Doing it in the Dark: Hybridization in Bats

Two studies explore possible hybridization events in bats.

Hybrid bats. It could be the title of a horror movie, but it is actually serious science. Two recent papers explored the genetics of several bat species to figure out whether there is (or has been) any hybridization. The findings were not too bat…

bat.jpg

A Natterer’s Bat (from: https://www.nederlandsesoorten.nl/)

 

Bat Complex

The first paper appeared in the Journal of Biogeography. Emrah Çoraman and his colleagues focused on the Natterer’s bat (Myotis nattereri) complex, which is distributed in Northwest Africa, Europe and parts of the Middle East. Based on one mitochondrial and four nuclear markers, the researchers reconstructed the evolutionary history of this bat complex. The analyses revealed four main groups. Let’s have a look at them. The colors in the text correspond to the figure below.

 

Four Lineages

A central lineage (yellow), ranging from Ireland to Ukraine, probably survived the last glacial period in three refugia: western Balkans, Greece and western Anatolia. After the ice ages, these populations expanded into Europe. We find the second lineage (blue) in Italy. Here, there are two separate groups: one in northern Italy and one in southern Italy. They meet and probably hybridize in the central Appenine Mountains. The third lineage (green) houses bats from the Iberian Peninsula and Northern Africa. Finally, the fourth lineage (reddish brown) represents the lesser known eastern part of the distribution. This lineage is comprised of four subgroups, which have an interesting history that we will explore in more detail.

bat phylogeny.jpg

The (a) evolutionary history and (b) distribution of the Myotis nattereri complex (from Çoraman et al. 2018 Journal of Biogeography)

 

Hybridization Events

As you can see in the figure above, the evolutionary history of this bat complex is quite…well…complex. It involves several hybridization events, resulting in a reticulated phylogeny. For example, there has been gene flow between the blue and yellow lineages. Probably, the blue lineage expanded out of Italy and come into contact with the yellow one.

Another hybridization event occurred in the eastern part. Expanding population first came into contact with the local lineage in Anatolia. They acquired the mtDNA from this lineage and continued their expansion to Israel where they obtained mtDNA from the resident populations. Interestingly, the Israeli populations went extinct but their genes live on in another species.

 

Cryptic Species

The second paper – in the journal Ecology and Evolution – delved into the genetics of three cryptic species of long‐eared bats (Plecotus auritus, P. austriacus, and P. macrobullaris). Cryptic species are reproductively isolated, but morphologically they are identical. Tommy Andriollo and his colleagues postulated that the extensive phenotypic overlap and observation of morphologically intermediate individuals may hide rampant hybridization. To test this idea, they sampled 349 individuals and genotyped them with a mitochondrial marker and microsatellites. Surprisingly, no sign of hybridization was detected suggesting that these three species are biologically separated. Hybridization is a common phenomenon, but that doesn’t mean you will always find it.

Plecotus auritus.jpg

A Brown Long-eared Bat (from: http://www.freenatureimages.eu/)

 

References

Andriollo, T., Ashrafi, S., Arlettaz, R. & Ruedi, M. (2019) Porous barriers? Assessment of gene flow within and among sympatric long‐eared bat species. Ecology and Evolution, 8(24):ece3.4714.

Çoraman, E., Dietz, C., Hempel, E., Ghazaryan, A., Levin, E., Presetnik, P., Zagmajster, M & Mayer, F. (2019) Reticulate evolutionary history of a Western Palearctic Bat Complex explained by multiple mtDNA introgressions in secondary contact. Journal of Biogeography, 46:343-354.

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A Little Brown Job: Unraveling the Chiffchaff complex

Genetic study clarifies relationships between different subspecies of the Common Chiffchaff.

