Endemic island songbirds as windows into evolution in small effective population sizes

Due to their limited ranges and inherent isolation, island species provide a unique opportunity to study the impact of non-adaptive forces on molecular evolution, especially how effective population size may influence the accumulation of deleterious mutations. By estimating nucleotide diversity at synonymous and non-synonymous sites for a large set of in-house and published whole genome sequences covering distant phylogenetic taxa (14 endemic insular and 11 mainland passerine bird species), we found support for contrasted patterns of molecular variation between endemic island and mainland species. Even after controlling for the phylogenetic inertia, we observed significantly lower nucleotide diversities, higher mutation loads and lower adaptive substitution rates in the endemic insular species. Overall, our results indicate that the smaller area available on islands likely constrains the upper bound of the effective population size, in such a way that demography has affected the ability of natural selection to efficiently remove weakly deleterious mutations. Endemic insular species genomes therefore represent excellent windows into evolution in small effective population sizes and we discussed the implications for both evolutionary and conservation biology.