Calculation of geographic and macroclimatic distances between
populations
We calculated the pairwise geographic distance between island
populations and between mainland populations as geodesic distance
(shortest distance on the WGS84 ellipsoid) based on the GPS coordinates
of the populations, using the “distGeo” function in thegeosphere package (Hijmans 2019) in R 3.4.4 (R Development Core
Team 2018). Geographic distances were similar between island and
mainland populations (Fig. S6.1).
To calculate the macroclimatic distance between each population pair, we
performed a Principal Component Analysis (PCA) of four climate variables
reflecting mean and variation in temperature and precipitation available
in CliMond V1.2 (Kritikos et al. 2012) at 10 minutes resolution: mean
annual temperature (Bio1), annual precipitation (Bio12), temperature
seasonality (CV) (Bio4) and precipitation seasonality (CV) (Bio15) using
the “prcomp function” in the stats package in R. For
populations where climate variables were not available on the global
climate maps mostly due to small island size not captured in CliMond, we
extracted data from the geographically closest grid cell with available
climate values, which was available within 3.5 km away from the focal
grid cell for all localities. Variables were centred on zero and scaled
to unit standard deviation prior to the analyses. Island and mainland
populations occupied a broadly similar climatic space and were best
represented in three regions of the PCA corresponding to i) wet, cold
climate with constant precipitation and seasonal temperature i.e.,
temperate oceanic climate, ii) dry climate and seasonal precipitation
i.e., temperate continental climate, and iii) wet, hot climate with
constant temperatures i.e., tropical oceanic climate (Fig. S3.1). We
calculated the pairwise macroclimatic distances between populations on
the first two axes of the PCA space using the “dist” function and
‘euclidean’ distance measure in R.