Phylogenetic signal
Among the available indices available to characterize phylogenetic
signals in trait data, Blomberg’s K is the most widespread and is
considered to capture the effect of trait evolution (Blomberg and
Garland 2003; Münkemüller et al. 2012). This is based on an approach in
which the magnitude of independent contrasts has smaller variance if
related species are similar to each other in trait character. Observed
versus expected contrast variances were compared under a null model
created by swapping the tips of the phylogenetic tree to test for
significance differences (Blomberg et al. 2003). When K approaches 1,
trait evolution follows a mode of evolution that is consistent with
Brownian motion (i.e. random walk), whereas for K > 1 and
<1, respectively, close relatives are more similar or less
similar than expected indicating a strong phylogenetic signal (Blomberg
et al. 2003). Using all the traits measured as continuous variables
(body mass, body size, bill length, tarsus length, and clutch size), we
calculated Blomberg’s K as reported by Münkemüller et al. (2012)
using the R package “phylosignal” (Keck et al. 2016). The
significance of K (p -value) was calculated by comparison
to a null distribution (Yang et al., 2014). We also used Moran’s
correlograms, plotted using the function “phyloCorrelogram” from the
package “phylosignal” (Keck et al. 2016), to assess how phylogenetic
autocorrelation changed across different phylogenetic distances.
Originally a measure of spatial autocorrelation, when used in
phylogenetic analysis Moran’s I assesses phylogenetic proximity among
species to describe the relationship between cross-taxonomic trait
variation and phylogeny.
All statistical tests were performed with R software version 3.6.2 (R
Core Team, 2019).