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