Looking for Migration Genes in the Willow Warbler Genome

“Here today, up and off to somewhere else tomorrow! Travel, change, interest, excitement! The whole world before you, and a horizon that’s always changing!”

– Kenneth Grahame (Wind in the Willows)


Migration is one of the most fascinating phenomena in ornithology. How do birds know which way to fly and how do they find their way back? It is well established that migratory behavior has a strong genetic component. Surprisingly, little is known about the exact genes that underlie migration. In principle, you could identify these ‘migration genes’ by comparing the genomes of closely related bird populations that follow different migration routes. And that is exactly what Max Lundberg and his colleagues did.


Migratory Divide

In their study, published in Evolution Letters, they focus on two subspecies of the Willow Warbler (Phylloscopus trochilus) in Europe. One subspecies (P. t. trochilus) migrates to the southwest to wintering areas in West Africa, whereas the other subspecies (P. t. acredula) migrates in a southeastern direction to winter in Eastern and Southern Africa. The two populations overlap and interbreed in Sweden giving rise to a particular kind of hybrid zone, a so-called migratory divide (you can read more about this situation here).


willow warbler.jpg

A Willow Warbler (from http://www.naturespot.org.uk)


A Lot of Data

The researchers performed extensive genetic analyses. They created a de novo assembly of the Willow Warbler genome (i.e. they assembled it from scratch), re-sequenced the whole genomes of nine samples from each population, and designed a molecular marker set (comprised of 6000 Single Nucleotide Polymorphisms or SNPs) for 1152 samples. This impressive dataset revealed that … there was almost no genetic differentiation between the populations. This suggests that these subspecies are at an early stage of divergence.


Migration Genes

Nonetheless, three genomic regions – on chromosomes 1, 3 and 5 – were highly differentiated (see the figure below). The genetic markers on chromosome 3 correlated with breeding altitude and latitude, while the regions on chromosomes 1 and 5 perfectly matched the differences in migration route. Zooming in on these regions revealed several genes that are involved in the synthesis of fatty acids.  This seems logical given that long-distance migrants mostly use fat as energy. Furthermore, the subspecies differ significantly in the distance they cover during migration. The authors admit that ‘it is tempting to speculate that these differences represent adaptations in fueling to their different routes.’


The three differentiated regions (highlighted in red) in the Willow Warbler genome. The remainder of the genome is largely undifferentiated (adapted from Lundberg et al. 2017)



Lundberg M, Liedvogel M, Larson K, Sigeman H, Grahn M, Wright A, Åkesson S, Bensch S 2017. Genetic differences between willow warbler migratory phenotypes are few and cluster in large haplotype blocks. Evolution Letters 1: 155-168.   http://onlinelibrary.wiley.com/doi/10.1002/evl3.15/abstract


The paper has been added to the Phylloscopidae page.


Promiscuous Petrels: Genetic Study Reveals Gene Flow on a Global Scale

A recent study, published in Molecular Ecology, shows inter-oceanic gene flow among five petrel species on a global scale.

Petrels, the narrow-winged seabirds that graciously soar across the world’s oceans, hold a peculiar contradiction. On the one hand, they disperse on a globally exploring distant waters, while, on the other hand, they display a strong instinct to return to their birth place during the breeding season (i.e. natal philopatry). In this respect, they resemble certain scientists who disperse widely, doing postdocs across the globe, only to return to the universities where they obtained their PhDs for a tenure position.


trindade petrel

A dark Trindade Petrel (from http://www.hbw.com)


Five Species – Three Oceans

How do these contrasting behaviors in petrels translate to their genetics? To explore this question, Katherine Booth Jones and her colleagues sampled no less than 1001 petrels (genus Pterodroma) on a worldwide scale. The sampled birds represented five distinct species, distributed across three oceans (Indian, Atlantic and Pacific):

  • Trindade Petrel (P. arminjoniana)
  • Herald Petrel (P. heraldica)
  • Kerdamec Petrel (P. neglecta)
  • Murphy’s Petrel (P. ultima)
  • Phoenix Petrel (P. alba)


Inter-oceanic Gene Flow

Hybridization among petrels is best studied on the Round Island in the Indian Ocean. Here, three species – Trindade, Kerdamec and Herald Petrel – hybridize extensively (read more about this case on the Procellariiformes page). However, the genetic analysis – based on microsatellites – in the present study shows that hybridization is not limited to this island. In fact, gene flow even occurrs between populations from different oceans, in other words: inter-oceanic gene flow.

