Genetic study of European coal tits reveals gene flow across a continental contact zone.
Hybridization is often studied in the context of hybrid zones. After a period of geographical isolation, populations might come into secondary contact and interbreed. In Europe, this scenario has been documented for numerous animal and plant taxa. The classical papers by Godfrey Hewitt (see for example here, here and here) describe an intriguing story of how populations were pushed in southern refugia by expanding ice sheets during the Quaternary (the geological period starting about 2.5 million years ago). Possible refugia included the Iberian peninsula, Italy and the Balkans. When the climate warmed and the ice retreated, these populations expanded from their refugia and recolonized the European continent. Organisms from different refugia met each other in different contact zones (see figure below).
German Coal Tits
The establishment of a secondary contact zone has several possible outcomes: (1) the formation of a narrow hybrid zone, (2) a broad zone of intergradation, or (3) merging of both lineages. A recent study in the Biological Journal of the Linnean Society explored which of these outcomes best described the situation of the coal tit (Periparus ater).
The genetic analysis of this small passerine revealed that all German populations were a mixture of southern and northern populations. This amalgamation of coal tit genes was not restricted to a narrow contact zone, but expanded across a wide area of intergradation. This genetic pattern is supported by morphological data from previous studies that show how different subspecies gradually change from north to south.
Isolated Island Populations
In addition, island populations of the coal tit were genetically distinct from the mainland birds. The population on Cyprus (subspecies cypriotes) is a genetically and morphologically distinctive form. It probably dates back to an ancient colonization event. The genetic differences between the populations on Corsica and Sardinia (subspecies sardus) and the mainland are more subtle. These islands might have received – or are still receiving – gene flow from continental birds.
Tritsch, C., Stuckas, H., Martens, J., Pentzold, S., Kvist, L., Lo Valvo, M., Giacalone, G., Tietze, D.T., Nazarenko, A.A. & Päckert, M. (2018) Gene flow in the European coal tit, Periparus ater (Aves: Passeriformes): low among Mediterranean populations but high in a continental contact zone. Biological Journal of the Linnean Society, 124(3):319-338.
The paper has been added to the Paridae page.