LBJ. Bird watchers will know what this abbreviation stands for: Little Brown Job. It refers to the large number of small brown passerines that are difficult to distinguish. A nice example of a LBJ is the “Chiffchaff complex”. This collection of small birds used to be considered one species, but detailed studies – based on genetics, acoustics and morphology – revealed the presence of four species: Common Chiffchaff (Phylloscopus collybita), Iberian Chiffchaff (P. ibericus), Mountain Chiffchaff (P. sindianus) and Canary Islands Chiffchaff (P. canariensis). In addition, these species can be divided into several subspecies. As the name already implied, the situation is quite complex. A recent study in the journal PLoS One tried to unravel the complicated relationships in this group of birds.

canary islands

The Canary Islands Chiffchaff (from: http://www.wikipedia.com/)

 

Two Genes

Marko Rakovic and his colleagues sampled across the range of the Chiffchaff complex, from western Europe to Siberia. Next, they sequenced two genes (the mitochondrial ND2 and the Z-linked ACO1I9). Analyses of these markers confirmed the delineation of the four recognized species.

However, some Z-linked alleles were shared between Common Chiffchaff and Canary Islands Chiffchaff. Moreover, based on the ND2-gene, one individual from the Canary Islands was located within the Common Chiffchaff group (see orange group in the figure below). Could it be that these species occasionally hybridize?

journal.pone.0210268.g003.PNG

Relationships within the Chiffchaff complex based on the ND2-gene. All four species form separate clades (from: Rakovic et al. 2019 PLoS One).

 

Common Subspecies

Let’s have a closer look at the patterns within these Common Chiffchaff. This species is generally divided into several subspecies:

  • collybita
  • abientus
  • tristis
  • brevirostris
  • caucasicus
  • menzbieri

Based on the mitochondrial ND2-gene, all these subspecies form distinct clades. However, this was not the case for the other genetic marker. Genetic variation is shared between the subspecies abientus and collybita, which are known to hybridize in Sweden (see for example Hansson et al. 2000). The data did not reveal hybridization between abientus and tristis, altough recently a hybrid zone was discovered in the Urals (see for example Shipilina et al. 2017 and this blog post).

common chiffchaff

The Common Chiffchaff (from: http://www.wikipedia.com/)

 

Southern Chiffchaffs

Less is known about the southern subspecies of the Common Chiffchaff. The Anatolian Peninsula of Turkey houses brevirostris, while caucasicus breeds more to the east in Armenia. Finally, menzbieri is known from eastern Iran and the border with Turkmenistan. The genetic analyses clustered brevirostris and caucasicus. In the eastern part of their range, they might hybridize with menzbieri.

map

Distribution of the Common Chiffchaff subspecies. Notice the overlap between the brevirostris/caucasicus group (light blue) and the mezbieri subspecies (dark blue). The size of the pie charts indicates the sample size (from: Rakovic et al. 2019 PLoS One).

 

Another Subspecies?

The researchers also sampled in a region that is normally not included in the distribution of the Common Chiffchaff: Mount Hermon (situated near the border between Lebanon and Syria). This population turned out to be a mix of brevirostris, caucasicus and colybita. Probably these birds use Mount Hermon as a wintering area. Interestingly, this population also contained alleles that were not present in any of the other subspecies. Perhaps this population might represent a yet unnamed taxon. Things just got a bit more complex again…

 

References

Rakovic, M., Neto, J.M., Lopes, R.J., Koblik, E.A., Fadeev, I.V., Lohman, Y.V., Aghayan, S.A., Boano, G., Pavia, M., Periman, Y., Kiat, Y., Ben Dov, A., Collinson, J.M., Voelker, G. & Drovetski, S.V. (2019) Geographic patterns of mtDNA and Z-linked sequence variation in the Common Chiffchaff and the ‘chiffchaff complex’. PLoS One 14(1):e0210268.

 

This paper has been added to the Phylloscopidae page.

Scientific Sherlocks: The Case of the Imperial Pheasant

How scientists figured out the origin of this mysterious bird.

In 1923, Jean Delacour stumbled upon a pair of captive pheasants during a trip in Vietnam. Unknown to science, these birds were shipped to Delacour’s estate in France where they reproduced. The species was named Imperial Pheasant (Lophura imperialis). After this discovery, several ornithologists set out to observe them in the wild, but failed. There was no sign of this peculiar bird species. Until 1990, when a bird was trapped in a lowland forest. About 10 years later, another individual was found. Only four specimens observed in nearly 100 years time, this must be a critically endangered species! Or is there something else going on here?