This results suggests that petrels migrate between oceans. To support this suggestion, the researchers also present tracking data from two individuals that both left the Indian Ocean. One individual traveled eastwards to the Pacific Ocean, whereas another bird explored the Atlantic Ocean in the west.



A Kerdamec Petrel (from http://www.hbw.com)


Three-way Hybrids

The genetic data also revealed possible hybrids among three different petrel species. In birds, most three-way hybrids are known from captivity (notably falcons), but there is a well-documented case in geese. Dreyen and Gustavsson (2010) report how a hybrid between Swan Goose (Anser cygnoides) and Snow Goose (A. caerulescens) paired up with a Barnacle Goose (Branta leucopsis) and produced offspring. A three-way intergeneric hybrid.


Threeway Goose Hybrid

The Swan Goose x Snow Goose hybrid (left) and its offspring with a Barnacle Goose standing on the right (from Dreyen & Gustavsson, 2010)


Think big

An important lesson to take away from this petrel study is to think beyond hybrid zones. Most studies focused on narrow contact zones between two species, which has led to important insights into speciation and hybridization. However, I think it is time to expand our view and sample widely to uncover unexpected patterns of gene flow on a global scale.



Booth Jones K.A., et al. (2017). Widespread gene flow between oceans in a pelagic seabird species complex. Molecular Ecology. 26:5716-5728. https://doi.org/10.1111/mec.14330

Dreyen P. & Gustavsson C. G. (2010). Photographic documentation of a Swan Goose x Snow Goose Anser cygnoides x Anser caerulescens hybrid and its offspring with a Barnacle Goose (Branta leucopsis) – a unique three-species cross. Ornithologischer Anzeiger. 49:41-52.


This paper has been added to the Procellariiformes page.

Throwback Thursday: Six Years with a Brewster’s Warbler.

It has been a while since I presented an old Avian Hybrids paper on Thursday. But the next paper, published in 1944 in The Auk, is a gem. It contains six years of observations and no statistics! Sounds like a great read, doesn’t it.

The hybrid in question concerns the so-called Brewster’s Warbler, a cross between Golden-winged (Vermivora chrysoptera) and Blue-winged Warbler (V. pinus). Hybrids between these species were first described as distinct species, namely Lawrence’s Warbler (V. lawrencei) and Brewster’s Warbler (V. leucobronchialis).

Here is the opening paragraph of the paper, written by T. Donald Carter.

In the ‘Auk,’ volume 40, July, 1923, R. H. Howland and I published an account of our discovery of a male Brewster’s Warbler, Vermivora pinus X chrysoptera, mated to a female Golden-winged Warbler, Vermivora chrysoptera. We located his nest and brood and later captured and banded him and three of his young. At the time of the writing we supposed that our story was closed, but it proved to be the first chapter of a six-year acquaintance with this same bird.

The remainder of the paper is a year by year description (running from 1922 to 1927) of the adventures of their Brewster’s Warbler in New Jersey. A fine piece of natural history. I could quote different sections from the paper, but I advice you to read the whole paper. Preferably close to a fire place with a hot beverage.

Click here to read the paper. If you cannot access it, feel free to contact me.


The young of a Brewster’s Warbler. Picture taken on June 10, 1922.


Carter, T. D. (1944). Six years with a Brewster’s Warbler. The Auk, 48-61.

Becoming Black: The Origins of Melanic Monarcha Flycatchers

Two populations of all black Monarcha Flycatchers might have independent origins.

In 1942, the German ornithologist Ernst Mayr published his seminal book on speciation: Systematics and the Origin of Species: from the Viewpoint of a Zoologist. In this book (which is still a nice read today), he argues that geographic isolation is the main driver of speciation. To support his claims, he discusses the subspecies of the Monarcha castaneiventris flycatcher that resides on the Solomon Islands (located east of Papua New Guinea). In this blogpost, I will focus on two subspecies: megarhynchus and ugiensis.


