Newly discovered hybrid zone between Colima and Virginia’s Warbler in Texas

Another case of hybridization in Wood-warblers.

If you want to study avian hybridization, the Parulidae family (Wood-warblers) is an obvious choice. Since 1980, hybrids have been documented in 24 out of 45 species (based on a 2014 paper). A recent study in Texas adds another hybrid to the list: Colima Warbler (Oreothlypis crissalis) x Virginia’s Warbler (O. virginiae).

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A Virginia’s Warbler (from:


Surveys in the Davis Mountains

In June 1999, extensive surveys in the Davis Mountains revealed the presence of singing Colima Warblers, the first documented occurrence outside of its main breeding distribution in the Chisos Mountains. More efforts were put into locating additional territories of this species. During this search it became apparent that some Colima Warblers showed plumage characteristics and song patterns that were similar to Virginia’s Warbler. Could these be hybrids?


A Colima Warbler (from:


Intermediate Plumage and Song Patterns

Several birds were caught on camera or trapped with mist nests. Morphological analyses confirmed previous observations: they exhibited plumage patterns that were intermediate between both parental species. Overall, the body plumage is darker than the gray of a typical Virginia’s Warbler, but the yellow tail coverts and the lighter gray head and underparts lead to a more Virginia-like appearance. There is, however, considerable variation in plumage patterns, suggesting some degree of backcrossing.

The morphological evidence was corroborated with acoustic data. Between 1999 and 2006, the songs of several birds were recorded. Interestingly, putative hybrids responded more strongly to ‘hybrid songs’ compared to recordings of pure species.

The morphological and acoustic data already point to hybrids in the Davis Mountains, but genetic analyses will be necessary to characterize this newly discovered hybrid zone in more detail.

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A hybrid Virginia’s x Colima Warbler (picture courtesy of Kelly B. Bryan)


Bryan, K.B. & Lockwood, M.W. (2018) Plumage Characteristics and Song Patterns of Presumed Colima x Virginia’s Warbler Hybrids in the Davis Mountains of Texas. North American Birds, 70(2): 142-154.


This paper has been added to the Parulidae page.

Thanks to Darren Irwin for pointing this study out to me and to Kelly Bryan for sending me the PDF and pictures!


What selection pressure is acting on hybrids between Audubon’s and Myrtle Warbler?

Another attempt to characterize the selection pressure acting on warbler hybrids.

What form of selection is acting on hybrids between Audubon’s Warbler (Setophaga coronata auduboni) and Myrtle’s Warbler (S. c. coronata)? It is a question that is haunting several American ornithologists. Genetic analyses of a hybrid zone between these species indicate that there is some selection against hybrids, but the exact mechanism remains a mystery.

Some weeks ago I wrote about a paper that tested whether parasites might be involved. The answer was negative, so the search for the exact selection mechanism continues. The latest attempt was recently published in the journal The Auk.


An Audubon’s warbler (from:


Comparing Classes

David Toews and his colleagues wanted to know if there are any differences in viability between male and female hybrids or between different age classes. If they uncover differences, this might provide important insights into what kind of selection is acting on the hybrids.

Females might be less fit compared to males. This pattern – known as Haldane’s Rule – has been observed in several bird species. If Haldane’s Rule also applies to these warblers, we would expect to find more early-generation male compared to female hybrids.

There might also be differences between age classes due to variation in migration strategies. If difference species follow distinct migration routes, hybrids might opt for an intermediate – and possibly sub-optimal – route. This could increase the mortality rate among hybrids. If this is the case, we would expect more early-generation hybrids before their migration.


A female Myrtle warbler (from:



To test these two expectations, the researchers used triangle plots (I have written about these before). Here is the crystal-clear explanation from the paper:

“In these plots, individuals near the top of the triangle are first-generation hybrids (i.e. they are heterozygous at nearly all the highly divergent sites). Individuals falling along the right and left edges of the triangle are backcrosses to the parental forms; individuals within the center of the triangle are F2, F3, and subsequent hybrid classes.”


