How mountains, rivers and hybridization drove the evolution of the Blue-crowned Manakin

Genetic study identifies four main lineages within this small songbird species.

Evolution never stops. Populations keep changing over time. Occasionally, a geographic barrier arises or a new behavior originates, resulting in some degree of reproductive isolation between the changing populations. Wait long enough and you might end up with different species. And then, the cycle repeats, culminating in the beautiful biodiversity we see today. A recent study in the journal Zoologica Scripta used genetic data to reconstruct the evolutionary history of the Blue-crowned Manakin (Lepidothrix coronata).

How many species of Blue-crowned Manakin are there? © Ltoniolo | Wikimedia Commons

 

Four Lineages

Camila Alves Reis and her colleagues analyzed several nuclear and mitochondrial genes to unravel the evolutionary relationships between Blue-crowned Manakins from different locations across South America. These analyses revealed four main lineages. How these lineages arose is an exciting story of rising moutains, expanding rivers and hybridization. Time for an evolutionary bedtime story.

Our story begins about 1.8 million years ago when the northern part of the Andes is forming. The slow build-up of this mountain chain splits the range of the ancestral Blue-crowned Manakin in two. A nothern population (dubbed TA, for Trans-Andean) is separated from a southern population, both going their own evolutionary ways. The TA lineage currently includes two subspecies (velutina and minuscula) based on mitochondrial DNA from another study.

The four main lineages (names TA, A, B and C) and the sampling locations. From: Reis et al. (2019) Zoologica Scripta

 

Rivers and Hybrids

Let’s follow the fate of the southern population, which consequently splits into three distinct lineages. Timing when these lineages formed and estimating their ancestral ranges indicates an important role for different Amazonian rivers. The authors nicely summarize the situation (also see the figure below): “(a) lineage A is delimited to the south by the left margin of the Japurá River, which corresponds to the Imeri and Jaú centres of endemism; (b) lineage B is found between the right (southern) margin of the Japurá River and the northern margin of the Solimões (Amazon), within the Napo centre of endemism; and (c) lineage C is located in the Inambari centre of endemism, to the south of the Amazon‐Solimões.”

There is, however, something peculiar about lineages B and C. The analyses in this study (based on nuclear and mtDNA) suggest that lineage C is the oldest group, whereas lineages A and B cluster together. Other studies (using only mtDNA) reported that lineages B and C are closely related. The disagreement between the nuclear and mitochondrial genes is most likely due to an ancient hybridization event. Birds from lineages B and C hybridized, resulting in the transfer of mtDNA between these lineages. This phenomenon – mitochondrial capture – has been described in numerous other bird species, including another group of manakins (see this blog post)

Reconstruction of ancestral ranges highlights the importance of Amazonian rivers. From: Reis et al. (2019) Zoologica Scripta

 

Plumage Patterns

Finally, the researchers uncovered some more fine-grained population structure within the different lineages. In contrast to the main lineages, which were delinated by the Andes or major rivers, these sublineages were not separated by obvious physical barriers. Interestingly, the two subgroups in lineage C (C1 and C2) show differences in plumage patterns: “Subclade C1 was represented primarily by males with completely green plumage or intermediate plumage, that is, typically green underparts and black upper body, whereas subclade C2 included males with completely green or black plumage (not co‐occurring), and some males with intermediate plumage.” These birds might be in the process of further diversification based on plulage patterns. Or they might form a secondary contact zone where two formerly distinct lineages are merging. More research in this area is needed to figure this out. The story continues…

A Blue-crowned Manakin with green plumage. © Michell Leon | Peruaves

 

References

Cheviron, Z., Hackett, S. J., & Capparella, A. P. (2005). Complex evolutionary history of a Neotropical lowland forest bird (Lepidothrix coronata) and its implications for historical hypotheses of the origin of Neotropical avian diversity. Molecular Phylogenetics and Evolution, 36(2), 338–357.

Reis, C. A., Dias, C., Araripe, J., Aleixo, A., Anciães, M., Sampaio, I., Schneider, A. & do Rêgo, P. S. (2020). Multilocus data of a manakin species reveal cryptic diversification moulded by vicariance. Zoologica Scripta, 49(2), 129-144.