Monarcha distribution

Distribution of the Monarcha castaneiventris subspecies (from: Cooper & Uy 2017)


Introducing the Islands

The first subspecies (megarhynchus) is found on the large island of Makira. These birds have chestnut bellies and iridescent blue-back upper parts. The second subspecies (ugiensis) is distributed on nearby satellite islands: Ugi and Three Sisters in the north and Santa Anda and Santa Catalina in the southeast. In contrast to the birds from Makira, this subspecies is entirely blue-black. A recent study in Molecular Ecology provides a genomic perspective on the evolution of this species complex.

monarcha subspecies

Two Monarcha subspecies: the chestnut-bellied megarhynchus and the entirely blue-black ugiensis (from: http://www.hbw.com/)


Hybridization or Independent Origins?

Elizabeth Cooper and Albert Uy used over 70,000 genetic markers (SNPs; Single Nucleotide Polymorphisms) to unravel the origin of the blue-black subspecies. Surprisingly, both melanic populations, which Ernst Mayr considered as a single subspecies, do not cluster together. The birds from the southeastern islands are more closely related to the chestnut-bellied individuals from Makira. This pattern can be explained in two ways: hybridization or independent origins.

To solve this riddle, Cooper and Uy performed isolation with migration analyses. The model with the highest likelihood pointed to migration between the large island of Makira and the satellite islands, but not between the satellite islands themselves. This results weakened a role for hybridization. It thus seems that the melanic populations have independent origins. This hypothesis is supported by the observation that different mutations underlie the black plumage color in each population, as shown in a previous study by Albert Uy and colleagues.

From a taxonomic point of view, the melanic populations should be considered separate subspecies. It turns out Ernst Mayr was wrong here. And it’s not the first time he made a mistake. In his 1963 book Animal Species and Evolution, Mayr stated that “the available evidence contradicts the assumption that hybridization plays a major evolutionary role.”



Cooper, E.A. and J.A.C. Uy, Genomic evidence for convergent evolution of a key trait underlying divergence in island birds. Molecular Ecology, 2017. 26(14): p. 3760-3774.

Mayr, E., Systematics in the origin of species : from the viewpoint of a zoologist. 1942, New York: Harvard University Press.

Mayr, E., Animal species and evolution. 1963, Cambridge: Belknap Press of Harvard University Press. xiv, 797 p.

Uy, J.A.C., et al., Mutations in different pigmentation genes are associated with parallel melanism in island flycatchers. Proceedings of the Royal Society B-Biological Sciences, 2016. 283(1834): p. 20160731.


This paper has been added to the Monarchidae page.

Che Figata! New studies on the ecology and evolution of a hybrid species, the Italian Sparrow

The Italian Sparrow (Passer italiae) is a curious little bird. The plumage of this sparrow, which occurs in Italy (what a surprise!) and several Mediterranean islands, seems to be a mixture of a House Sparrow (P. domesticus) and a Spanish Sparrow (P. hispaniolensis). Biologists speculated that the Italian Sparrow is the result of hybridization between these species. A recent study in Science Advances provides a detailed look into the genome of this hybrid species.


italian sparrow

An Italian Sparrow (from http://www.gobirding.eu)


Genetic Mixture

As expected, the genome of the Italian Sparrow is a mixture of the two parental species. Some parts of the DNA resemble those of House Sparrow, while others are closer to Spanish Sparrow. In general, however, the Italian Sparrow is genetically most similar to the House Sparrow. It could be that there were just more House Sparrows around during the first wave of hybridization with Spanish Sparrows, leading to an excess of the former species DNA.

But it is also possible that House Sparrow genes provide a selective advantage. This hypothesis is supported by the similar ecology of House Sparrow and Italian Sparrow. Both species live in cities and villages where they follow a diet of grains, rice and insects. The Spanish Sparrow, which mostly occurs in dry areas, has a different diet, consisting primarily of insects. A previous study in the journal Heredity already showed that these differences in diet lead to divergence in beak morphology. The new study supports this conclusion with genetic data: genes that influence beak morphology are significantly different between Italian Sparrow and Spanish Sparrow.