The Imperial Pheasant, an extremely rare species?

The Imperial Pheasant, an extremely rare species?

 

Gathering Evidence

A interesting hypothesis was raised in 1997, what if the Imperial Pheasant was a hybrid between Silver Pheasant (L. nycthemera) and Edwards’ Pheasant (L. edwardsi), two species that co-occur in Vietnam? This idea was tested following three lines of inquiry: morphology, experiments and genetic analyses.

All known museum specimens of the Imperial Pheasant and numerous photographs were gathered. Alain Hennache started a hybridization experiment at the Zoological Park of Clères in France (nice detail: Delacour’s former estate). And Ettore Randi took care of the genetic analyses.

Silver Pheasant (above) and Edwards'  Pheasant

Silver Pheasant (above) and Edwards’ Pheasant

 

Captivating Consilience

All studies pointed towards a hybrid origin of the Imperial Pheasant. The museum specimens were intermediate in size between Silver Pheasant and Edwards’ Pheasant. Similarly, the hybridization experiment produced Imperial Pheasant-like birds that closely matched the original descriptions by Delacour. Finally, the genetic analyses showed that the Imperial Pheasant shared alleles (i.e., particular versions of genes) with its hypothesized parent species. The conclusion was clear: the Imperial Pheasant is not a species, but a hybrid of Silver Pheasant and Edwards’  Pheasant.

This beautiful study also exemplifies a characteristic of evolutionary theory, namely consilience of evidence. The principle that evidence from independent, unrelated sources leads to the same conclusions. In evolution, many disciplines (e.g., paleontology, genetics, morphology, embryology, biogeography, …) support the idea that life has evolved. Doubting this would just be silly….

 

References

Hennache, A., Rasmussen, P., Lucchini, V., Rimondi, S. & Randi, E. (2003). Hybrid origin of the imperial pheasant Lophura imperialis (Delacour and Jabouille, 1924) demonstrated by morphology, hybrid experiments, and DNA analyses Biological Journal of the Linnean Society, 80 (4), 573-600 DOI: 10.1111/j.1095-8312.2003.00251.x

Why are more sex-linked genes flowing from Thrush into Common Nightingale? Exploring the faster Z-effect in birds

Strong postcopulatory selection influences patterns of gene flow in two Nightingale species.

Why are some hybrid sterile or nonviable? This is still one of the burning questions in speciation research. Hybrid sterility and non-viability can be caused by genetic incompatibilities between mutations that arose after two species diverged. Let’s say that in species A the mutation BRAD arises, while in species B another mutation ANGELINA emerges. When these two species come into secondary contact and interbreed, hybrids will have the unfavorable combination BRAD-ANGELINA which leads to sterility. This intuitive model – known as the Bateson-Dobzhansky-Muller (BDM) model – nicely explains the occurrence of hybrid dysfunctions.

 

Large X/Z-effect

Interestingly, these hybrid incompatibilities are more common on the X and Z sex chromosomes compared the autosomes (i.e. a chromosome that is not a sex chromosome). This phenomenon is known as the large X/Z-effect and the underlying mechanism is still not understood. One explanation is a faster rate of evolution on sex chromosomes. This fast evolution can be due to more efficient selection or higher levels of genetic drift on sex chromosomes. A recent study in the journal Heredity tried to disentangle the mechanisms behind fast X/Z-effect in two Nightingale species.

common nightingale.jpg

Common Nightingale (from: http://www.vogelwarte.ch/)

 

Nightingale Sperm

Common Nightingale (Luscinia megarhynchos) and Thrush Nightingale (L. luscinia) diverged about 1.8 million years ago and currently hybridize in Central and Eastern Europe. Female hybrids are sterile. Václav Janoušek (Charles University Prague) and his colleagues studied the Z-chromosomes of these species to understand the evolution of hybrid sterility in this system.