A triangle plot to deduce the genetic ancestry of hybrids (from Toews et al. 2018, The Auk)


And the selection pressure is …

These triangle plots were compared between the different classes (males vs. females and pre-migration vs. post-migration) and revealed…no differences. There seems to be no selection against a particular sex or age class. Hence, the quest for the hybrid warbler selection pressure will continue.



Toews, D.P.L., Lovette, I.J., Irwin, D.E. & Brelsford, A. (2018) Similar hybrid composition among different age and sex classes in the Myrtle-Audubon’s warbler hybrid zone. The Auk, 135: 1133-1145.


The paper has been added to the Parulidae page.

Crisscrossing Europe: The genetics of crossbills in the western Palearctic

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.


Different groups of crossbills are adapted to different species of pine (from: Price 2008).


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.”


Geographical Isolation

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.


Two crossbills on a pine tree (from:


Resource Competition

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.


Parrot Crossbill

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!


A parrot crossbill – picture by Tom Melling (from:



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


Are yellow-rumped warbler hybrids more susceptible to parasite infections?

Do parasites drive avian speciation?

Life as a hybrid is not always easy. They might be infertile or unattractive to potential partner. Or they might have a higher chance of dying. In other words, hybrids often have lower fitness compared to ‘pure’ species. But what factors determine this decrease in hybrid fitness? A recent study in the journal Ecology and Evolution focused on parasites.


One subspecies (coronata) of the Yellow-rumped Warbler complex (from:


Yellow-rumped Warblers

The Yellow-rumped Warbler (Setophaga coronata) complex comprises four subspecies (coronataauduboninigrifrons and goldmani). The first two interbreed along a narrow hybrid zone in British Columbia. Previous work indicated that there is selection against hybrids, but the exact mechanisms could not be unraveled. Camille-Sophie Cozzarolo (University of Lausanne) and her colleagues tested the hypothesis that hybrids are more susceptible to parasite infections. Specifically, they focused on haemosporidian parasites of the genera Haemoproteus, Leucocytozoon, and Plasmodium that are transmitted by dipteran flies.



On the one hand, hybrids might be an easy target for parasites because they lack the resistance that ‘pure’ species have evolved (this has been shown in black duck x mallard hybrids). On the other hand, the increased genetic diversity in hybrid genomes might confer an advantage when they get infected. In the present study, the authors expected that hybrids between coronata and auduboni will be infected more. In this way, parasites are (partly) driving the speciation process.


The other subspecies (auduboni) of the Yellow-rumped Warbler complex (from:



Contrary to their expectations, the researchers found no support for their parasite-driven speciation hypothesis. Hybrids did not have a higher infection prevalence. Instead elevation was the most important predictor of prevalence. This is probably because the vectors of certain parasites (particularly Leucocytozoon) thrive at higher elevations where they find suitable water bodies for reproduction.

In the case of Haemoproteus parasites, there was an effect of hybrid index (i.e. the genetic background of the birds). The probability of infection in coronata strongly increased with elevation. Possibly, coronata individuals do not cope well with high elevations, making them more vulnerable for infection.


Black flies (familu Simuliidae) are the main vectors of Leucocytozoon parasites (from:


Publication Bias

So, it seems unlikely that haemosporidian parasites play a key role in selecting against coronata auduboni hybrids. Although the outcome of this study is negative (it would have been very cool if parasites are driving bird speciation), it is important that this paper has been published. Only reporting positive results will lead to a publication bias and a skewed perspective on the role of parasites in bird evolution.



Cozzarolo, C.S., Jenkins, T., Toews, D.P.L., Brelsford, A. & Christe, P. (2018) Prevalence and diversity of haemosporidian parasites in the yellow-rumped warbler hybrid zone. Ecology and Evolution


This paper has been added to the Parulidae page.

Mixing in the Marshes: King rail and clapper rail hybridize in Virginia

Cryptic introgression between two rail species in Virginia.

