Are there really two species of red panda?

How strong is the case for splitting the red panda into a Himalayan and a Chinese species?

On several popular science websites you can read that the red panda actually consists of two species: the Himalayan red panda (A. fulgens) and the Chinese red panda (Ailurus styani). When I checked the original publication in Science Advances, I became skeptical of this conclusion. The title states that there is “genomic evidence for two phylogenetic species”. Do the researchers split the red panda solely based on the phylogenetic species concept? So, this morning, I grabbed a nice cup of coffee and started reading the paper. Let’s have a look at the evidence!

The red panda: one or two species? © H. Zell | Wikimedia Commons

 

Species Criteria

In another blog post, I explained my views on the species problem. There is an important distinction between the theoretical question of what species are (i.e. species concepts) and the ways in which species can be delimited in practice (i.e. species criteria). From a theoretical point of view, we can define species as separately evolving metapopulation lineages. But how do we delineate these lineages in practice? Taxonomy has become pluralistic and combines several lines of evidence from different species criteria (e.g., morphology, behavior, genetics, reproductive isolation, …). In some cases, all criteria agree and it is easy to classify species. In other cases, different criteria result in different taxonomic decisions. For example, taxa might be genetically distinct, but morphologically indistinguishable (i.e. cryptic species).

 

Three lines of evidence

In this study, the authors mention three species criteria that support a split into two species. First, there are clear morphological differences between the Himalayan and the Chinese red panda (i.e. morphological species criteria):

Morphologically, the Chinese subspecies has much larger zygomatic breadth, the greatest skull length, stronger frontal convexity, more distinct tail rings, and redder face coat color with less white on it.

Second, the genetic data (based on whole genomes, the Y-chromosome and mtDNA) nicely separate the two proposed species (i.e. genetic species criteria):

On the basis of the whole-genome SNPs, the phylogenetic tree, principal components analysis (PCA), and ADMIXTURE results revealed substantial genetic divergence between the two species, providing the first genomic evidence of species differentiation.

Third, demographic analyses reveal that both species experienced drastically different population dynamics (possibly due to differences in geography and climate). This line of evidence correspond to the Evolutionary Species Concept which states that “A species is an entity composed of organisms which maintains its identity from other such entities through time and over space, and which has its own independent evolutionary fate and historical tendencies”. The figure below nicely illustrates that both species have followed an independent evolutionary trajectory.

The demographic history of the two red panda species is quite different. From: Hu et al. (2020) Science Advances

 

Conservation

Combining the three lines of evidence – morphology, genomics and demography – it is safe to say we are indeed dealing with two different species. My initial skepticism was roused by the conservation status of the red panda (endangered). By splitting this species in two, the researchers automatically create two even more endangered species (which could be used to justify funding more research).

In fact, the genomic analyses revealed that the Himalayan red panda has experienced three bottlenecks, resulting in very low genetic diversity and demanding rapid conservation efforts. If the red panda had not been split into two separate species, this reduction in genetic diversity might not have been picked up. This finding thus illustrates the importance of good taxonomic decisions for conservation.

 

References

Hu et al. (2020) Genomic evidence for two phylogenetic species and long-term population bottlenecks in red pandas. Science Advances, 6(9):eaax5751.

Ottenburghs, J. (2019). Avian species concepts in the light of genomics. In Avian Genomics in Ecology and Evolution (pp. 211-235). Springer, Cham.

Ecological speciation in the saltmarshes

Scientists search the genomes of Saltmarsh and Nelson’s Sparrow (A. nelsoni) for candidate genes under divergent ecological selection.

There are numerous speciation models, but recently ecological speciation has caught the attention of several evolutionary biologists. In this model, populations diverge as they adapt to different ecological conditions, eventually resulting in reproductive isolation. Ideally, the traits under divergent ecological selection are correlated with mating traits (e.g., beak morphology). Although this model makes intuitive sense, it is difficult to disentangle ecological factors from non-ecological ones, such as genetic drift. One way around this challenge is to study species along an environmental gradient where divergent selection is very pronounced. A recent study in the journal Ecology and Evolution used this approach to understand the evolutionary history of Saltmarsh (Ammospiza caudacutus) and Nelson’s Sparrow (A. nelsoni).