First, the researchers assessed postcopulatory sexual selection in both species. This type of sexual selection concerns competition between males after copulation, namely between their sperm cells. Female birds often mate with multiple males and the sperm cells of these males consequently race each other to the egg cell. The length of sperm cells is a good indicator of postcopulatory sexual selection. Measurements revealed that sperm cells were significantly longer in Common Nightingale compared to Thrush Nightingale, suggesting stronger sexual selection in the former species.

thrush nigthingale.jpg

Thrush Nightingale (from http://www.hbw.com/)

 

Genetic Drift

Next, let’s have a look at the genetic diversity on the Z-chromosomes. They found that the Z chromosome had lower levels of genetic diversity in the Common Nightingale than in the Thrush Nightingale. This result indicates that genetic drift is probably stronger in the Common Nightingale. Hence, mutations (including potential hybrid incompatibilities) will accumulate faster on the Z-chromosome of the Common Nightingale. This concept is illustrated in the figure below: the less diversity in the population, the easier the blue balls increase in frequency.

genetic drift

A cartoon illustrating genetic drift. The lower the genetic diversity, the faster mutations can increase in frequency.

 

Patterns of Gene Flow

The faster accumulation of hybrid incompatibilities in Common Nightingales has an effect on the patterns of gene flow between these species. Because hybrids that have a Common Nightingale Z-chromosome have a higher chance of suffering from these incompatibilities, you expect lower levels of gene flow from Common Nightingale into Thrush Nightingale. And that is exactly what this study found.

To finish, let us put this all together: stronger postcopulatory selection in Common Nightingale leads to lower levels of genetic diversity on the Z-chromosome, which results in a lower effective population size, leading to a faster accumulation of hybrid incompatibilities due to genetic drift and lower levels of gene flow into Thrush Nightingale. Did I just manage to summarize the paper in one sentence?

nightingale figure

A figure summarizing the line of reasoning in the paper by Janoušek et al. (2018).

References

Janoušek, V., Fischerová, J., Mořkovský, L., Reif, J., Antczak, M., Albrecht, T. & Reifová, R. (2018) Postcopulatory sexual selection reduces Z-linked genetic variation and might contribute to the large Z effect in passerine birds. Heredity 

 

This paper has been added to the Muscicapidae page.

 

Who runs the world? Hybrid zone dynamics in a bird species where females compete for males

Introgression of a female competitive trait across a Jacana hybrid zone.

The only law in biology is that there is always an exception.” This was one of the propositions of my PhD thesis. A few weeks ago, I wrote about how sexual selection is mainly driven by male-male competition and female choice. Well, there are exceptions. In some mating systems, such as polyandry, the female defends a harem of males. Here, competition between females can determine reproductive success. One nice example of such as system are Jacanas, Neotropical shorebirds. A recent study in the journal Evolution explored the consequences of a polyandrous mating system on hybrid zone dynamics.

Northern_Jacana.jpg

Northern Jacana (from: http://www.audubon.com/)

 

Hybrid Zone

Northern Jacana (Jacana spinosa) and Wattled Jacana (J. jacana) interbreed along a narrow hybrid zone in Central America. Previous work indicated that there is some gene flow between these species. To see if the peculiar mating system of these birds has an effect on the patterns of introgression, Sara Lipshutz and her colleagues characterized the hybrid zone with about 13,000 molecular markers (SNPs).

They also measured several morphological characters, such as body mass and the length of wing spurs. Northern Jacanas have larger body mass, longer wing spurs and are more aggressive compared to Wattled Jacanas. The wing spurs are used during fighting and in threat displays.

Wattled-Jacana.jpg

Wattled Jacana (from: https://landsinlove.com/)

 

Clines

The researchers performed cline analyses on the genetic and morphological data. If you need to refresh your cline theory, you can read this blog post. But I will briefly explain the concept. A cline shows the distribution of character along a geographical transect. Clines can show smooth, continuous gradation in a character, or they may show more abrupt changes in the trait from one geographic region to the next.

The shape and the position of a cline can provide information about the biological processes underlying it. For example, a steep cline suggests a potential role in reproductive isolation or local adaptation because the species have totally different characters.

