Coloring outside species boundaries: A ‘super’-cool example of adaptive introgression in butterflies

Although the main topic on this blog is avian hybridization, I occasionally venture into the world of other creatures (see for example here and here). Surprisingly, I have not covered hybridization in butterflies yet. But a recent paper in Current Biology is just too “super” to ignore.


Wondrous Wing-patterns

The butterfly Heliconius numata flutters around Amazonian forests in a variety of color morphs. Each wing-pattern closely matches the colors and shapes of other toxic butterflies. A textbook example of Batesian mimicry. But what is the genetic basis of this color palette? Previous work showed that the wing-pattern morphs are associated with large chromosomal rearrangements. The reshuffling of the genome has brought together several genetic loci involved in the build-up of color patterns, culminating in an inversion that can considered a supergene.



Just one of the many color morphs of Heliconius numata (from


Helpful Hybridization

Paul Jay and his colleagues reconstructed the evolutionary history of this supergene. First, they looked for this particular inversion in several closely related species. They found it only in H. pardalinus. Interestingly, numata and pardalinus are not sister species. How did they come to share the same inversion? Could it be (wait for it…) introgressive hybridization?!

Further analyses confirmed this suspicion. The inversion probably introgressed from pardalinus into numata about 2.3 million years ago. Quite a long time after these species diverged around 3.5 million years ago. After the introgression event, additional rearrangements in adjacent positions contributed to the variety of color morphs we see today. This study highlights the potential evolutionary significance of introgression. The authors conclude:

Beyond suggesting a mechanism for supergene evolution, these findings demonstrate how introgression, when involving structural variants, can trigger the emergence of novel genetic architectures.



The inversion (P1 – the purple boxes) was found in H. pardalinus (blue) and H. numata (red). Further analyses revealed hybridization was involved (from Jay et al. 2018 Current Biology)


A Major Evolutionary Force

Ernst Mayr once wrote that ‘the available evidence contradicts the assumption
that hybridization plays a major evolutionary role.’ I guess it is safe to say he was wrong. Hybridization is an important player that evolutionary biologists need to take into account.



Jay, P., Whibley, A., Frezal, L., Rodriguez de Cara, M.A., Nowell, R.W., Mallet, J., Dasmahapatra, K.K. & Joron, M. (2018) Supergene Evolution Triggered by the Introgression of a Chromosomal Inversion. Current Biology, 28:1-7.

Why Nightingales Don’t Mix: Interspecific Competition and Sterile Females

Recent studies illuminate the nature of reproductive isolation between two Nightingale species.

‘There is no competition of sounds between a nightingale and a violin,’ wrote the Serbian poet Dejan Stojanovic. But there is competition between the Common Nightingale (Luscinia megarhynchos) and the Thrush Nightingale (L. luscinia). Two new studies reveal how these species can live side-by-side despite hybridizing.


Secondary Contact

About 1.8 million years ago the two Nightingale species diverged, only to come into secondary contact later on. At the moment, these birds interbreed across a hybrid in Central and Eastern Europe. Where their ranges overlap, they are distributed in a mosaic fashion, resulting in allotopic sites where only one species occurs and syntopic sites where both species can be observed.


common nightingale

A Common Nightingale (from


Habitat Segregation

Camille Sottas and her colleagues took advantage of this situation to see how the mosaic distribution affects the Nightingales ecology. The researchers showed that habitat use differs between the species in the allotopic sites: Common Nightingales are found more in drier areas at a slightly higher elevation. There is no difference in syntopic sites. In addition, bill size was more divergent between Common Nightingales and Thrush Nightingales in allotopic sites compared to syntopic sites.

These results suggest that competition is driving these species into different habitats. Furtermore, the habitat-specific food supply is probably leading to divergence in beak morphology. This could enhance reproductive isolation between these species: when you don’t meet each other, you are less likely to interbreed.



A Thrush Nightingale (from


Sterile Females

Despite this developing habitat segregation, Common and Thrush Nightingale do occasionally hybridize. Previous work has shown that females are sterile. But why is this? What genetic mechanism underlies this sterility? To figure this out, Libor Morkovsky and his colleagues compared the genomes of both species. They looked for regions of high differentiation and high divergence.