 

This paper has been added to the Pipridae page.

 

Shake your tail feather! Courtship displays and genetic analyses uncover extensive hybridization between Allen’s and Rufous Hummingbird

Two recent studies explore a hummingbird hybrid zone from morphological, behavioral and genetic perspectives.

Allen’s Hummingbird (Selasphorus sasin) and Rufous Hummingbird (S. rufus) are almost indistinghuisable in the field. Identifying these fast-flying birds might even be more tricky than expected, because several authors argued that these species hybridize on the west coast of the US. A recent study by Brian Myers and his colleagues confirmed these suspicions: using behavioral and morphological data, they showed that Allen’s Hummingbird and Rufous Hummingbird interbreed along a contact zone in Oregon.

A Rufous Hummingbird © Ed Post | Flickr

 

Courtship Displays

Hummingbirds are known for their acrobatic courtship displays. And the Selasphorus hummingbirds are no exception. Males fly in complicated patterns above potential mates, trying to woo them with daring dives, zig-zags and special sounds. Allen’s Hummingbird and Rufous Hummingbird produce fluttering sounds with their tail feathers, and they do this in slightly different ways. Rufous Hummingbird uses the tip of the second tail feather (or retrix 2) while Allen’s Hummingbird relies on the third tail feather (or retrix 3). This difference strategy is also reflected in the morphology of these feathers. A useful feature to discriminate between these species.

Courtship displays can be broken up into several elements, such as dives, shuttles and pendulums (illustrated in the figure below). These elements often occur in a species-specific order. For instance, if an Allen’s Hummingbird performs a series of pendulums, it often ends this series with a dive. Interestingly, birds from the contact zone performed displays that differed from both parental species. Some putative hybrids showed a series of pendulums but instead of ending with a single dive, they finished with multiple dives. This move is characteristic for Rufous Hummingbirds.

The different elements of hummingbird courtships: dives (D), shuttles (S) and pendulums (P). From Myers et al. (2019) The Auk

 

Gene Flow

Using  this morphological and behavioral information, the researchers were able confidently identify hybrids and pinpoint the exact location of hybrid zone. But these results raise another question: does hybridization also result in introgression (i.e. gene flow between the species)? You could imagine that females are not impressed by the abnormal courtship displays of the hybrids. So, hybrid males might be unable to find a partner and backcross.

This question can be resolved with genetic data. And that is exactly what Christopher Battey did in a recent Evolution paper. He collected DNA samples from both species and performed a suite of genetic analyses to reconstruct the evolutionary history of these hummingbirds. He concluded that “demographic models, introgression tests, and genotype clustering analyses support a reticulate evolutionary history consistent with divergence during the late Pleistocene followed by gene flow across migrant Rufous and Allen’s Hummingbirds during the Holocene.”

The location of the hybrid zone based on morphological and behavioral data (left) and the admixed genetic make-up of individuals from both species (right).

 

Z-chromosome

The high levels of introgression lead to another mystery: how do these hummingbird species remain distinct in the face of gene flow? To unravel this mystery, Battey compared the level of genetic differentiation across the genomes of these species. Genomic regions that are highly differentiated might hold genes related to reproductive isolation. This search revealed some interesting regions – so-called islands of differentiation – on the Z-chromosome. Moreover, these regions showed an unexpected decrease in genetic diversity, suggesting strong linked selection.

To explain the background about these patterns, I will quote from an excellent digest (i.e. the news-articles accompanying some Evolution papers) that was written about this paper by… me.

[Battey] uncovered several islands of differentiation on the Z-chromosome, which were accompanied by a significant drop in nucleotide diversity, suggesting that they were formed by linked selection. However, the Z-chromosome is expected to show lower nucleotide diversity compared to autosomes (Irwin 2018). This sex chromosome has a lower effective population size (3/4 of the autosomes) and is heavily influenced by differential reproductive success between males (who carry two Z-chromosomes) and females (who carry one Z-chromosome). If the diversity on the Z-chromosome is affected only by its lower effective population size, the diversity ratio between Z-chromosome and autosomes would be around 0.75. Taking into account variation in reproductive success can, in theory, push this ratio down to 0.56 (Charlesworth 2001). In the hummingbirds, the ratio was even lower (ranging from 0.44 to 0.58), indicating that additional linked selection might have occurred.