A Saltmarsh Sparrow © Dominic Sherony | Wikimedia Commons

 

Divergent Regions

Jennifer Walsh and her colleagues scanned the genomes of 20 individuals (10 of each species), looking for particular regions that showed markedly higher divergence compared to the rest of the genome. Based two methods – using either Fst or the number of fixed SNPs – the researchers uncovered 33 highly divergent regions that were examined in greater detail. This approach culminated in a list of candidate genes under divergent selection. Let’s have a look at the most interesting genes…

Regions of elevated divergence (indicated with stars) in the genomes of Saltmarsh Sparrow and Nelson’s Sparrow. The different tracks correspond to relative divergence Fst (blue), absolute divergence Dxy (green) and number of fixed SNPs (pink). From: Walsh et al. (2019) Ecology and Evolution

 

Candidate Genes

The first candidate gene is SLC41A2, a transporter that moves magnesium-ions across the cell membrane. Hence, this gene plays an important role in osmoregulation, a crucial mechanism when you live in a salty environment. Moreover, SLC41A2 popped up in another study on these sparrows, showing signs of reduced introgression and increased selection. 

Candidate gene number two listens to the name CRY1 and is associated with the circadian clock. How does circadian rhythm relate to saltmarshes? Well, the marshes are affected by flooding during spring when the birds are breeding. To avoid nest failure, it is important to synchronize the nesting cycle (which takes between 23 and 26 days) with the 28-day tidal cycle.

Finally, there is TYRP1. This gene is part of the melanin biosynthesis pathway which determines plumage color. Saltmarsh Sparrow and Nelson’s Sparrow differ in plumage patterns: Saltmarsh Sparrow have darker breast and flank streaking compared to Nelson’s Sparrow. The darker coloration is saltmarshes might be beneficial because it helps the birds avoid predators by blending in with the muddy background. Or the darker feathers might be more resistant to degradation (more melanin slows down degradation by salt-tolerant bacteria).

A Nelson’s Sparrow © Andy Reago & Chrissy McClarren | Wikimedia Commons

 

Genetic Drift

These divergent regions (and their candidate genes) indicate that divergent ecological selection has shaped the genomes of these sparrows. Importantly, one of the candidate genes (TYRP1) affects plumage coloration which could be used in mate choice, thus providing a link between ecological selection and reproductive isolation.

However, the researchers found that genetic drift has also influenced the evolutionary history of Saltmarsh Sparrow and Nelson’s Sparrow. Demographic analyses pointed to reductions in population size, possibly related to glacial cycles. Smaller populations are more prone to genetic drift, which can also lead to divergence in particular genomic regions. Hence, the evolution of these species is best explained by a combination of genetic drift and strong ecological selection. The most likely scenario was nicely outlined in the introduction of the paper:

The prevailing evolutionary hypothesis suggests a history of vicariance for the saltmarsh and Nelson’s sparrow, whereby an ancestral population spanning a coastal to interior range was split by Pleistocene glaciation, resulting in an isolated interior population. Following differentiation, this interior population then spread eastward back toward the Atlantic coast after recession of the Wisconsin ice mass, making secondary contact with ancestral coastal populations and establishing the current ranges and ecotypes within this species complex.

The demographic history of Saltmarsh Sparrow (orange) and Nelson’s Sparrow (purple) shows reductions in effective population size, resulting in higher levels of genetic drift. From: Walsh et al. (2019) Ecology and Evolution

 

References

Walsh, J., Shriver, W. G., Olsen, B. J., & Kovach, A. I. (2016). Differential introgression and the maintenance of species boundaries in an advanced generation avian hybrid zone. BMC Evolutionary Biology, 16(1):65.

Walsh, J., Clucas, G. V., MacManes, M. D., Thomas, W. K., & Kovach, A. I. (2019). Divergent selection and drift shape the genomes of two avian sister species spanning a saline–freshwater ecotone. Ecology and Evolution, 9(23):13477-13494.

 

This paper has been added to the Passerellidae page.

Observations confirm intergeneric hybridization between Common Black Hawk and Red-Shouldered Hawk

A putative hybrid sighting in California is confirmed with careful field observations of the nesting birds.

