The evolution of this species complex does not follow the strict definition of a ring species.
If you enjoy watching educational videos on YouTube, I can recommend the channel Crash Course. Over the years, they have produced numerous online courses, ranging from artificial intelligence to world history. Currently, conservationist and ecologist Rae Wynn-Grant is hosting an interesting course on Zoology. The latest episode on species concepts (see video below) included a discussion on ring species, using the Herring Gull complex (Larus argentatus) as an example. While watching this video, I remembered a paper in the Proceedings of the Royal Society B showing that the Herring Gull complex is not a ring species. But why?
The concept of a ring species developed from a speciation model involving isolation-by-distance in which the most distant populations differentiate despite a chain of interconnected populations that continue to exchange genes. A special case of this “speciation by distance” concerns ring species, in which the chain of populations is wrapped around a geographical barrier and the populations at the end meet without interbreeding. The Herring Gull complex was often presented as an example of a ring species around the Arctic circle.
Two Scenarios
In his book Systematics and the Origin of Species, Ernst Mayr described his version of the evolutionary history of the Herring Gull complex. He envisioned that ancestral populations in Europe extended in two directions: One group moved west across Scandinavia towards Britain and Iceland differentiating into dark-mantled lesser black-backed gulls (fuscus, intermedius and graellsii), while another group went east, giving rise to the progressively paler-mantled forms taimyrensis (Taimyr), birulai and vegae (eastern Siberia), and into North America (smithsonianus). Later on, the North American herring gulls crossed the North Atlantic and invaded Europe, giving rise to argentatus and argenteus, which now overlap with lesser black-backed gulls. This scenario depicted a circumpolar chain of populations connected by gene flow, resulting in the reproductively isolated herring gulls and the lesser black-backed gulls at the ends of the circle in Europe. A ring species. (Note that the Crash Course video does not accurately depict this scenario).
In 2004, Dorit Liebers and her colleagues tested this scenario with genetic data. Using mitochondrial DNA, they reconstructed the evolutionary history of the Herring Gull complex. Their analyses uncovered a more complicated picture compared to the ring species scenario described by Mayr. During the Ice Ages, two ancestral lineages originated in a North Atlantic refugium and a continental Eurasian refugium. From these locations, different gull populations spread across Eurasia and North America. These genetic patterns do not fit a model of speciation by distance. Indeed, the authors write that “not isolation by distance, but vicariance and subsequent range expansion […] were the processes that played the decisive role in the evolution of the herring gull complex.” The situation is even more complicated due to gene flow between several populations, as shown by a more recent genetic study.
Moreover, reproductive isolation in regions of overlap is not the outcome of isolation by distance, but is mainly due to genetic differentiation in allopatry. For example, marinus was probably isolated in northeastern North America before making secondary contact with smithsonianus in North America and with argentatus and fuscus in Europe.
Avian Examples?
So, the Herring Gull complex is not a ring species. Other avian examples of ring species have been shown to not adhere to the strict definition of a ring species (i.e. isolation by distance and reproductive isolation between the end points of the ring), such as the Great Tit (Parus major) in Eurasia and the Greenish Warbler (Phylloscopus trochiloides) in Asia. These findings suggest that ring species are probably a rare phenomenon, as nicely described in the paper on the Herring Gull complex.
In conclusion, although ring speciation is theoretically possible, the few well-studied examples suggest that it occurs infrequently, because the dynamics of species’ ranges are more likely to result in fragmentation, i.e. periods of allopatry, before the slow process of isolation by distance leads to sufficient divergence to allow for circular overlap.
References
Alcaide, M., Scordato, E. S., Price, T. D., & Irwin, D. E. (2014). Genomic divergence in a ring species complex. Nature, 511(7507), 83-85.
Kvist, L., Martens, J., Higuchi, H., Nazarenko, A. A., Valchuk, O. P., & Orell, M. (2003). Evolution and genetic structure of the great tit (Parus major) complex. Proceedings of the Royal Society of London. Series B: Biological Sciences, 270(1523), 1447-1454.
Liebers, D., De Knijff, P., & Helbig, A. J. (2004). The herring gull complex is not a ring species. Proceedings of the Royal Society of London. Series B: Biological Sciences, 271(1542), 893-901.
Martens, J., & Päckert, M. (2007). Ring species–do they exist in birds?. Zoologischer Anzeiger-A Journal of Comparative Zoology, 246(4), 315-324.
Sonsthagen, S. A., Wilson, R. E., Chesser, R. T., Pons, J. M., Crochet, P. A., Driskell, A., & Dove, C. (2016). Recurrent hybridization and recent origin obscure phylogenetic relationships within the ‘white-headed’gull (Larus sp.) complex. Molecular phylogenetics and evolution, 103, 41-54.
Featured image: Herring Gull (Larus argentatus) © Ввласенко | Wikimedia Commons
Avian Hybrids 