 

Species Status

Z-linked genes involved in reproductive isolation mechanisms, such as male plumage traits and female preference, have been found in other bird species. Whether the Z-chromosome of these hummingbirds also harbors such barrier loci remains to be determined. If so, these genomic regions can be used as an extra line of evidence to support the species status of Allen’s Hummingbird and Rufous Hummingbird.

 

References

Battey, C. J. (2019). Evidence of linked selection on the z chromosome of hybridizing hummingbirds. Evolution.

Myers, B. M., Rankin, D. T., Burns, K. J., & Clark, C. J. (2019). Behavioral and morphological evidence of an Allen’s× Rufous hummingbird (Selasphorus sasin× S. rufus) hybrid zone in southern Oregon and northern California. The Auk, 136(4), ukz049.

Ottenburghs, J. (2020). Exploring genomic islands of differentiation on the Z‐chromosome of hummingbirds. Evolution.

 

These papers have been added to the Apodiformes page.

Are there still “pure” Hawaiian Ducks?

Recent study explored the genetics of this species across the Hawaiian Islands.

The Hawaiian Duck or Koloa (Anas wyvilliana) is a special bird. This species is endemic to the Hawaiian Islands (as you might have guessed) and probably originated about 3000 years ago from a hybridization event between Mallard (A. platyrhynchos) and Laysan Duck (A. laysanensis). In other words, the Hawaiian Duck is a hybrid species (you can read this blog post for more on hybrid bird species). Unfortunately, this unique species is threatened with extinction by extensive hybridization with feral Mallards. An ironic twist for this hybrid species.

Feral Mallards were introduced on the Hawaiian Islands in the 1800s, mainly for food and hunting. During the 1930s and 1940s, several “wild” feral populations were established on different islands. The increasing numbers of Mallards resulted in hybridization with the local Hawaiian Ducks. Genetic analyses confirmed gene flow between these species, raising the fear that the Mallard is genetically swamping the Hawaiian Duck. A recent study in the journal Molecular Ecology assessed the current situation and genetically characterized several island populations.

A pair of Hawaiian Ducks © Dick Daniels | Wikimedia Commons

 

Kaua‘i

Caitlin Wells, Philip Lavrestky and their colleagues sampled birds throughout the Hawaiian archipelago and sequenced their DNA using a ddRAD-approach. The genetic patterns that emerged from the analyses painted a dual picture of hope and despair. On the positive side, the island of Kaua‘i (which supports the biggest population of Hawaiian Ducks) contained few hybrids and seemed practically pure.

The lack of hybrids on Kaua‘i can probably be explained by the skewed sex ratio of three males for every one female. Hybridization might occur when females have a difficult time finding a mate from the same species. In the end, they settle for a male from another species (i.e. the desperation hypothesis). This situation does not occur on Kaua‘i because there are plently of Hawaiian males to choose from.

The Hawaiian Ducks on Kaua’i (in yellow) show little admixture from Mallards (blue). This island houses the largest population of “pure” birds in the archipelago. From: Wells et al. (2019) Molecular Ecology

 

Hybrid Swarms

The other islands, however, showed different degrees of hybridization between Hawaiian Ducks Mallards. The first-generation hybrids interbreed with each other or backcross into feral Mallards. The result is a collection of hybrid swarms that blur the line between the parental species. The fitness of effects of this genetic mash-up remain to be investigated. The authors list potential consequences of feral genes in Hawaiian Ducks, including higher growth rates, smaller digestive organs and lower survival rates.

From a conservation point of view, the reseachers provide clear guidelines: “our results suggest that the removal of feral mallards is critical and should be considered a management priority to limit the chance that remaining koloa will be lost to hybridization.” Let’s hope we can save this hybrid species from genetic extinction.