A weird bird might be a hybrid. But it could also be a color variant or an individual in bad condition. So, you have to be careful and not jump to conclusions. This is nicely illustrated by a recent case in California when people found a dead raptor on the roadside. Plumage and other measurements were intermediate between Red-tailed Hawk (Buteo jamaicensis) and Red-shouldered Hawk (Buteo lineatus), leading to the hypothesis that the bird was a hybrid. However, genetic analyses revealed that it was a Red-shouldered Hawk, probably of eastern origin. This example shows the importance of genetic testing of hybrid hypotheses. Another possibility is careful observation of the nesting site, as illustrated by another recent case.

Interactions between a female Common Black Hawk and a male Red-shouldered Hawk in California. From: Moore & Coulson (2020) Journal of Raptor Research

 

An Intergeneric Hybrid?

In 2012, Lisa Hug reported the sighting of a putative hybrid between Red-shouldered Hawk and Common Black Hawk (Buteogallus anthracinus). This observation was questioned by some people, who suggested it might be a juvenile Common Black Hawk. The debate prompted Stan Moore to investigate the case in greater detail. He decided to observe the nest of a female Common Black Hawk in California.

 

Field Observations

These field observations revealed that the bird was frequently engaging in breeding attempts with a Red-shouldered Hawk. In 2014, for example, “Stan Moore (SM) and property owner Guy Smith observed the male Red-shouldered Hawk mounting and copulating with the perched Common Black Hawk on the morning of 8 April. SM observed a second copulation that evening. The pair also vocalized frequently to one another. SM observed the Common Black Hawk collecting nest material on 18 March, and nest-building on 8 and 11 April.”

The copulation was successful, because a nestling was observed in May. The juvenile’s plumage showed characteristics of both species. For instance, the streaked brown crown and nape are reminiscent of Red-shouldered Hawk, while the dark brown stripes on the side of the neck point to Common Black Hawk. The behavioral and morphological observations thus support the conclusion that we are dealing with an intergeneric hybrid.

A blood sample was collected in 2014, which could be used to confirm the hybridization event genetically. But I would say the evidence is already quite convincing…

Several pictures of the hybrid highlight the characteristics of both species. From: Moore & Coulson (2020) Journal of Raptor Research

 

References

Clark, W. S., Galen, S. C., Hull, J. M., Mayo, M. A., & Witt, C. C. (2017). Contrasting molecular and morphological evidence for the identification of an anomalous Buteo: a cautionary tale for hybrid diagnosis. PeerJ, 5, e2850.

Moore, S., & Coulson, J. O. (2020). Intergeneric Hybridization of a Vagrant Common Black Hawk and a Red-Shouldered Hawk. Journal of Raptor Research, 54(1), 74-80.

 

Thanks to Jennifer Coulson for sending me this paper, which has been added to the Accipitriformes page.

Choosing between Cerrado and Chaco: Which corridor connected the Andean and Atlantic forest populations of the Buff-browed Foliage Gleaner?

Genetic study provides several lines of evidence for a Cerrado connection.

The Buff-browed Foliage Gleaner (Syndactyla rufosuperciliata) is a small passerine that occurs in the tropical forests of South America. You can find it in the west (Andean forests) and in the east (Atlantic forests) of this continent. In the center, however, this species is absent: the dry and open vegetation functions as a barrier between the Andean and Atlantic forests (the so-called Open Vegetation Corridor). This distribution suggest that eastern and western populations of the Buff-browed Foliage Gleaner were connected at some point in time.

There are two possible connections between the Andean and Atlantic forests: the Chaco in the south and the Cerrado in the north (see map below). Several botanical studies indicated that these regions were forested during the Pleistocene, allowing birds to travel between the Andean and Atlantic forests. But which route did the Buff-browed Foliage Gleaner use? A recent study in the journal Molecular Phylogenetics and Evolution provides evidence for a Cerrado connection.