 

Big Birds

The cline analyses of the Jacana hybrid zone revealed a striking pattern. The cline for female body mass was shifted to the east relative to the center of the hybrid zone. This suggests asymmetric introgression of this trait from the larger Northern Jacana into Wattled Jacana. Increased competition between females might select for bigger body mass, because larger females can defend larger harems of males. So, Wattled Jacanas that got the “bigger-body-genes” from Northern Jacanas might be more successful.

Similar patterns have been reported for male traits, such as red plumage in Fairy-wrens and yellow plumage in Manakins. This study provides the first example of introgression of a female trait from one species into another.

cline.jpg

The cline for female body mass (purple) is shifted to the east relative to the general pattern (grey), suggesting introgression of this female trait (from Lipshutz et al. 2019 Evolution).

 

References

Lipshutz, S.E., Meier, J.I., Derryberry, G.E., Miller, M.J., Seehausen, O. & Derryberry, E.P. (2019) Differential introgression of a female competitive trait in a hybrid zone between sex-role reversed species. Evolution.

 

This paper has been added to the Charadriiformes page.

Hybridization contributes to rapid and repeated evolution of cave fish

Genetic analyses uncover gene flow between different cave populations.

Evolution tends to repeat itself. A classical example of repeated adaptation to similar environments are cave fish (Astyanax mexicanus). Genetic analyses revealed that populations in caves are related to different surface populations. So, these fish have colonized caves on multiple occasions. A recent study in Molecular Ecology shows that hybridization has also played a role in the evolution of these cave fish.

mexican tetra.jpg

Two blind cave fish or Mexican tetras (from: http://www.youtube.com/).

 

Two Lineages

Adam Herman and his colleagues sampled five populations of the Mexican tetra or blind cave fish in Mexico. The first comparison revealed two lineages: a “new lineage” with populations from the Molino cave and the Río Choy surface, and an “old lineage” with populations from the Pachón cave, the Tinaja cave and the Rascon surface. The fact that cave and surface populations cluster together supports the notion of repeated evolution.

 

Underground Gene flow

Next, the researchers applied a suite of statistical techniques (including F-statistics and demographic modelling) to infer whether there has been gene flow between different populations. The results demonstrate “recent and historical gene flow between cave and surface populations both within and between lineages.”

Surprisingly, the analyses suggested subterranean gene flow between all three caves (Molino, Pachón and Tinaja) although the entrances are separated by more than 100 kilometers. Such transfer might seem unlikely, but it does occur. For example, many cave-dwelling insects have been able to migrate over long distances.

treemix.jpg

A Treemix-result showing gene flow between the different populations (colored arrows). Notice how different cave and surface populations cluster together, indicating repeated evolution of cave fish (from: Herman et al., 2018 Molecular Ecology)

 

Rapid Adaptation

But what does this gene flow mean for cave fish evolution? It might have sped up adaptation to the cave environment by distributing genetic variation among the populations. Also, genes involved in adaptation to caves might have been exchanged, resulting in almost immediate adjustment to a new environment. For instance, genes involved in pigmentation and eye development show signs of gene flow.

Blind cave fish are an excellent system to study repeated evolution and the role of hybridization therein. I would keep an eye on these fish!

 

References

Herman, A. et al. (2018) The role of gene flow in rapid and repeated evolution of cave-related traits in the Mexican tetra, Astyanax mexicanusMolecular Ecology 27:4397-4416.

Are Snow Bunting and McKay’s Bunting different species?

A genetic study explores the demographic history of these white passerines.

The description of McKay’s Bunting (Plectrophenax hyperboreus) on Wikipedia reads: “This species closely resembles Snow Bunting (P. nivalis) in all plumages, but is whiter overall.” The close resemblance of these passerine species makes you wonder if they are not different morphs of the same species. A recent study in the journal PeerJ explored the species status of these birds using genetic techniques.