The remaining islands show extensive hybridization: all birds show ancestry of Hawaiian Duck (yellow) and Mallard (blue). From: Wells et al. (2019) Molecular Ecology

 

References

Wells, C. P., Lavretsky, P., Sorenson, M. D., Peters, J. L., DaCosta, J. M., Turnbull, S., Uyehara, K. J., Malachowski, C. P., Dugger, B. D., Eadie, J. M. & Engilis Jr, A. (2019). Persistence of an endangered native duck, feral mallards, and multiple hybrid swarms across the main Hawaiian Islands. Molecular Ecology, 28(24), 5203-5216.

 

This paper has been added to the Anseriformes page.

Beyond genetics: How different are Kentish and White-faced Plover?

Researchers provide multiple lines of evidence to assess the species status of these plovers.

The recent surge in genomic resources allows ornithologists to uncover extremely fine-grained population structure. In a book chapter that I wrote with several colleagues, you can read that “In the past, statistical power from only a small number of genetic markers from distant regions of the genome has often been insufficient to unveil weak population structure, and increasing the number of markers has clearly shown that more markers give better signals.” This rise in resolution raises an important issue: Will a more fine-grained picture of population structure lead to the splitting of more and more species? Several recent papers on plovers (genus Charadrius) highlight this conundrum.

A Kentish Plover in India © David Raju | Wikimedia Commons

 

Genetic Differentiation

Are the Kentish Plover (C. alexandrinus) and the White-faced Plover (C. dealbatus) different species? Opinions about the species status of these two plovers vary and several researchers have attempted to answer this question using genetic data. In 2011, Frank Rheindt and colleagues used three mitochondrial and seven nuclear DNA markers to describe the genetic situation among these plovers. Despite clear morphological differences, they failed to find genetic differentiation between Kentish and White-faced Plover. They speculated that “diagnostic phenotypic characters may be encoded by few genes that are difficult to detect” or that “gene expression differences may be crucial in producing different phenotypes”.

Fast forward to 2019, when Xuejing Wang, Pinjia Que and co-workers revisited these plovers using three mitochondrial and sixteen nuclear loci. Their analyses revealed a clear genetic difference in the mitochondrial DNA: the species formed distinct groups in the haplotype networks. The nuclear networks, however, could not confidently discriminate between the putative species. This result is in line with the speculation from the 2011 study, namely that diagnostic phenotypic characters may be encoded by few genes.

Haplotype networks of (a) mitochondrial and (b) nuclear markers for Kentish (blue) and White-faced (yellow) plover. From: Wang et al. (2019) BMC Evolutionary Biology

 

Enter Genomics

To really know whether the morphological differences between these birds are encoded by a few genes, there is only one solution: sequence the whole genome! And this is exactly what Xuejing Wang, Kathryn Maher, Nan Zhang and colleagues did. As expected, comparing the genomes of Kentish and White-faced Plover showed low levels of genome-wide differentiation. However, the researchers did find a few differentiated genomic regions (so-called “islands of differentiation”) which might hold the key to the morphological differences. Which genes are in these regions and how they might contribute to species-specific differences remains to be investigated.

An example of a genomic island of differentiation (in blue, based on relative genetic differentiation Fst). Notice that this regions corresponds to a decrease in Dxy (absolute divergence) and π (genetic diversity), suggesting strong linked selection. From: Wang et al. (2019) Frontiers in Genetics

 

Multiple Lines of Evidence

In summary, the genetic differences between Kentish and White-faced Plover are concentrated in a few differentiated islands while the rest of the genome is largely undifferentiated (genome-wide Fst = 0.046). To come back to the question posed in the beginning: Are these genetic differences sufficient to consider these plovers distinct species? As I have argued in other blog posts (see for example here, here and here), the answer is clearly no. Taxonomy has become pluralistic, requiring multiple lines of evidence to justify lumping or splitting species. Luckily, the authors of the papers mentioned above provided some insights into the morphology, ecology and demography of Kentish and White-faced Plover.

There are clear morphological differences between these birds: White-faced Plovers lack the dark eye ring found in Kentish Plovers. Plumage-wise, they show lighter patterns and a brighter cinnamon cap. Moreover, Whiter-face Plovers have, on average, longer beaks, longer wings and a larger body mass. The differences in bill size suggest that Kentish and White-faced Plover use different food sources. This suggestion is supported by a stable isotope analysis which provides insights into the diet of these birds. It turned out that White-faced Plovers probably feed on a higher energy diet compared to Kentish Plovers.  Finally, the genomic analyses mentioned above revealed distinct demographic histories. The population size of Kentish Plover increased, while the number of White-faced Plovers decreased during the Last Glacial Period.