A Buff-browed Foliage Gleaner in Brazil © Cláudio Dias Timm | Wikimedia Commons

 

Evidence for the Cerrado Connection

Gustavo Cabanne and his colleagues collected DNA-samples for all known subspecies of the Buff-browed Foliage Gleaner. A suite of genetic analyses all pointed to a Cerrado connection. Here is the brief summary:

1.  Isolation-with-Migration analyses uncovered a clear split between the Andean and Atlantic populations that occurred during the Pleistocene. Moreover, there were high levels of gene flow between the populations in northern Bolivia and the Atlantic forest population.
2. A cluster analysis (based on the mitochondrial gene cytb) grouped the northern Andean individuals (from Peru) with the North Atlantic population.
3. Demographic modelling rejected the Chaco-corridor model and supported a Cerrado connection.

The Buff-browed Foliage Gleaner occurs in the Andean and Atlantic forests. These populations have been connected in the past through the Cerrado corridor (arrow I in figure A). There was less evidence for a Chaco connectoin (arrow II in figure A).  From: Cabanne et al. (2019) Molecular Phylogenetics and Evolution

 

Taxonomic Issues

As mentioned above, the researchers found a clear split between the Andean and Atlantic populations. They suggest to treat these populations as distinct species. The Andean species would be comprised of three subspecies (oleaginea, cabanisi and similis) while the Atlantic species holds two subspecies (acrita and rufosuperciliata).

Interestingly (especially for this website), the analyses also revealed gene flow between the Buff-browed Foliage-gleaner and its sister species, the Russet-mantled Foliage-gleaner (Syndactyla dimidiata). These species overlap – and apparently interbreed – in Paraguay and Brazil. Another hybrid zone to study!

The Russet-mantled Foliage-gleaner is hybridizing with the Buff-browed Foliage-gleaner © Hector Bottai | Wikimedia Commons

 

References

Cabanne et al. (2019) Phylogeographic variation within the Buff-browed Foliage-gleaner (Aves: Furnariidae: Syndactyla rufosuperciliata) supports an Andean-Atlantic forests connection via the Cerrado. Molecular Phylogenetics and Evolution, 133: 198-213.

 

This paper has been added to the Furnariidae page.

The variable beak of the finch: How hybridization increases variation in beak morphology

Darwin’s Finches show more variation in beak morphology due to introgressive hybridization.

In 2015, I attended the workshop “Evolutionary Biology in Guarda”. The idea of this workshop is to develop a research project on an evolutionary topic under the guidance of several well-known evolutionary biologists. That year, the teaching staff consisted of Peter and Rosemary Grant, Richard Lenski, Dieter Ebert and Sebastian Bonhoeffer. It was an inspiring time: thinking about evolution in the idyllic Swiss mountains.

I remember one session where Peter and Rosemary Grant visited our working group. After listening to our ideas, Peter stared out the window and said: “Look at that red flower. Why is it red? You can think of several explanations. For example, it might be red to attract insects for pollination. That would be your hypothesis. Now you could devise an experiment to test this hypothesis. From the experiment, you can then test several predictions.” With this short musing, Peter wanted us to think about the difference between hypotheses and predictions. And it is nice to see that he has followed his own advice and published a study in the journal PNAS (together with Rosemary Grant, obviously) in which a clear hypothesis is formulated and certain predictions are tested.

The Large Cactus Finch © Harvey Barrison | Wikimedia Commons

 

A Hypothesis on Hybridization

Let’s look at the hypothesis first, outlined in the following scenario: “When populations begin to diverge, an exchange of genes may be frequent but will have little effect on the variation of each. As morphological divergence proceeds further to a point at which the populations become biological species—they seldom interbreed but suffer little or no loss of fitness when they do—phenotypic and genetic effects of gene mixing are expected to be greater, and at some point reach a maximum. Thereafter, population variation declines, caused by strengthening of premating isolating mechanisms and hence increased rarity of interbreeding, and/or by the accumulation of incompatible alleles through mutations that reduce or prevent exchange.”

From this scenario, we can formulate a general prediction: the population variation in traits affected by hybridization should increase with time, reach a peak and then decline. Peter and Rosemary tested this prediction using data from Darwin’s Finches on the Galapagos Islands. They studied data on beak morphology from several populations across the archipelago. When they plotted the average coefficient of variation in beak length versus the age of the species, the predicted pattern arose.

The variation in beak length shows the expected pattern in accordance with the scenario of hybridization dynamics. From: Grant & Grant (2019) PNAS

 

Common Cactus-finch

Apart from the general prediction, we can also focus on more specific cases. Does the variation in beak morphology also increase on a species level? To answer this question, the researchers turned to the small island of Daphne Major where they have been monitoring several hybridizing species.

