Mitochondrial introgression from Red-billed into Black-billed Gulls

But no signs of nuclear introgression (yet).

A few years ago, I wrote a blog post about the genetic population structure of Black-billed Gulls (Chroicocephalus bulleri) in New Zealand. Analyses of the mitochondrial DNA (mtDNA) revealed two major groups, of which one clustered with the Red-billed Gull (C. novaehollandiae scopulinus). This pattern suggests hybridization, but can also be explained by ancestral variation that has not been sorted into both species (i.e. incomplete lineage sorting, see this blog post for a detailed explanation). In other words, these gull species might share mitochondrial variants that were present in their common ancestor.

A recent study in the journal Ibis revisited this conundrum and investigated the genetic make-up of Black-billed and Red-billed Gulls with mitochondrial and nuclear markers. Are we dealing with introgression or incomplete lineage sorting?

Dilution Effect?

When Andrew Given and his colleagues inspected the genetic make-up of 26 Black-billed Gulls, they found six individuals with mtDNA from Red-billed Gulls. In contrast, all Red-billed Gulls possessed mtDNA from their own species. This pattern argues against incomplete lineage sorting where you would expect shared mitochondrial variants in both species. Moreover, the comparison of different demographic models provided statistical support for a model of strict isolation followed by secondary contact. Hybridization is thus the most likely explanation.

Interestingly, there were no signs of introgression in the nuclear DNA (based on six microsatellites). The researchers suggest that extensive backcrossing with Black-billed Gulls has diluted the introgression signature in the nuclear DNA. However, it is also possible that the microsatellites – which only cover a small section of the genome – were not powerful enough to pick up subtle signals of past gene flow (see for example this blog post). A genomic analysis is thus warranted here.

A phylogenetic tree of mitochondrial sequences shows a clear split between both species. However, some Black-billed Gulls carry mtDNA from Red-billed Gulls (highlighted in grey boxes). From: Given et al. (2023).

Desperate Females

The observation that only Black-billed Gulls have acquired foreign mtDNA can tell us something about the behavior of these birds. As you probably know, mtDNA is transferred through the maternal line. Hence, hybridization mainly occurred between female Red-billed Gulls (which supply the mtDNA) and male Black-billed Gulls. This mating pattern could be explained by the female-biased sex ratio in colonies of Red-billed Gulls. Males fledglings and adults show lower survival rates, resulting in an imbalanced sex ratio. Because there are insufficient males for Red-billed Gull females, they turn to Black-billed Gulls. A nice example of Hubbs Principle, which I covered in another blog post.

Despite clear genetic evidence for introgressive hybridization, field observations of hybrids are rare. During more than 50 years of monitoring, the authors found no mixed pairs in the large Kaikoura colony of Red-billed Gulls. And only five cases of hybridization have been reported in literature (see this paper). Nonetheless, you only need a few hybridization events for the exchange of genetic material between species. Or the transfer of mtDNA happened in the distant past. A genomic analysis might provide the answer.


Given, A. D., Mills, J. A., Momigliano, P., & Baker, A. J. (2023). Molecular evidence for introgressive hybridization in New Zealand masked gulls. Ibis165(1), 248-269.

Featured image: Black-billed Gull (Chroicocephalus bulleri) © Paul Davey | Wikimedia Commons

The Herring Gull complex is not a ring species

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.

Two hypotheses about the differentiation and colonization history of the herring gull complex based on (a) Mayr (1942) and (b) Liebers et al. (2004). Current ranges are shown in green (from an Atlantic refugium, green circle) and brown (Aralo-Caspian refugium, brown circle). The checkerboard pattern indicates areas of overlap.

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.


Alcaide, M., Scordato, E. S., Price, T. D., & Irwin, D. E. (2014). Genomic divergence in a ring species complex. Nature511(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 Sciences270(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 Sciences271(1542), 893-901.