Distinguishing between king rail (Rallus elegans) and clapper rail (R. crepitans) is challenging, to say the least. These secretive birds have similar diets, calls and morphology. In general, however, king rails are slightly larger and have a more deep rust-colored plumage compared to clapper rails. Despite their morphological differences, genetic studies indicated that these rails do represent distinct species.

king rail.jpg

A king rail (from: © Luke Seitz



King and clapper rail prefer different kinds of wetland. King rails are mostly found in a diverse range of habitats from freshwater to brackish marshes, whereas clapper rails live exclusively in brackish and saltwater marshes. But where they overlap, they interbreed (as exemplified by the picture below showing a mixed pair). Stephanie Coster (West Virginia University) and her colleagues wanted to know if occasional hybridization results in gene flow between these species. Their results recently appeared in the journal Ecology and Evolution.

The researchers sampled birds across a salinity gradient on the east coast of the US. The genetic analyses – based on mtDNA and 13 nuclear markers (SNPs) – revealed several admixed individuals in Virginia. Most of these birds were backcrosses to clapper rail, indicating that hybrids are fertile.


A mixed pair of clapper rail (left) and king rail. Picture taken by Robert Ostrowski (from: Coster et al., 2018)


Wave-front Dynamics

The uncovered genetic patterns fit an evolutionary history proposed by Storrs L. Olson in 1997. He envisioned that an ancestral population of king rails was adapted to freshwater. Birds at the periphery became isolated due to rising sea levels and adapted to salty environments. These birds would evolve into clapper rails. Later on, this new lineage expanded into brackish habitats, thereby displacing the resident king rails.

Initially, the expanding clapper rails are outnumbered by king rails, leading to hybridization. As the expansion proceeds, king rails and previously produced hybrids are engulfed by clapper rails, thereby overturning the numerical imbalance. Consequently, hybrids have a higher chance of backcrossing with clapper tails, resulting in a genetic wake of introgressed genes following the wave-front of the expanding clapper rails.


A clapper rail (from:



Coster, S.S., Welsh, A.B., Costanzo, G., Harding, S.R., Anderson, J.T., McRae, S.B. & Katzner, T.E. (2018) Genetic analyses reveal cryptic introgression in secretive marsh bird populations. Ecology and Evolution.


The paper has been added to the Gruiformes page.

Looking for genetic differences between Araripe and helmeted manakins in Brazil

Is it possible to detect genetic differences between Antilophia manakins?

One of the outstanding questions in biology is the relationship between genotype and phenotype: how does the genetic make-up of an individual translate into physical characteristics? This question becomes even more daunting when comparing different species. Some species pairs are genetically distinct but look almost exactly the same (so-called cryptic species), whereas other species pairs are genetically nearly identical but look totally different. A recent study in Molecular Ecology explores the latter situation in Antilophia manakins.


Black and White

In Brazil, you can find two species of manakin that look quite distinct. Males of the Araripe manakin (A. bokermanni) are white, whereas the plumage of Helmeted manakin males (A. galeata) is black. Despite the obvious color differences, genetic studies – using mtDNA and introns – could not detect any population structure. This finding could be explained by (1) recent divergence with gene flow or (2) lack of statistical power of these studies.

araripe manakin.jpg

The white Araripe manakin (Antilophia bokermanni) – from:


Conservative Measures

To differentiate between these two possibilities, Fabio Raposo do Amaral and his colleagues focused on another molecular marker: ultraconserved elements (UCEs). Analyses of these genomic regions revealed clear differentiation between Araripe and helmeted manakin. It thus seems that the statistical power of the previous studies was just too low to pick up any signal of population structure. In addition, demographic modelling indicated that there has been no (or very little) gene flow between these two species.

helmeted manakin.jpg

The black helmeted manakin (Antilophia galeata) – from:


Male Plumage

Araripe and helmeted manakin are genetically distinct, but how did the males of these species evolve different plumage patterns? Perhaps these colors have been under strong sexual selection, with Araripe females preferring white and helmeted females preferring black males. Or maybe the white plumage of Araripe males simply increased in frequency due to genetic drift in a small isolated population.

To answer these questions, a genomic approach might be warranted. Indeed, the authors write that “resequencing of their complete genomes will offer exciting opportunities to identify functionally important regions evolving under selection, which may include candidate genes related to the plumage differences between those two species.”