 

High Latitude Birds

Kevin Winker (University of Alaska) and his colleagues studied population genomics of Snow Bunting and McKays Bunting, using ultraconserved elements (UCEs). I will discuss these molecular makers below, but let’s first take a closer look at these buntings. McKay’s Buntings breed on remote islands in the Bering Sea, while Snow Buntings are found throughout the rest of the Holarctic at high latitudes. Previous work suggests that they diverged during the last glacial maximum, about 18,000 to 74,000 years ago.

snow bunting

A Snow Bunting (from: http://www.allaboutbirds.com/)

 

Gene Flow

The researchers tested different demographic models and checked which model fits their genetic data. In the end, the best model indicated a scenario of divergence-with-gene-flow. So, over time these birds became different while still exchanging some genes. In fact, possible hybrids have been reported relatively recently (see for example Sealy 1969), so the gene flow might still be ongoing.

But are they different species? Although they are genetically different, they still share a large amount of genetic variation. The genetic markers used in this study revealed no fixed differences. The researchers think that “given phenotypic differences between the taxa, it seems likely that there are fixed allelic differences in portions of the genome not included in our data.” More genomic analyses are needed to sort out the species question.

mckays bunting

A McKay’s Bunting (from: http://www.birdsna.com/)

 

Ultraconserved Elements

As mentioned above, this study used ultraconserved elements or UCEs. These conserved sequences are shared among divergent animal genomes and are probably involved in controlling gene expression. They have mostly been used to uncover evolutionary relationships at deeper levels, but this study shows that they can also be applied to population genomic questions.

The results obtained using UCEs are different from a previous study that relied on mtDNA and AFLPs (Amplification Fragment Length Polymorphism). Specifically, effective population sizes were lower, divergence was deeper (about 241,000 years) and gene flow levels were lower. These discrepancies are probably due to the differences in effective population sizes of mtDNA and UCEs. Moreover, these markers are subject to quite different selection regimes. Good to keep in mind for further analyses.

 

References

Winker, K., Glenn, T.C., Faircloth, B.C. (2018) Ultraconserved elements (UCEs) illuminate the population genomics of a recent, high-latitude avian speciation event. PeerJ 6:e5735.

 

The paper has been added to the Calcariidae page.

How healthy are juvenile eagle hybrids?

Study of feathers reveals nutritional status of hybrid and pure eagles.

“The available evidence contradicts the assumption that hybridization plays a major
evolutionary role [in animal evolution].” These are the words of biologist Ernst Mayr. At the time of writing (1963, in his book Animal Species and Evolution), he had a point.  Animal hybrids were rarely observed and often turned out to be sterile. These observations led to the idea that hybrids are generally less fit compared to “pure” species. But is this really the case? A recent study in the Journal of Raptor Research assessed this claim for two hybridizing eagle species.

greater_spooted_eagle_707copy_copy

A Greater Spotted Eagle (from: http://www.hbw.com/)

 

Eastern Poland

Hybrids between Greater Spotted Eagle (Clanga clanga) and Lesser Spotted Eagle (C. pomarina) have been reported for over 150 years. For the past 30 years, researchers studied the breeding populations of these species in Eastern Poland. They discovered a substantial amount of mixed pairs. In fact, the proportion of broods producing hybrids increased by over 30% between 1996 and 2012. Hybrids seem to be doing quite well, but are they faring better than their “pure” counterparts?

 

Ptilochronology

To quantify the fitness of the birds, Gregorz Maciorowski and his colleagues relied on ptilochronology. This technique is based on the assumption that each growth bar on a feather represents a 24 hour period. The width of the growth bar is determined by amount of energy and nutrients invested during the growth process. So, the width of the growth bar indicates the nutritional status of an individual bird. The researchers also checked the feather for fault bars. These transparent bands are produced in times of stress.

sedge_warbler

The fault bars on the tail of a Sedge Warbler (from: http://www.birdguides.com/)

 

Water Regime

The feathers from nestlings of 54 Lesser Spotted Eagles, 42 Greater Spotted Eagles and 23 hybrids were compared.  There were no significant differences in the number of fault bars or the average width of growth bars. Hence, there seems to be no difference in nutritional status between the hybrid and the “pure” nestlings.

