Evolution of wing length and melanin‐based coloration in insular populations of a cosmopolitan raptor

Abstract

Aim Insular populations face different conditions than those living on continents, thus, resulting in the evolution of typical insular phenotypes, like smaller body sizes or reduced colourations. However, the generality of the so‐called “island rule” has been questioned, and intraspecific analyses on the effects of insularity on cosmopolitan species are lacking. Here, we tested the predictions of the island rule in the cosmopolitan common barn owl group. Location World. Taxon Barn owl species complex. Methods We compared wing and bill length, as well as melanin‐based plumage traits, between thousands of insular and continental barn owls living in the Afro‐Palearctic region (Tyto alba), in the Americas (Tyto furcata) and in Australasia (Tyto javanica). We also tested whether the difference between insular and continental populations in these phenotypic traits varies among islands/archipelagos of different size and isolation. Results In all the regions, we found differences between insular and continental owls in all the traits but bill length, with insular populations convergently evolving shorter wings and paler colourations. In addition, the difference in wing size between insular and continental populations is particularly marked on small and remote island systems, while melanin‐based traits are less expressed especially on large islands. Main conclusions We thus provide unprecedented evidence that insular conditions drive predictable phenotypic variations, even at the intraspecific level in different biogeographic regions, possibly promoting speciation events. In addition, our results also indicate that selective advantages of a given colouration can arise as the by‐product of positive selection on individuals displaying phenotypic traits which can favour island colonization and are genetically linked to melanization.

Unexpected post-glacial colonisation route explains the white colour of barn owls (Tyto alba) from the British Isles

“…Therefore, it appears the white insular morph can be most parsimoniously explained by relaxation or absence of selective pressure in contrast to the mainland. Such a pattern is actually common among insular birds which, due to relaxed selection, tend to display less colourful plumage than their mainland counterparts (Doutrelant et al, 2016;Grant, 1965), as also observed in the barn owl worldwide (Romano, Séchaud, & Roulin, 2021). This early history of colonisation of the British Isles inferred here from whole-genome sequences and supported by SDM projections on past climatic features is apparently unique among terrestrial vertebrates, but it is far from the first to deviate from the most common colonisation route over Doggerland (e.g.…”

Unexpected post‐glacial colonisation route explains the white colour of barn owls (Tyto alba) from the British Isles

“…Therefore, it appears the white insular morph can be parsimoniously explained by relaxation or absence of selective pressure in contrast to the mainland. Such a pattern is actually common among insular birds which, due to relaxed selection, tend to display less colourful plumage than their mainland counterparts (Doutrelant et al, 2016 ; Grant, 1965 ), as also observed in the barn owl worldwide (Romano et al, 2021 ).…”

Genomic bases of insularity and ecological divergence in barn owls (Tyto alba) of the Canary Islands

“…This classification is based on its smaller size and even on colouration pattern, although the latter is contested by ornithologists and inconsistent with reported phenotypical measurements (Burri et al 2016). The reduction in size is actually a common pattern in insular barn owls (Romano et al 2021), and could be an adaptation to nesting in very small cavities (i.e. cracks in lava walls) and/or to better navigate the strong winds in the eastern islands (Siverio 2007).…”