Some Population Genetics

Wait a minute, the observant reader might say, what is the difference between differentiation and divergence? I am glad you asked. Population geneticists calculate differentiation using a statistic called Fst. This measure compares the amount genetic variation within a population with the variation between subpopulations. The outcome is an indication of population structure. If Fst is close to zero, the subpopulations are freely interbreeding. But if Fst is closer to one, the subpopulations do not share much genetic diversity.

If you calculate Fst for different parts of the genome, you get an idea of regions that are similar (low Fst) and different (high Fst) between species. Regions of high differentiation could be the result of selection or low levels of gene flow. Unfortunately, Fst cannot distinguish between these two processes. That is where divergence comes in, measured using Dxy. This statistic just calculates the number of pairwise differences between two DNA sequences. In other words, absolute sequence divergence. Dxy is expected to be higher in genomic regions with low gene flow.


Mistakes in Meiosis

Calculating these statistics for the Nightingale genomes uncovered many differentiated regions. Within these regions, the researchers found several genes related to female meiosis, an important process in the production of eggs. Previous work has shown that female hybrids do not lay eggs, suggesting that egg production is hampered by incompatible genes in the differentiated regions. Compare this situation to trying to put two pieces from different puzzles together. No matter how much you press, they won’t fit. Something similar happens when you try to combine divergent genomic regions from different species in a hybrid female. It just won’t click.




Sottas, C., Reif, J., Kuczynski, L. & Reifova, R. (2018) Interspecific competition promotes habitat and morphological divergence in a secondary contact zone between two hybridizing songbirds. Journal of Evolutionary Biology.

Morkovsky, L., Janousek, V., Reif, J., Ridl, J., Paces, J., Choleva, L., Janko, K., Nachman, M.W. & Reifova, R. (2018) Genomic islands of differentiation in two songbird species reveal candidate genes for hybrid female sterility. Molecular Ecology, 27:949-958.

These papers have been added to the Muscicapidae page.

What keeps bird species distinct in Amazonian hybrid zones?

A study on Woodcreepers and Antbirds tries to answer this question.

A few years ago, Jason Weir and colleagues published a paper in the journal Evolution comparing seven bird species pairs that meet at Amazonian headwaters (see a short blog post about it here). They clearly showed that these species pairs are hybridizing, but what prevents these pairs from merging into one? To figure this out, they focus on two specific cases in a new paper:

  • Elegant (Xiphorhynchus elegans) and Spix’s (X. spixii) Woodcreepers
  • Scale-backed (Willisornis poecilonotus) and Xingu (W. vidua) Antbirds



A Elegant Woodcreeper (from


Reproductive Isolation Mechanisms

Before we continue our expedition into the Amazonian forest, I will need to introduce some reproductive isolation mechanisms. Biologists discriminate between prezygotic and postzygotic barriers. This distinction refers to processes that work before (prezygotic) or after (postzygotic) fertilization.

Prezygotic barriers can be behavioral. Members from different species don’t see each other as potential mates because they look or sound too different. When behavioral isolation is imperfect and copulation does occur, fertilization might still fail. Perhaps the key does not fit the lock. Or maybe the sperm and egg are incompatible.


An overview of different prezygotic isolation mechanisms (from my PhD thesis)

Postzygotic isolation mechanisms act after fertilization and can be either intrinsic or extrinsic. Intrinsic mechanisms lead to sterility or inviability of the offspring, while extrinsic  mechanisms encompass lower fitness of the offspring for ecological or behavioral reasons, not developmental defects.

For more information about these mechanisms you can check chapter 2 from my PhD dissertation (see here).



In birds, prezygotic and extrinsic postzygotic isolation are generally seen as the most important species barriers. Does this also hold for Amazonian species? To test this, Paola Pulido-Santacruz, Alexandre Aleixo and Jason Weir looked into the DNA of birds from the hybrid zones. Based on two statistics – heterozygosity and hybrid index – they constructed triangle plots. Below you can see the result. Pure individuals are located in the lower corners, while first generation hybrids are at the top. The sides of the triangles (D1 and D2) indicate backcrosses.

triangle plots

Triangle plots based on genetic data. Pure individuals are in the lower corners, while hybrids are at the top. Backcrosses line up along the sides. (from Pulido-Santacruz et al. 2018)

Next, the researchers simulated data under different scenarios of pre- and postzygotic isolation. Comparing the simulations with the actual data suggested that reproductive isolation in these hybrid zones is mainly maintained by postzygotic isolation mechanisms.