An additional discovery of this genetic study was the presence of parental mitochondrial DNA (mtDNA) in some individuals. Normally, mtDNA is only passed on to the offspring through the maternal line. The mtDNA from the father is destroyed when the sperm cell reached the egg. Other studies have shown that this mechanism sometimes fails in hybrids. This might explain why some Italian Sparrows inherited mtDNA from their fathers.


spanish sparrow

A Spanish Sparrow (from http://www.hbw.com)



Another recent study, published in the Proceedings of the Royal Society B, focused on the interaction between the Italian Sparrow and one of its parents, the Spanish Sparrow. In 2013, Spanish Sparrows invaded a population of Italian Sparrows in the nature reserve station of Lago Salso (Italy). The arrival of the Spanish ‘invaders’ led to a change in feeding behavior. Before 2013, Italian Sparrows would feed on cereal fields close to the nature reserve station. By 2015, the Spanish Sparrows has almost completely monopolized this field, forcing their Italian cousins to feed in other locations. This shift in habitat use by Italian Sparrows led to a significant drop in body condition, suggesting a negative impact of competition with Spanish Sparrows.

The changes in habitat use and diet did not result in morphological changes, but were nonetheless noticeable on a genetic level. There was an clear increase in genetic divergence between the two species following the arrival of Spanish Sparrows. Interestingly, four highly divergent genes are known to be involved in cellular processes linked to learning and brain development. The precise mechanism behind these patterns remains to be investigated. But one thing is certain, the Italian Sparrow is a goldmine for geneticists!


sparrow songs from around the world.JPG



Elgvin, T. O., C. N. Trier, O. K. Tørresen, I. J. Hagen, S. Lien, A. J. Nederbragt, M. Ravinet, H. Jensen and G.-P. Sætre (2017). The genomic mosaicism of hybrid speciation. Science Advances 3(6): e1602996.

Eroukhmanoff, F., J. S. Hermansen, R. I. Bailey, S. A. Saether and G. P. Saetre (2013). Local adaptation within a hybrid species. Heredity 111(4): 286-292.

Saetre, G. P., A. Cuevas, J. S. Hermansen, T. O. Elgvin, L. P. Fernandez, S. A. Saether, C. L. Cascio Saetre and F. Eroukhmanoff (2017). Rapid polygenic response to secondary contact in a hybrid species. Proc Biol Sci 284(1853).


These papers have been added to the Passeridae page.

Splitting Buntings in the Sahara

The House Bunting complex houses two distinct species, but might have had a history of hybridization.

In taxonomy, you have lumpers and splitters. Lumpers like to keep everything together, while splitters don’t mind dividing a species into two. The Saharo-Arabian desert belt is a splitters dream. This region extends from the Sahara in North Africa over the Arabian Peninsula into Pakistan and India. Several bird species have distinct populations in the eastern and western part of this arid environment. Distinct populations? Geographically separated, you say? Let’s start splitting!

Indeed, in recent years, it has been proposed to elevate some populations to species status. For example, the Houbara Bustard (Chlamydotis undulata) and MacQueen’s Bustard (C. macqueenii), and the African Desert Warbler (Sylvia deserti) and the Asian Desert Warbler (S. nana). In a new study, Manual Schweizer and colleagues focus on another species complex in the Saharo-Arabian desert belt: the House Bunting (Emberiza striolata) complex.


house bunting

House Bunting (from http://www.hbw.com)


Recent taxonomic studies have culminated in the proposal to recognize two species in this complex: the House Bunting (E. sahari) in the west and the Striolated Bunting (E. striolata) in the east. In addition, morphologically intermediate birds have been found in Sudan and Chad. Might these be hybrids?

The east-west divergence is supported by two mitochondrial markers, suggesting that the split is justified. The intermediate birds, however, reveal a striking pattern. Morphologically, these specimens resemble the eastern species (E. striolata), but genetically they are closer to the western species (E. sahari). Incongruence between mtDNA and morphology has been reported in other studies as well and can be explained by incomplete lineage sorting or hybridization. The researchers speculate that ‘introgressive hybridization in a secondary contact zone is the most likely explanation.’ But to test this hypothesis, more genetic data is needed.

