the landscape scale
translate to finer-scale or deeper phylogeographic structure across all
lineages, we regressed (glm) lineage area and age (obtained from the
mitochondrial DNA BEAST chronograms for each genus; Suppl. Mat. Figure
S1-S4) against mean Ho , IBD slope and Tajima’s D as predictor
variables. Knowing that recent range expansion (as inferred by Tajima’s
D) could influence the relationship of range size with Ho , we
also performed a generalized linear regression using Tajima’s D values
as a covariate. We also tested for effects of habitat preference and
lizard group on mean Ho and Tajima’s D values.
Results