Evolutionary change is driven by a combination of neutral and selective processes. Selection is the prevailing evolutionary force sifting through genetic variation allowing organisms to adapt and seize novel opportunities (positive selection), but likewise to maintain systemic functionality (purifying selection). As selection efficacy strongly depends on the effective population size (Ne), its interplay with genetic drift needs to be considered. However, owing to a historical lack of genome-wide data, our knowledge about the relative importance and strength of selection acting on the genome is limited.
Comparative genomic approaches are powerful to address this question. We generated genomic resources for several species within the avian genus Corvus (crows, rooks, and jackdaws). Here we focus on a species pair with a strong contrast in expected population sizes using population genomic re-sequencing data from an island species, the New Caledonian crow (C. moneduloides), and from a widespread species, the European crow (C. corone). We compare estimates of the distribution of fitness effects of new mutations (DFE), and of levels of positive selection using McDonald-Kreitman type statistics. The marked difference in Ne between the species under investigation allows us to isolate the effect of genetic drift on the selective genomic landscape, thus addressing a central question arising from evolutionary theory.