The researchers noticed a large number of fault bars, suggesting that the nestlings experienced stress. This could be due to the extreme environmental fluctuations in the Biebrza River valley. Variability in water levels could negatively affect the eagles because changing water levels may result in lower prey availability or less suitable foraging areas. How these changes will impact on the hybridization dynamics remains to be investigated.

lesser

A Lesser Spotted Eagle (from http://www.wikipedia.com/)

 

References

Maciorowksi, G., Yosef, R., Väli, Ü & Tryjankowski, P. (2018) Nutritional Condition of Hybrid Nestlings Is Similar To That of Pure-Species Offspring of Spotted Eagles (Clanga clanga x C. pomarina).  Journal of Raptor Research 52(4):484-490.

 

This paper has been added to the Accipitriformes page.

Why do different populations of the Greenish Warbler sing other songs?

The dynamics of sexual selection revealed by the songs of the Greenish Warbler.

Males fight and females choose. That is sexual selection in a nutshell. Charles Darwin called upon sexual selection to explain certain phenomena that didn’t make sense in a natural selection context. For example, why did the peacock have such an elaborate tail? This would make it an easy target for predators. Unless something else is going on. What if females prefer males with long tails? Birds that manage to survive with such cumbersome tails should be amazing partners. In addition, sexual selection also clarified other exaggerated male traits, such as the big antlers of red deer. These over-sized hatstands come in handy when fighting off male rivals in the conquest for females. Darwin explored sexual selection in his book The Descent of Man, and Selection in Relation to Sex and the concept is still widely studied by evolutionary biologists.

Sexual selection.jpg

The peacock’s tail and the red deer’s antlers. Textbook examples of sexual selection.

 

Two Sides of the Sexual Selection Coin

Female choice and male competition are two sides of the same coin. But these two processes do not always act in concert. Let’s explore a hypothetical example. Females might prefer males with long, colorful tails. In order to maintain their beautiful tails, males need high quality habitats with plenty of food. To defend their habitats males will have to fight off competitors, for example by chasing away intruders. But flying around your territory is much easier with a shorter tail. So, female choice selects for longer tails and male competition selects for shorter ones. What will be the outcome of this conflict?

A similar conflict is also apparent in bird song. In many species, females choose males with long, complex songs. But males need to produce short, simple songs to defend their territories. Elizabeth Scordato explored this conundrum in the Greenish Warbler (Phylloscopus trochiloides) and recently reported her results in the journal Evolution.

 

Three Populations

The Greenish Warbler consists of a chain populations around the Tibetan plateau. The ends of this chain meet in Siberia where the birds from both sides are reproductively isolated. A nice example of a ring species (but see Alcaide et al., 2014). The ecological conditions along this chain vary: in the southern populations density is high and food is scarce, while the northern populations are less densely populated and have abundant food. Could these ecological conditions have an impact on the songs of these birds? To answer this question, Elizabeth Scordato studied populations in India, Kyrgyzstan and Siberia.

greenish warbler.jpg

A Greenish Warbler (from: http://www.wikipedia.com/)

 

Population Density and Song Length

In all populations, females preferred males with longer, more complex songs and large repertoires. However, males in the south (India) sang shorter songs compared to males in the north (Kyrgyzstan and Siberia). This latitudinal pattern reflects the degree of male competition. In the south, population density is higher, leading to more interactions between the males and consequently strong selection for shorter songs. In the north, competition between males is weaker and there is a stronger selection by females for longer, complex songs.

explanation_scordato

A graphic showing how population density influences the conflict between male competition and female choice, culminating in different songs. (Drawings from Handbook of Birds of the World and sonograms from Scordato, 2018 Evolution)

 

Resolving Conflict

There is clearly a conflict between female choice (long songs) and male competition (short songs) here. So, how do males cope with this situation? First, they adjust their song depending on the time of the year. Early in the season, when birds establish their territories, they sing short songs. When females are fertile, males produce longer songs. Later on, after most females have mated, males switch back to shorter songs. The figure below shows this pattern in the three populations.

scordati.jpg

Males sing longer songs when female are fertile (grey area in the graph). The pattern holds for the three populations. (from: Scordato, 2018 Evolution)

Another strategy that the males used to resolve the conflict between female choice and male competition was to aim particular components of the song at different receivers. Short, simple songs were directed towards competing males, whereas long, complex songs were sang to attract females. The analyses suggested that not only song length, but also repertoire size was subject to female choice. Males with a larger repertoire had a higher pairing success and fostered larger chicks. By contrast, competing males were not really impressed by the repertoire of their rivals.

