Calculation of pairwise distances between population parameters
To test how genetic and phenotypic measures vary within island and
mainland systems, we calculated the pairwise difference between
population mean values for each phenotypic and genetic measure for each
species 1) between island populations and 2) between populations on the
mainland. To allow comparisons across the range of different measures,
the pairwise difference between populations was expressed as the log
response ratio of the paired measures, as follows:\(x_{\text{ij}}=\ \log_{10}(\frac{m_{\text{ij}}}{n_{\text{ij}}})\),
where m = larger population mean value and n = smaller
population mean value, i = population measure type , j =
species. Higher log ratios indicate more variability between sites for
phenotypic or genetic values (cf. Hedges et al. 1999). In this metric
there was no directional structuring of the pairwise distances between
populations, and values ranged between \(\log_{10}\)(1) and\(\log_{10}\)(∞). This dimensionless metric did not require further
standardisation across different types of genetic and phenotypic
variability measures, enabling meaningful comparisons between
populations. The pairwise distance in genetic diversity between
populations bears the following specificities: i) It quantifies
difference in genetic diversity (diversity of genotypes in a
population), not genetic differentiation (difference between genotypes)
e.g., two strongly differentiated genotypes can have identical genetic
diversity and thus, low value of the metric; ii ) It quantifies
the scale of the difference, not the level of diversity e.g., comparing
equally high diversity populations can be identical to comparing equally
low diversity populations.
The non-independence of pairwise distance measures was accounted for in
the statistical analysis (see below).