Martens, J., & Päckert, M. (2007). Ring species–do they exist in birds?. Zoologischer Anzeiger-A Journal of Comparative Zoology246(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

Black-and-white birds: The influence of climatic conditions on gull plumage

Thermoregulation and protection against solar radiation determine black plumage patterns in gulls.

When you pay close attention to the distribution of animals across the globe, some interesting patterns emerge. For example, some animals tend to be darker in warm and humid areas. This pattern – known as Gloger’s Rule – has been described for several animal groups, but the underlying mechanisms are still a matter of debate. Proposed explanations include camouflage, protection against parasites and dealing with solar radiation (recently reviewed by Delhey 2019). A related pattern is Bogert’s Rule which states that darker animals occur in colder regions because dark coloration absorbs more solar radiation and thus ensures proper thermoregulation. In both cases, we have clear predictions that can be tested with climatic data on solar radiation, temperature and precipitation. All we need is a group of animals that shows variation in dark plumage patterns…

A recent study in the journal Global Ecology and Biogeography found the ideal study system for this challenge: gulls. In these black-and-white birds, the researchers decided to focus on the mantle color and the proportion of black on the wing tips. These traits are sexually monochromatic (i.e. males and females look alike), suggesting that there is little sexual selection for certain plumage patterns that could complicate the analyses. They quantified these plumage colors for 80 subspecies (representing 52 species) and correlated them with several climatic variables. Did the results follow Gloger’s Rule of Bogert’s Rule?

An overview of the dataset on gull plumage patterns. The researchers focused on mantle color (KGS) and proportion of black on the wingtips (PB). From: Dufour et al. (2020) Global Ecology and Biogeography

Thermoregulation and Solar Radiation

Statistical analyses revealed that climatic conditions experienced during the non-breeding season had a stronger effect than those of the breeding season. Let’s start with the mantle color. It turned out that darker mantle coloration was negatively correlated with air temperature, and positively with solar radiation (see figures below). These findings can be explained by the fact that darker-mantle species winter in colder regions and experience more solar radiation compared to lighter species. Their black plumage allows the gulls in these regions to retain heat in freezing conditions (i.e. black objects trap heat better). And the dark plumage protects against solar radiation because melanin pigments increase the resistance of feathers against harmful UV radiation.

Similarly, the proportion of black color on the wingtips was mostly influenced by the positive interaction between solar radiation and migration distance. In other words, gulls migrating over long distances and overwintering in areas with high level of solar radiation have more black on wing tips, whereas sedentary species in areas of low solar radiation have less black feathers on their wingtips. The researchers explain these results are follows: “Species with a high proportion of black are thus more likely to tolerate long migration distances, which may cause mechanical damage, and more likely to spend the winter in highly insolated conditions where UV radiation can also damage their plumage.”

The statistical analyses revealed a positive relationship between darker mantle color and solar radiatin (figure a) and a negative relationship with temperature (figure b). The black lines between the dots represent phylogenetic relationships. From: Dufour et al. (2020) Global Ecology and Biogeography

Immature Birds

In summary, the black coloration of gull plumage plays a crucial role in both thermoregulation and protection against solar radiation. The reported patterns are in line with Bogert’s Rule which predicted darker animals in colder regions. In contrast, there was little support for Gloger’s Rule (darker animals in warm and humid areas) because there was no clear relationship with precipitation. The present study focused on plumage patterns of adult birds. Hence, these results remain to be confirmed with immature plumage patterns. But as any birdwatcher knows, immature gull plumage is more complex (just open any field guide to experience the mindboggling variation) and will be more daunting to analyze. I wish the researchers that will pick up this challenge the best of luck.


Dufour, P., Guerra Carande, J., Renaud, J., Renoult, J. P., Lavergne, S., & Crochet, P. A. (2020). Plumage colouration in gulls responds to their non‐breeding climatic niche. Global Ecology and Biogeography29(10), 1704-1715.

Featured image: Great Black-backed Gull (Larus marinus) © Ken Billington | Wikimedia Commons