What drives genetic differentiation in European crossbills?
Crossbills are a textbook example of how adaptation to different resources can result in genetic differentiation. I remember reading a 2003 paper by Craig Benkman during my Masters in Antwerp. This article, entitled ‘Divergent selection drives the adaptive radiation of crossbills‘, featured a figure showing how groups of crossbills with different beak morphologies are adapted to different species of pine (see below). Later research showed that these distinct beak shapes result in several call types, which consequently leads to assortative mating (i.e. birds prefer a partner with the same call). In the end, this culminates in the build-up of genetic differentiation between the call types, the onset of ecological speciation!
This fascinating system has been studied for years in North America. But what about the crossbills in Europe? Do they show similar patterns? Thomas Parchman and his colleagues crossed the Atlantic to figure this out. Their findings recently appeared in the Journal of Evolutionary Biology.
Should I stay of should I go now?
The European subspecies of the common crossbill (Loxia curvirostra) mostly feed on the Aleppo pine (Pinus halepensis), which occurs around the Mediterranean. Three subspecies are sedentary: balearica on Mallorca, poliogyna in northern Africa and hispana in Spain. The northern subspecies curvirostra, however, often undertakes long-distance movements when food resources are sparse. These nomadic ventures can be quite impressive, as noted by the English monk Matthew Paris:
“In 1254, in the fruit season, certain wonderful birds, which had never before been seen in England, appeared, chiefly in the orchards. They were a little bigger than Larks, and eat the pippins of the apples [pomorum grana] but no other part of them… They had the parts of the beak crossed [cancellatas] by which they divided the apples as with a forceps or knife. The parts of the apples which they left were as if they had been infected with poison.”
The occasional movements of curvirostra into the distribution of other subspecies could potentially lead to hybridization and gene flow. To test this idea, the researchers compared genetic data from these European subspecies using a genotyping by sequencing (GBS) approach. The results indicated that balearica and poliogyna were clearly different from the other subspecies, probably because they are geographically isolated in Mallorca and northern Africa, respectively.
The degree of genetic differentiation between the Spanish hispana and the northern curvirostra suggests that other factors than geographical isolation are at play. The Spanish birds differ in beak morphology from their northern relatives because they are adapted to the local pine trees, which provide a stable food source. When nomadic curvirostra crossbills arrive in Spain, they would be outcompeted by the locally adapted hispana birds. This might prevent interbreeding and thus promote genetic divergence.
Finally, the researchers also compared the northern subspecies with the parrot crossbill (L. pytyopsittacus), a species that occurs in the same area. Previous analyses found no clear genetic differences between these species. The present study did uncover some genetic differentiation, suggesting that a few genomic regions are responsible for the morphological differences between common crossbill and parrot crossbill. A pattern that has been observed in other bird species as well (see for example wagtails and crows). Pinpointing these differentiated regions – and checking if they are related to adaptation to local pine species – is a promising next step. Fingers crossed!
Parchman, T.L., Edelaar, P., Uckele, K., Mezquida, E.T., Alonso, D., Jahner, J.P., Summers, R.W. & Benkman, C.W. (2018) Resource stability and geographic isolation associated with genome divergence in western Palearctic crossbills. Journal of Evolutionary Biology