 

Speciation

All in all, this study shows how the strength of male competition can influence the degree of female choice. In populations with weak competition between males, birds can produce longer songs, displaying their large repertoire. The distinct selective pressures in the different populations result in song divergence and might ultimately contribute to the origin of new species.

 

References

Scordato, E. (2018) Male competition drives song divergence along an ecological gradient in an avian ring species. Evolution 72(11):2360-2377.

 

The paper has been added to the Phylloscopidae page.

Unraveling the history (or histories?) of the Red-bellied Woodpecker

Reconstructing the history of the Red-bellied Woodpecker.

Florida is not only a popular holiday destination, it also houses a famous suture zone. What is a suture zone, you ask? It is a term from phylogeography, the study of the historical processes that culminated in the present distributions of organisms. In 1968, Remington defined a suture zone as “a band of geographic overlap between major biotic assemblages, including some pairs of species or semispecies which hybridize in the zone.” In other words, it is a region that houses numerous contact zones between various organisms.

suture zones

The expansion from Pleistocene refugia (indicated by the letters) let to the formation of numerous secondary contact zones. Regions where multiple contact zones cluster are called suture zones (from: Swenson & Howard 2005 The American Naturalist)

 

Ice Age Legacy

Suture zones are the outcome of post-Pleistocene expansion. During the Pleistocene, most of North America was covered with ice sheets, pushing animals and plants into southern refugia. Once the ice sheets melted, the organisms expanded from their refugia and recolonized North America. In this process, populations from different refugia came into secondary contact (I have written about this before, see here and here). As a consequence, populations on both sides of the contact zone are genetically differentiated.

Avian examples include Carolina Chicadee (Poecile carolinensis) and Barred Owl (Strix varia). Another putative instance of this scenario is the Red-bellied Woodpecker (Melanerpes carolinus). A recent study in The Wilson Journal of Ornithology investigated whether this species conforms to the expected pattern.

red-bellied woodpecker.jpg

A Red-bellied Woodpecker (from: http://www.allaboutbirds.com/)

 

Different Histories

George Barrowclough (American Museum of Natural History) and his colleagues collected samples from across the range of the Red-bellied Woodpecker. Analyses of the mitochondrial gene ND2 revealed striking patterns. Populations outside of Florida housed one common genetic variant (or haplotype) and a few uncommon ones. This genetic distribution indicates an expanding population. Statistical tests, such as Fu’s Fs, supported this conclusion. Populations in Florida, by contrast, showed little genetic variation and tested negative for population expansion.

The authors state that “these alternate haplotype frequencies and associated demographies suggest that the populations have had separate evolutionary histories and are now in secondary contact in a well-known suture zone.”

 

Plumage Patterns

The genetic results are supported by morphological data. The researchers examined 204 adult males. Each individual was scored for the presence or absence of a distinct tan-colored forehead band between the nostrils and the eyes. This plumage pattern is used to diagnose the subspecies perplexus, which occurs in Florida. Mapping the scores for this characteristic on a map revealed a gradual decrease of this band from south to north.

Taken together, the geographic patterns in mtDNA and plumage might lead to the recognition of the Florida populations as a separate species: the Florida Red-bellied Woodpecker. Another reason to book a holiday to Florida if you are an avid bird watcher.

woodpeckers

Specimens of the Florida subspecies perplexus (two on the left) and the nominate subspecies carolinus (two on the right). Notice the tan-colored forehead on the perplexus individuals (from Barrowclough et al. 2018, The Wilson Journal of Ornithology)

 

References

Barrowclough, G.F., Groth, J.G., Bramlett, E.K., Lai, J.E. & Mauck; W.M. (2018) Phylogeography and geographic variation in the Red-bellied Woodpecker (Melanerpes carolinus): characterization of mtDNA and plumage hybrid zones. The Wilson Journal of Ornithology, 130(3): 671-683.

 

This paper has been added to the Piciformes page.