Amazonian Hybrid Zones

In contrast to hybrid zones in North America and Europe, it seems that postzygotic isolation is more important than prezygotic isolation in the Amazonian forest. However, the species pairs in this study are already quite old. Elegant and Spix’s Woodcreeper diverged about 2.5 million years ago. Scale-backed and Xingu Antbird are even older, having split ways around 4 million years ago. It remains to be tested if the same patterns hold for younger species pairs.



A Scale-backed Antbird (from:



Pulido-Santacruz, P., Aleixo, A. & Weir, J.T. (2018) Morphologically cryptic Amazonian bird species pairs exhibit strong postzygotic reproductive isolation. Proceedings of the Royal Society B, 285:20172081.

Weir, J. T., Faccio, M. S., Pulido-Santacruz, P., Barrera-Guzman, A. O. & Aleixo, A. (2015). Hybridization in headwater regions, and the role of rivers as drivers of speciation in Amazonian birds. Evolution 69, 1823-1834.


This paper has been added to the Furnariidae and Thamnophilidae pages.

A Dutch mystery: Can you guess which duck species have hybridized?

A hybrid duck on Schiermonnikoog requires expert identification skills.

Hybrid ducks are common. But when you encounter one in the wild, it can be tricky to figure out which species have intermingled. A recent paper in Dutch Birding describes the quest of several Dutch birders to identify a hybrid duck on the island of Schiermonnikoog. Let’s see if you can guess the right answer. Here is the duck:

hybrid duck.jpg

A mystery duck on Schiermonnikoog (picture by Thijs Glastra)


An Escapee?

This picture (among others) was shared on social media and led to the suggestions that it might be an Australasian Shoveler (Anas rhynchotis). This species is known to escape from captivity in the Netherlands. Have a look at a picture of an Australasian Shoveler. Could the mystery bird belong to this species?

australasian shoveler

An Australasian Shoveler (from


Or a Hybrid?

There are some similarities, but it is obvious that our mystery bird is not an Australasian Shoveler. Two other birders decided to have a closer look at the bird on Schiermonnikoog. They managed to lure it to the shore by playing the sound of a Northern Shoveler (A. clypeata). Based on the combination of characters, they quickly concluded that it concerns a hybrid. An obvious candidate was Northern Shoveler x Blue-Winged Teal (A. discors).


The species that might have produced the mystery bird: Norther Shoveler (left) and Blue-winged Teal. (from


But which hybrid?

Mystery solved? Not quite. This hybrid is just one of many other possible combinations. A Northern Shoveler might have crossed with a Cinnamon Teal (A. cyanoptera) or Red Shoveler (A. platalea). It could even be a backcross… Clearly, a difficult decision. But the authors are quite convinced about this conclusion:

There are several constant differences between hybrids Northern Shoveler x Blue-winged Teal and Northern Shoveler x Cinnamon Teal that we believe permits robust identification. The former shows a white to pale cream-coloured breast, strong spotted/marbled pattern on breast, dark markings on flank and clean white patch on rear flank, and the iris colour is typically darker. Hybrids involving two shoveler species (Red x Northern Shoveler, Australasian x Northern Shoveler) are also different from Northern Shoveler x Blue-winged Teal, in particular as they are heavy-headed birds with bill and body size similar to Northern.

Do you agree? Feel free to comment below.

hybrid cinnamon.jpg

A hybrid between Northern Shoveler and Cinnamon Teal (picture by Rob Fowler)



van Bemmelen, R.S.A., Lehmhus, J. & Mlodinow, S.G. (2018) Hybrid Northern Shoveler x Blue-winged Teal on Schiermonnikoog, Netherlands, in May 2014, and identification and WP occurrence. Dutch Birding40:71-81.


The paper has been added to the Anseriformes page.

What Happens When Two Badger Species Meet in Russia?

Two Badger species meet in a contact zone along the Volga and Kama Rivers in Russia. But do they also interbreed?