Amaral, F.A., Coelho, M.M., Aleixo, A., Luna, L.W., Rego, P.S., Araripe, J., Souza, T.O., Silva, W.A.G. & Thom, G. (2018) Recent chapters of Neotropical history overlooked in phylogeography: shallow divergence explains phenotype and genotype uncoupling in Antilophia manakins. Molecular Ecology.

Salty Genes: Hybridization between saltmarsh and Nelson’s sparrow results in exchange of adaptive genes

Genomic study uncovers candidate genes for adaptive introgression in saltmarsh and Nelson’s sparrow.

Hybridization can work as an evolutionary stimulus. For example by transferring beneficial genetic variants from one species to another. This process, adaptive introgression, has been described for numerous taxa, such as butterflies and snowshoe hares. However, examples in birds are quite rare. A recent study in the journal Evolution provides evidence for adaptive introgression between two sparrow species.


Hybrid Zone

Jennifer Walsh (Cornell University) has published a nice series of papers on between saltmarsh sparrow (Ammodramus caudacutus) and Nelson’s sparrow (A. nelsoni) that hybridize along the coast of New England (see the Emberizidae page for an overview). The saltmarsh sparrow is a specialist of – you guessed it – saltmarshes, whereas Nelson’s sparrow has a broader ecological niche that includes grasslands and brackish marshes. Using microsatellites, she was able to document gene flow between these species. But could this exchange of genetic material be adaptive? Time to bring out the big guns: genome sequences!

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A saltmarsh sparrow (from:


Salty Genes

By comparing the genomes of 36 individuals, Jennifer and her colleagues were able to pinpoint several genomic regions that have been exchanged between the sparrows. Further exploration of these regions uncovered several genes that could be important for life in the saltmarshes.

[Sixteen] of these 24 putative candidate genes confer potential adaptations to tidal marsh environments, including genes with links to osmotic regulation, response to salt stress, response to water deprivation, and muscle development. The remaining 8 candidates include additional regions, including four genes related to DNA repair and several genes with a range of putative adaptive functions (visual perception, response to pH).

It can be tempting to tell an adaptive story for each gene, but it is important to keep in mind that these are “candidate genes” for adaptation. They should be the starting point for further research, not the final answer. So, you can expect more sparrow studies in the near future.



Walsh, J., Kovach, A.I., Olsen, B.J., Shriver, W.G. & Lovette, I.J. (2018) Bidirectional adaptive introgression between two ecologically divergent sparrow species. Evolution


This paper has been added to the Emberizidae page.


Where did all these fish come from?! A genomic perspective on the explosive diversification of cichlids

Cichlids provide important insights into the drivers of explosive speciation.

There are more than 1700 species of cichlid fishes, 90% of which can be found in the Great Lakes of East Africa. Since the 19th century, biologists have been visiting the Lakes Victoria, Malawi and Tanganyika to explore the diversity of cichlid species. In 1931, Woltereck introduced the term ‘Artexplosion’ (now known as explosive speciation) to describe the situation. In a recent review paper, Walter Salzburger (University of Basel) puts the cichlid explosion into perspective:

To put cichlid radiations into a temporal context, during the evolutionary time span of our own species, starting with the split between chimpanzees and humans some 5–7 million years ago, approximately 2,000 species of cichlid fish evolved in East Africa, the geographic region where the chimpanzee–human split initially occurred. Within the time span that it took for 14 species of Darwin’s finches to evolve on the Galapagos archipelago, about 1,000 cichlid species evolved in Lake Malawi alone. In addition, since the last ice age, which is when sticklebacks began to diverge into replicate species pairs in the Northern hemisphere, hundreds of cichlid species evolved in Lake Victoria.


It’s complicated

To sum up the introductory paragraph of this blog post: that is a lot of fish in a short amount of time. Reconstructing the evolutionary history of such rapidly diversifying species groups is challenging. There are multiple biological processes that hamper the construction of phylogenetic trees. Figure 3 in Salzburgers review paper provides a nice overview of these processes (honestly, one of the best pictures I have come across). The most important ones to keep in mind are incomplete lineage sorting (d) and introgression (e). Incomplete lineage sorting occurs when lineages fail to coalesce in the ancestral population (I have explained this process in more detail previously), while introgression refers to the exchange of genetic material by means of hybridization and backcrossing (see practically all posts on this website…).