Here, the Common Cactus-finch (Geospiza scandens) interbreeds with the Medium Ground-finch (Geospiza fortis). Because the Common Cactus-finch is the largest species, we expect its beak to become smaller over time (i.e. more like the beak of a Medium Ground-finch). In addition, the variation in beak morphology is expected to increase over time due to introgressive hybridization. And that is exactly what we see: beak length clearly decreases (first figure below) and variation in beak length increases (the black line in the second figure below) over time.

Interestingly, the effect on beak depth is a bit different. This implies that the effect of hybridization on beak length and beak depth (the red line in the second figure below) is uncoupled. These traits are correlated, but seem to be evolving independent to some degree.

The effects of hybridization on beak morphology. The beak length decreases (top) and the variation in beak length increases (bottom, black line) over time. The sudden shift around the year 2000 is due to an El Niño-effect when there was no breeding on the island. From: Grant & Grant (2019) PNAS

 

Empirical Evidence

This study nicely illustrates the power of empirical data. It makes intuitive sense that introgression can result in increased variability. Indeed, several modelling studies have illustrated these predictions (see for example this blog post). However, actually testing the predictions with field data is another story. Peter and Rosemary Grant show how the yearly meticulous collection of population-level data (from 1973 to 2012!) can help evolutionary biologists answer outstanding questions.

In addition, they illustrate the evolutionary importance of hybridization in creating variation. The final sentence of the paper nicely captures this conclusion: “Hybrids are more live paths to the future than dead ends.”

The attendants and teaching staff at the workshop in Guarda. Can you find Peter and Rosemary Grant? © Jente Ottenburghs

 

References

Grant, P. R., & Grant, B. R. (2019). Hybridization increases population variation during adaptive radiation. Proceedings of the National Academy of Sciences, 116(46), 23216-23224.

 

This paper has been added to the Thraupidae page.

Contemplating Kingfishers in Indonesia: The story of a “Great Speciator”

Study describes how two species are at different stages of diversification.

A few months ago, I wrote about the “Paradox of the Great Speciator”, which applies to species that diversify quickly while colonizing a vast geographical range (you can read the blog post or my digest for Evolution). Incipient species are expected to come into secondary contact during their rapid colonization. Next, hybridization and consequent gene flow prevent further divergence and effectively reverse the speciation process. So, how can you reconcile widespread distributions with diversification?

One possible solution to this paradox concerns the taxon cycle concept. First, a taxon colonizes a new area. As time progresses, the taxon shows a decrease in vagility (i.e. the ability to spread across the environment), eventually culminating in an endemic taxon. A recent study in the journal Ibis provides evidence for this scenario for a particular great speciator: the Kingfisher genus Todiramphus.

A Collared Kingfisher in Fiji © Tom Tarrant | Wikimedia Commons

 

Two Kingfishers

Darren O’Connell and his colleagues studied two species of Todiramphus Kingfisher in southern Sulawesi (Indonesia). The first species – the Collared Kingfisher (T. chloris) – is actually a species complex that comprises numerous lineages (five of which have recently been elevated to species rank). For this story, we can forget about most of these lineages and just focus on one subspecies: chloris. The second species is the Sacred Kingfisher (T. sanctus). Both species occur on the mainland of Sulawesi and the neighboring Wakatobi Islands. The main difference between these species is their migratory behavior: the Collared Kingfisher is sedentary while the Sacred Kingfisher migrates.

Two Sacred Kingfishers in Australia © J.J. Harrison | Wikimedia Commons

 

Taxon Cycle

Analyses of mitochondrial DNA and morphology of these species revealed some interesting patterns. The Collared Kingfishers from mainland Sulawesi and from the Wakatobi Islands were genetically distinct (about 0.4% difference for the ND2-gene). Moreover, birds from the islands were significantly larger. The situation for the Sacred Kingfisher was completely different: there were no genetic or morphological differences between the mainland and the island birds.