Luckily for the splitters, these results do not impinge (a wonderful word the authors used in the abstract) on the decision to recognize two species.


striolated bunting

Two Striolated Buntings (from http://www.birdforum.net)



Schweizer, M., H. Shirihai, H. Schmaljohann and G. M. Kirwan (2017). Phylogeography of the House Bunting complex: discordance between species limits and genetic markers. Journal of Ornithology: 1-15.


This paper has been added to the Emberizidae page.

More Species Than Meets the Eye: A Genetic Look at the Red-eyed Vireo Complex

The Red-eyed Vireo complex houses more than the five species that are currently recognized. And of course, there has been some hybridization.

The taxonomic world of birds is full of species complexes, groups of closely related species that are so similar in appearance that the boundaries between them are fuzzy. And often these ‘species’ are also interbreeding, rendering the situation even more complex. Some examples include Redpolls (genus Acanthis) and Bean Geese (Anser fabalis, see here). These species complexes provide great opportunities to study hybridization and speciation in birds.

In a recent study, published in Molecular Phylogenetics and Evolution, C.J. Battey and John Klicka focus on the Red-eyed Vireo (Vireo olivaceus) species complex. This group of passerine birds is currently divided into five species:

  • Red-eyed Vireo (V. olivaceus)
  • Noronha Vireo (V. gracilirostris)
  • Yellow-green Vireo (V. flavoviridis)
  • Black-whiskered Vireo (V. altiloquus)
  • Yucatan Vireo (V. magister)

The researchers obtained material of all species, except for the Noronha Vireo, a species endemic to the Noronha Island (Brazil). A DNA analysis of the other four species led to some interesting findings. Let’s have look!


Cryptic Species

The genetic data revealed that there are more species than meets the eye. The Red-eyed Vireo consists of Northern and Southern hemisphere populations, whereas the Yellow-green Vireo is structured into Eastern and Western populations. For the Red-eyed Vireo this distinction is quite obvious and there does not seem to be any gene flow between the populations. The situation for the Yellow-green Vireo, however, is less clear. It appears that this species is currently stuck in the “species/subspecies conundrum.” More data are warranted here.



Red-eyed Vireo (from: http://www.wikipedia.com/)


Ancient Gene Flow

Two species pairs show evidence for gene flow. The Northern population of the Red-eyed Vireo has exchanged genes with the Western population of the Yellow-green Vireo. And the Southern population of the Red-eyed Vireo has interbred with the Black-whiskered Vireo. The very low levels of introgression suggest that it probably concerns historic gene flow.



Yellow-green Vireo (from: http://www.hbw.com/)


Recent Hybrids?

Interestingly, populations of Red-eyed Vireo on the island of Trinidad seem to be hybrids between Red-eyed Vireo and Black-whiskered Vireo. However, more extensive sampling is needed to check whether there is a hybrid zone on this island.


Black-whiskered Vireo (from http://www.birdspix.com/)


Speciation via Migration Loss

Finally, the Yucatan Vireo might represent an example of speciation via loss of migration. This speciation model states that a group of individuals ‘decides’ to stop migrating and settle down, while the remainder of the population continues its migratory habits. Over time, the resident and migratory populations diverge until they can be considered two separate species. Two observations support this scenario for the Yucatan Vireo: (1) its ancestors are mainly migratory and (2) its current range is close to a stopover site for the Black-whiskered Vireo, which is its closest relative.


yucatan vireo

Yucatan Vireo (from http://www.hbw.com/)



Battey, C. J. and J. Klicka (2017). Cryptic speciation and gene flow in a migratory songbird Species Complex: Insights from the Red-Eyed Vireo (Vireo olivaceus). Molecular Phylogenetics and Evolution 113: 67-75


This paper has been added to the brand-new Vireonidae page

Mottled Mallards on the Western Gulf Coast

The Mallard (Anas platyrhynchos) is one of the most active species when it comes to hybridization. This familiar duck species interbreeds with numerous other duck species, including the Mottled Duck (A. fulvigula).