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Several biological processes that can complicate the estimation of phylogenetic trees (from: Salzburger 2018 Nature Reviews Genetics)


Untangling Tanganyika

Because of these processes, it has been difficult to produce a completely resolved phylogenetic tree of cichlid fish. However, Iker Isarri, Pooja Singh and colleagues managed to disentangle the relationships between all cichlid species of Lake Tanganyika. In addition, they uncovered introgression that the base of this explosive radiation, suggesting that ‘hybridization might have facilitated these speciation bursts.’

This suggestion was supported by further analyses, revealing that several genes related to key innovations in cichlids, such as dietary adaptations and color vision, showed signs of positive selection and introgression. This is nicely illustrated by the opsin-genes, which shape the visual system of these fish. In cichlids that feed on plankton the UV-sensitive opsin sws1 shows faster evolution, facilitating foraging because plankton is more visible under UV-light. Fish that graze on green algae, on the other hand, show faster evolution in the green-sensitive opsin rh2α-α.

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The resolved cichlid phylogeny. Check out the original paper for a higher quality image: Irisarri et al. (2018) Nature Communications


Multiple Opportunities

So, does that answer our question? Did we get all these fish because of introgressive hybridization? It is definitely one of the factors that has fueled cichlid speciation, but other factors also need to be taken into account, such as ecological opportunity: the African Great Lakes are conducive for diversification by providing a plenitude of ecological niches to fill. Genomic analyses of several cichlids also revealed that the genomes of these fish have certain features that could potentially facilitate explosive speciation (I cannot go into great detail here, but I have provided links to relevant literature for interested readers):

  1. Accumulation of standing genetic variation before the radiation (partly supplied by introgressive hybridization)
  2. Increased rate of gene duplications which speeds up the process of genes acquiring new functions (i.e. neo-functionalization)
  3. Greater dynamics in gene regulatory processes compared to other fish species (check out this review if you want to know more about gene regulation and speciation).
  4. Elevated levels of coding sequence evolution (see examples of opsin-genes above)
  5. Three waves of transposable element expansion that might have sped up cichlid evolution (see these recent reviews the role of transposable elements in speciation and adaptation)

Clearly, the explosive diversification of cichlids is a complex interplay of several factors. Disentangling these processes – and possibly discovering new ones – will likely fuel the research community for some time.



Irisarri, I., Singh, P., Kolbmuller, S., Torres-Dowdall, J., Henning, F., Franchini, P., Fischer, C., Lemmon, A.R., Lemmon, E.M., Thalinger, G.G., Sturmbauer, C. & Meyer, A. (2018) Phylogenomics uncovers early hybridization and adaptive loci shaping the radiation of Lake Tanganyika cichlid fishes. Nature Communications 9, 3159.

Salzburger, W. (2018) Understanding explosive diversification through cichlid fish genomics. Nature Reviews Genetics,



The eagle has landed: Tracking the migration of hybrids between lesser and greater spotted eagles

Following hybrid eagles to their wintering grounds in Africa.

Bird migration is on of the big mysteries in biology. How do birds know where to fly to each winter? For small songbirds, a genetic basis for migration strategies has been uncovered, perhaps regulated by a so-called ‘migratory gene package‘. In larger birds that travel in flocks, such as geese and cranes, social factors are probably more important as birds learn the migration routes from their parents and relatives.


Hybrid Migration

But how to disentangle genes from culture? To answer this nature-vs-nurture question on bird migration, you can turn to hybrids. One of the first studies to use this strategy focused on European blackcap warblers (Sylvia atricapilla). These small passerines migrate either southwest or southeast. Hybrids between birds that use different migratory strategies direct their migration intermediate, namely south. Similar studies have been conducted on other bird species, such as Swainson’s trush (Catharus ustulatus) and willow warbler (Phylloscopus trochilus).

lesser spotted eagle.jpg

A lesser spotted eagle (from: http:/


Tracking Eagles

But what about birds of prey? Researchers have suggested that these big soaring birds learn their migration routes although they usually migrate alone. As the work on passerines indicates, just check the migration of hybrids. And that is exactly what Ülo Väli and his colleagues did: they tracked the migration routes of 62 lesser spotted eagles (A. pomarina), greater spotted eagles (A. clanga) and their hybrids.