These results could be explained by the taxon cycle outlined above. The Sacred Kingfisher represents a species early in the cycle. It still has a high level of vagility (as evidenced by its migratory behavior) and it probably colonized the Wakatobi Islands recently. The Collared Kingfisher, on the other hand, has completed the first part of the taxon cycle and consists of several diverging populations. The genetic differences between the mainland and island birds can be due to genetic drift, a founder effect or different selection pressures. The authors suspect that the latter process plays an important role here: the environment on the islands is quite different from the mainland, forcing the Collared Kingfishers to change their habitat use to a more generalist niche. Indeed, in other species, a generalist lifestyle has led to increases in body and bill size.

It would be interesting to expand this line of research to more members of the Todiramphus genus. Can we find representatives for each part of the taxon cycle?

Scatterplot of kingfisher morphology for the Sacred Kingfisher (open circles), the Collared Kingfisher mainland population (triangles) and the Collared Kingfisher Wakatobi Islands population (filled circles). From: O’Connell et al. (2019) Ibis

 

References

O’Connell, D. P., Kelly, D. J., Lawless, N., Karya, A., Analuddin, K., & Marples, N. M. (2019). Diversification of a ‘great speciator’in the Wallacea region: differing responses of closely related resident and migratory kingfisher species (Aves: Alcedinidae: Todiramphus). Ibis, 161(4), 806-823.

Isolation by distance, adaptation or environment: What explains genetic patterns in the Red-crowned Ant-tanager?

Ornithologists compiled a large genetic and morphometric data set to answer this question.

Here is a fun fact about the history of population genetics: one of the most important papers in this field was published in the Journal of Eugenics. The journal does not exist anymore (for obvious reasons), but the papers are still accessible for “scholarly use”. Luckily, because in this paper, Sewall Wright introduced the still widely used F-statistics for quantifying population structure.  Another landmark paper by Wright appeared a few years earlier under the simple title “Isolation by Distance”, introducing another fundamental concept in population genetics. The idea behind isolation by distance (or IBD) is simple: it describes the process of increasing genetic differentiation correlated with increasing geographic distance. It makes intuitive sense: the farther apart two populations are, the less likely they will be connected by gene flow.

The Red-crowned Ant-tanager © Hector Bottai | Wikimedia Commons

 

IBD, IBA or IBE?

However, genetic differentiation can also be due to other processes, such as isolation by adaptation (IBA) or isolation by environment (IBE). The difference between these two processes is subtle, but a recent study in the journal Ecology and Evolution explained it nicely:

Isolation by adaptation is defined as the effect of environmental gradients that results in divergent natural selection that can lead to adaptive phenotypic divergence between populations, resulting in a positive correlation between genetic divergence and adaptive phenotypic differentiation. Isolation by environment is defined as the occupation of two populations in different points on the ecological gradient. This process is observed when the phenotypic target of selection is unknown or is not easily measured, and then, the environmental variation can be used as a proxy and a positive correlation between genetic divergence and environmental dissimilarity is expected.

This study focused on the Red‐crowned Ant-tanager (Habia rubica), a bird species that can be found from central Mexico to northeastern Argentina and southeastern Brazil. What processes can explain the genetic patterns in this passerine: IBD, IBA or IBE?

 

mtDNA

Sandra Ramírez‐Barrera and her colleagues collected 124 mitochondrial DNA (mtDNA) sequences across the range of the Red‐crowned Ant-tanager. These sequences could be divided into seven phylogroups (based on previous analyses)  The researchers correlated the genetic data with various morphological and environmental variables. They used a Multiple Matrix Regression with Randomization (MMRR) approach which estimates the independent contributions of environment and geography on genetic and phenotypic variation.

The broad distribution and different phylogroups in the Red-crowned Ant-tanager (NP, northern pacific of Mexico; SP, southern pacific of Mexico; GM, Gulf of Mexico; SE, southeastern Mexico and northern Central America; PA, Panama; WS, western South America and ES, eastern‐northwestern South America). From Ramírez‐Barrera et al. (2019) Ecology and Evolution

 

Regressions

The results from the regression analyses were clear: only geographic distance had a significant relationship with genetic distance, indicating that isolation by distance (IBD) is the main process driving genetic differentiation in the Red-crowned Ant-tanager. Does this mean that the environment has no effect on the genetic structure of this tropical species? Not necessarily, the researchers might have missed some environmental factors that shaped the genetic patterns in Red-crowned Ant-tanager. More research is clearly warranted here.