In January 2017, I wrote about Mottled Ducks in North America (see here). This blogpost focused on the genetic divergence between two Mottled Duck populations in Florida and the Western Gulf Coast. I briefly mentioned hybridization with Mallards.

In a recent study, published in The Condor, Robert Ford (Louisiana State University) and colleagues investigate the hybridization dynamics between these duck species in the Western Gulf Coast populations. Using microsatellites, they estimate levels of hybridization between 5 and 8%, which is lower compared to Florida (about 9%).


A pair of Mottled Ducks (from http://www.birdzilla.com)


Limited Interactions

Why are these levels of interbreeding so low? Probably because Mallards and Mottled Ducks do not meet that often. Mottled Ducks are not migratory and remain all year at the Western Gulf Coast, whearas Mallards do migrate. In addition, Mottled Ducks form pairs in fall, starting as early as August. When the Mallards arrive, most Mottled Ducks have already found a partner. But despite these limited opportunities for hybridization, hybrids do occur. This can probably be explained by such behavioural processes as interspecific nest parasitism and extra pair copulations (see here for an overview in geese). Hybridization might also be due to feral Mallards that have escaped from game breeders.


No Conservation Issue (Yet?)

For now, it is unclear whether duck hybridization in the Western Gulf Coast will be a conservation issue in the future. The most important factor seems to be habitat loss, which could drive Mottled Ducks into urban areas where feral Mallards could welcome them with open arms (or wings)…



Ford, R. J., Selman, W. & Taylor, S. S. (2017). Hybridization between Mottled Ducks (Anas fulvigula maculosa) and Mallards (A. platyrhynchos) in the western Gulf Coast region. The Condor 119, 683-696.


Thanks to Robert Ford for sending me a copy of the paper. This study has been added to the Anseriformes page.


Hybridization in Primates (and yes, that includes you!)

A recent paper in Current Opinion in Genetics and Development gives a concise overview of hybridization in primates, including humans.

Let’s start with some numbers. The genetic contribution of one primate species to the genome of another by means of ancient gene flow:

  • Tibetan Macaque genomes contain 1-8% DNA from Rhesus Macaques
  • Chimpanzee genomes harbor about 1% Bonobo DNA
  • Non-African humans have 1 to 5% of Neanderthal DNA (Africans never came into contact with Neanderthals and hence to not have their DNA)
  • Modern-day people from Oceania have 4-6% Denisovian (an extinct group of archaic humans) ancestry in their genomes

These figures clearly indicate that gene flow has occurred during primate evolution. The obvious question is: did it matter? Did ancient hybridization influence the evolutionary history of primates? For Neanderthal genes it has been shown that they contribute to depression and the immune system. For non-human primates, the question remains unanswered for now. But the advent of more genomic data hold promise to tackle this issue.


A Tibetan Macaque. Understandably irresistible to ancient Rhesus Macaques.

Hybrid Fertility

An interesting observation by the authors (Jenny Tung and Luis Barreiro) is absence of sterility or low viability of primate hybrids. Most hybrids, including those involving modern humans are alive and kicking (i.e. fertile). Some exceptions are hybrids in captivity, such as a Baboon x Rhesus Macaque cross, and hybrids between Black Howler Monkeys and Mantled Howler Monkeys in Mexico. In the latter case, only female hybrids have been reported, suggesting that male offspring are not viable. This is in line with the predictions of Haldane’s Rule.


A Black Howler Monkey. Hybrids between this species and Mantled Howler Monkeys are always female.

Evolutionary Ghosts

Another striking discovery is the existence of so-called ‘ghost lineages’. By sequencing ancient DNA researchers have found indications of gene flow in Africa between anatomically modern humans and older lineages that are now extinct. A similar patters was uncovered for African Baboons of the genus Papio. This seems to suggest that the African continent is home to several primate ghosts lineages. Scary! But seriously, this ancient DNA approach could lead to the discovery of unknown extinct species.


As if this guy isn’t scary enough with his long canines. His ancestors might have interacted with a ‘ghost lineage’.