In the children’s novel “Wind in the Willows“, Mole and Rat visit Badger, who lives deep in the Wild Wood. They are expecting a grumpy black-and-white carnivore, because the rumor goes that Badger does not like visits. However, Badger warmly welcomes both visitors into his large and cosy underground home. This story shows that Badgers can do unexpected things. Perhaps they can even hybridize?


Russian Rivers

A recent study in Mammalian Biology focused on a contact zone between two Badger species in Russia. Along the Volga and Kama rivers, European Badger (Meles meles) and Asian Badger (M. leucurus) occasionally bump into one another. Emi Kinoshita and colleagues wanted to know if these encounters also lead to interbreeding.

So, they screened 71 samples with several genetic markers. No less than 17 individuals showed signs of admixture. The genetic make-up of these individuals suggests that backcrossing occurs.

european badger

A European Badger wandering around (from


Westward Expansion

Fossil evidence indicates that the Asian Badger has expanded westwards during the Holocene (the geological epoch from about 12,000 years ago until present). The current hybrid zone probably formed as recent as 100 to 200 years ago. Interestingly, there are contact zones with another species – the Southwest Asian badger (M. canescens) – in the Tian Shan Mountains and the Caucasus. Could there also be hybridization?

It seems that Badgers have moved around more than we (or at least I) expected. But you could have known this if you read “Wind in the Willows”. Here is a quote from Badger himself:

People come—they stay for a while, they flourish, they build—and they go. It is their way. But we remain. There were badgers here, I’ve been told, long before that same city ever came to be. And now there are badgers here again. We are an enduring lot, and we may move out for a time, but we wait, and are patient, and back we come. And so it will ever be.

asian badger

An sniffing Asian Badger (from:



Kinoshita et al. (2018) Hybridization between the European and Asian badgers (Meles, Carnivora) in the Volga-Kama region, revealed by analyses of maternally, paternally and biparentally inherited genes. Mammalian Biology.

Dumb, dumber, hybrids? Chickadee hybrids show lower intelligence

Hybrid Chickadees perform worse on associative learning and problem-solving tasks compared to their parental species.

Life as a hybrid can be difficult: infertility, low fitness, not being able to find a mate. These are just a few of the challenges that hybrid individuals face during their struggle for existence. In the end, hybrids might not succeed in surviving or reproducing. This kind of selection against hybrids is known as postzygotic isolation. A recent study in Evolution explored a peculiar postzygotic isolation mechanism: intelligence. Maybe hybrids are just too stupid?



Michael McQuillan and his colleagues assessed the intelligence of hybrids between Black-capped (Poecile atricapillus) and Carolina (P. carolinensis) Chickadee. These small songbirds are scatter hoarders, they store food items in countless places. During harsh winters they rely on these hidden treasures for survival. Remembering where you put your food (or car keys) can be crucial for survival.


Black-capped Chickadee (Poecile atricapillus) and Carolina Chickadee (P. carolensis)


Two Experiments

The researchers performed two experiments to gauge the intelligence of both Chickadee species and their hybrids. In the first experiment, they placed a wax worm in a hole and covered it with a white “pom-pom” ball. After the birds removed the ball, they could eat the worm. However, the hole with the worm was surrounded with other holes (also covered with balls). For several days, the same challenge was presented to the birds. Would they figure it out and remember in which hole they could find a worm?

The second experiment was a real problem-solving test. The birds had to move a circular nylon washer to obtain a reward (again a wax worm). The researchers noted down how many birds were able to solve the puzzle.

nylon washer

Nylon washer were used to create a challenging puzzle.


Dumb and Dumber?

The results were clear: hybrids are not that clever. They needed more days to figure out the location of the worm and not all hybrids could solve the puzzle. Interestlingly, female hybrids performed worse. This finding is in line with Haldane’s Rule, which states that the sex to suffer first from hybrid breakdown is the one with different sex chromosomes (in birds, the female has ZW chromosomes, while male has two Z chromosomes). Why hybrid chickadees have a bad memory and lower intelligence remains to be investigated. But this study could be the start of an interesting exploration into the mind of hybrids.


Outcome of the problem-solving experiment: less hybrids (red) could solve the challenged compared to the pure species.



McQuillan, M., Roth II, T.C., Huynh, A.V. & Rice, A.M. (2018) Hybrid chickadees are deficient in learning and memory. Evolution


This paper has been added to the Paridae page.