The importance of isolation by distance highlights the potential role of geographic barriers in this particular species. Indeed, five Mesoamerican phylogroups are located in the northern (NP) and southern (SP) regions of the Mexican Pacific coast, on the slope of the Gulf of Mexico (GM), from Southeastern Mexico to Costa Rica (SE), and Panama (PA). This region is known as a highly fragmented topographic complex area that has been strongly influenced by climatic and geological events. Moreover, the other two phylogroups occur in Western (WS) and Eastern/Northwestern (ES) South America where they are separated by a diagonal strip of dry vegetation (I have written about this Open Vegetation Corridor before). Whether isolation by distance has rendered these phylogroups so genetically distinct that they should be considered separate species remains open for debate…

The analyses only found support for isolation by distance (red box) in shaping the genetic patterns in the Red-crowned Ant-tanager. From Ramírez‐Barrera et al. (2019) Ecology and Evolution

 

References

Ramírez‐Barrera, S. M., Velasco, J. A., Orozco‐Téllez, T. M., Vázquez‐López, A. M., & Hernández‐Baños, B. E. (2019). What drives genetic and phenotypic divergence in the Red‐crowned Ant tanager (Habia rubica, Aves: Cardinalidae), a polytypic species?. Ecology and Evolution, 9(21), 12339-12352.

 

Genetic study uncovers viability selection in the Imperial Eagle

Genome-wide heterozygosity provides insights into potential inbreeding effects. 

You might remember the terms homozygote and heterozygote from your high school biology classes. They refer to the genetic variants (or alleles) at a certain position (or locus) in the genome. Imagine a gene with two alleles: A and a. Because you have two copies of each chromosome, several situations are possible. Both chromosomes might carry the same allele (AA or aa), making you homozygous for this locus. Alternatively, each chromosome might carry a different allele (Aa), leading to a heterozygous setting. Not to difficult, right? As Mendel would say: “easy peasy.”

Heterozygosity is good proxy for inbreeding. When relatives mate, there is a higher chance that they carry the same alleles, resulting in homozygous offspring. Most genetic diseases are recessive, meaning that they are only expressed in the homozygote state. A low heterozygosity across the genome might indicate a higher susceptibility for such diseases (or other deleterious effects) and consequently a lower chance of survival. This relationship between genetic variation and survivorship can lead to so-called viability selection in which genetically poor individuals (i.e. low heterozygosity) die early in life. This selection should become apparent when comparing the heterozygosity in juveniles and adults. A recent study in the Journal of Heredity tested this idea for three raptor species.

Is there viability selection in the Imperial Eagle? © AngMoKio | Wikimedia Commons

 

Two Eagles and a Falcon

Jacqueline Doyle and her colleagues investigated the genome-wide heterozygosity in the Golden Eagle (Aquila chryseatos), the Imperial Eagle (Aquila heliaca) and the Prairie Falcon (Falco mexicanus). When they compared their measurements between juveniles and adults, the researchers found no differences in the Golden Eagle and the Prairie Falcon. In the Imperial Eagle, however, there was a small (but significant) difference: juveniles has lower levels of heterozygosity compared to adults.

Interestingly, the Imperial Eagle has the lowest population size of the three investigated species. This suggests that there might be higher levels of inbreeding, resulting in highly homozygous juveniles with reduced chances of survival. In other words, there could be viability selection in the Imperial Eagle. This finding has important implications for the conservation of this species, which is listed as “vulnerable” on the IUCN-list.

Genome-wide heterozygosity in three raptor species. Comparing adults (blue) with juveniles (green) shows a significant difference in the Imperial Eagle, suggesting viability selection in this species. From: Doyle et al. (2019) Journal of Heredity

 

References

Doyle, J. M., Willoughby, J. R., Bell, D. A., Bloom, P. H., Bragin, E. A., Fernandez, N. B., Katzner, T. E., Leonard, K. & DeWoody, J. A. (2019). Elevated heterozygosity in adults relative to juveniles provides evidence of viability selection on eagles and falcons. Journal of Heredity, 110(6), 696-706.