Neanderthal DNA

The popular press has focused most on ancient gene flow between humans and Neanderthals. For some reason, intercourse between these two closely related species captivates the attention of the general public. Commercial companies that offer personal genome sequencing, such as 23andMe, even provide you with an estimate of Neanderthal ancestry in your own genome. I had my genome sequences this year. The result: 3.0% Neanderthal!

Neanderthal posing

A Neanderthal kindly posing for a picture.


Tung, J. and L. B. Barreiro (2017). The contribution of admixture to primate evolution. Current Opinion in Genetics & Development 47: 61-68.

Arctic Sharks: A Story of Ice-olation and Hybridization

Genetic study finds hybridization between Greenland Sharks and Pacific Sleeper Sharks. The researchers propose a model of ‘ice-olation with migration’.

Although this blog is about hybridization in birds, I will occasionally write about other taxonomic groups. This week, I came across a genetic shark study by Ryan Walter (California State University) and colleagues in the journal Ecology and Evolution. For several reasons (see further down), I could not resist fabricating a blog post on this shark story.


A Shark Tale

The Simniosidae is a family of sharks known as ‘sleeper sharks because of their apparent slow swimming. The recent study focuses on two species: the Greenland Shark (Somniosus microcephalus) and the Pacific Sleeper Shark (S. pacificus). The Greenland Shark ranges from the Canadian Arctic to the south of Norway, while the Pacific Sleeper Shark occurs from the Bering Street throughout the Pacific (hence the name) into the Southern Ocean. Previous studies, based on mitochondrial DNA (mtDNA) showed that both species can be distinguished from one another despite their physical resemblance.


greenland shark

A Greenland Shark (Simniosus microcephalus)


The researchers collected samples of 247 sharks across their extensive distribution. They sequences mtDNA and several nuclear markers. The genetic data confirmed previous studies; the species could be easily kept apart. However, some individuals were a genetic mixture of both species, suggesting hybridization. Further analyses (Isolation-with-Migration models for the curious readers) confirmed this suggestion. It turned out that genes have been flowing from Pacific Sleepers Sharks into Greenland Shark populations.

To explain this findings, the authors introduce a model of ‘ice-olation with migration’. Apart from being a great play of words, this model nicely places the genetic data in a climatic context. About 2.5 million years ago, a drastic reduction in global temperatures occurred. This decline resulted in sea ice formation in polar regions. The spreading of thick sea ice in combination with the submerged mountains of the Arctic probably reduced the connectivity between different shark populations. During this isolation, the shark populations diverged genetically. In periods when the climate warmed and the ice melted, the sharks re-established contact and hybridized.


seal broken

Another wordplay…


Why this blog post?

This cool story caught my eye for several reasons:

  1. Sharks! Who doesn’t like sharks? Okay, they are not birds, but they are sharks!
  2. After my PhD, I briefly worked for the science department at a Dutch newspaper (De Volkskrant). One of my most successful stories was about the Greenland Shark which can live up to 400 years! Here is the link (in Dutch).
  3. The climatic conditions that influenced the evolutionary history of these sharks is very similar to the conditions that drove the evolution of geese. Here is a quote from one of my papers on geese: “The approximate date of diversification coincides with the beginning of a period of climatic oscillations between 3.2 and 1.9 million years ago. This period was part of a fast global cooling trend, following the closure of the Panama Seaway and the uplifting of the Tibetan Plateau around four million years ago. This resulted in the formation of permanent Northern Hemisphere ice sheets, the establishment of a circumpolar tundra belt and the emergence of temperate grasslands, which opened up new ecological niches in which new groups of animals and plants were able to spread.” Do you notice the resemblance.
  4. And of course, the great wordplay in the title (Ice-olation). Priceless!



Walter, R. P., D. Roy, N. E. Hussey, B. Stelbrink, K. M. Kovacs, C. Lydersen, B. C. McMeans, J. Svavarsson, S. T. Kessel and S. Biton Porsmoguer (2017). Origins of the Greenland shark (Somniosus microcephalus): Impacts of ice‐olation and introgression. Ecology and Evolution.