These two raptors – that breed in eastern Europe – have very different migration routes. Greater spotted eagles migrate over short distances to winter in southern Eurasia and northeast Africa. The lesser spotted eagle, on the other hand, is a long distance migrant that flies all the way to southern Africa.


A greater spotted eagle (from:


Mixed Strategies

The results, based on GPS-telemetry, indicated that the timing of the hybrids was similar to lesser spotted eagles while the wintering destinations were similar to greater spotted eagles. The map below shows the migration routes of lesser spotted eagles (blue), greater spotted eagles (yellow) and their hybrids (red). These mixed patterns suggests that there is some genetic influence on the migration strategy of these eagles. But this doesn’t mean that it is all genetics, the researchers write: ‘these results suggest a strong genetic influence on migration strategy via a trait-dependent dominance effect, although we cannot rule out the contribution of social interactions.’

migration eagles

the migration routes of lesser spotted eagles (blue), greater spotted eagles (yellow) and hybrids (red). From: Väli et al. (2018).



Väli, U., Mirski, P., Sellis, U., Dagys, M. & Maciorowski, G. (2018) Genetic determination of migration strategies in large soaring birds: evidence from hybrid eagles. Proceedings of the Royal Society B 285,20180855.


This paper has been added to the Accipitriformes page.

Wall lizards form hybrid swarms in German cities

Genetic study reveals extensive mixing of wall lizard lineages in German cities.

Lately, I have been writing quite a lot about hybridization in South America, featuring among others jacamars, siskins, and ruddy ducks. But you don’t need to venture into the dense jungle of Colombia or Bolivia to see hybrids. Sometimes you can find them right under your nose, in the city for example. A recent study in Proceedings of the Royal Society explored hybrid lizards in German cities.


Wall Lizards

Apart from pigeons and jackdaws, you can also find reptiles in cities, such as the common wall lizard (Podarcis muralis). Joscha Beninde and his colleagues collected no less than 826 of these little critters in four German cities: Trier, Saarbrucken, Freiburg and Mannheim. They genotyped all of these individuals using a mitochondrial marker (cytochrome b) and 17 microsatellites.


A picture of a wall lizard that I took myself in 2014 during a conference on Speciation Genomics in Fribourg (Switserland).


Hybrid Swarms

The wall lizard comprises a number of distinct genetic lineages that originated from multiple regions in the Mediterranean and spread across Europe. The researchers wanted to know how many lineages can be found in each of the cities and if individuals from different lineages are interbreeding.

Each city houses a native lineage (the so-called ‘Eastern France’ lineage), but in several cities you can also find some non-native lineages. In Mannheim, for instance, the researchers found representatives of the Southern Alps and the Venetian lineages. More detailed genetic analyses revealed that these lineages are mixing, giving rise to hybrid swarms (i.e. a population comprised of two or more genetic lineages).



The researchers also applied some landscape genetic analyses, to explore how cityscape structures influence patterns of gene flow. Water bodies turned out to be strong barriers, whereas railway tracks are conducive to gene flow. Surprising, only the genes of the admixed populations flow along railway tracks. It thus seems that non-native lizards are spreading by railway, not by taking the train but by travelling along the railway enbankments.

It will be interesting to see how the genetic make-up of lizards in these cities will develop. Indeed, the authors conclude that: ‘cities are likely to become major playgrounds for hybridization.’


The ideal habitat for non-native wall lizards?



Beninde, J., Feldmeier, S., Veith, M. & Hochkirch, A. (2018) Admixture of hybrid swarms of native and introduced lizards in cities is determined by the cityscape structure and invasion history. Proceedings of the Royal Society B 285, 20180143.