Synthesizing all the results (based on genetics, morphology, ecology and demography), we can conclude that Kentish and White-faced Plover can be considered “independently evolving metapopulation lineages” and are thus – according to the General Lineage Concept – distinct species!

A Kentish Plover (left) and White-faced Plover (right) at Tanjung Tokong in Malaysia. © D.N. Bakewell | PLoS One

 

References

De Queiroz, K. (1999). The general lineage concept of species and the defining properties of the species. Species: new interdisciplinary essays. MIT Press, Cambridge, MA.

Ottenburghs, J., Lavretsky, P., Peters, J.L., Kawakami, T. & Kraus, R.H.S. (2019) Population genomics and phylogeography. Avian Genomics in Ecology and Evolution – From the lab into the wild (Edited by Kraus, R.H.S.), pp. 237-265, Springer Nature.

Rheindt, F. E., Szekely, T., Edwards, S. V., Lee, P. L., Burke, T., Kennerley, P. R., et al. (2011). Conflict between genetic and phenotypic differentiation: the evolutionary history of a ‘lost and rediscovered’ shorebird. PLoS One, 6(11).

Wang, X., Que, P., Heckel, G., Hu, J., Zhang, X., Chiang, C. Y., et al. (2019). Genetic, phenotypic and ecological differentiation suggests incipient speciation in two Charadrius plovers along the Chinese coast. BMC Evolutionary Biology, 19(1), 135.

Wang, X., Maher, K. H., Zhang, N., Que, P., Zheng, C., Liu, S., et al. (2019). Demographic histories and genome-wide patterns of divergence in incipient species of shorebirds. Frontiers in Genetics, 10, 919.

 

These papers have been added to the Charadriiformes page.

Different migration strategies contribute to reproductive isolation between Barn Swallow subspecies

Recent study tests three predictions to link migratory divides with reproductive isolation.

Bird migration is a fascinating phenomenon. And it becomes even more interesting when you throw hybridization into the mix. At migratory divides, populations with different migration strategies interbreed. For example, in central Europe one population of Blackcaps (Sylvia atricapilla) migrates to the southeast, while the other population prefers the southwest. Hybrids between both populations show an intermediate migration route: straight to the south.

Migratory divides can generate reproductive isolation between the interacting populations, potentially culminating in speciation. If individuals with different migration strategies arrive at distinct times, assortative mating can arise (i.e. prezygotic isolation). Or hybrids might show suboptimal migration routes – such as the Blackcaps – and suffer reduced survival (i.e. postzygotic isolation). The connection between migratory divides and reproductive isolation seems clear, but surprisingly nobody has tested this relationship in detail. However, a recent study in the journal Ecology Letters did just that.

A Barn Swallow in Germany © Andreas Eichler | Wikimedia Commons

 

Three subspecies and three predictions

Elizabeth Scordato and her colleagues focused on three subspecies of Barn Swallow (Hirundo rustica) that hybridize in Asia: rustica, tytleri and gutturalis. Two subspecies pairs (namely rustica-tytleri and rustica-gutturalis) show migratory divides, while a third combination (tytleri-gutturalis) does not. This set-up allowed the researchers to test three predictions:

1. Hybridization is less common in the migratory divides.
2. The migratory phenotype explains more genetic variance compared to other traits in the migratory divides.
3. Assortative mating is stronger in the migratory divides.

 

Prediction 1: Less hybrids

The first prediction was confirmed using genetic data. The researchers reported that they “found limited hybridisation between rustica‐tytleri and rustica‐gutturalis, but extensive admixture between tytleri and gutturalis.” Indeed, there were only a few first-  or later-generation hybrids in the first two migratory hybrid zones and many multi-generation hybrids in the third non-migratory hybrid zone.

In addition, a geographic cline analysis revealed very steep clines in the rustica‐tytleri and rustica‐gutturalis contact zones, pointing to strong reproductive isolation. The clines in the tytleri-gutturalis zone, on the other hand, were much wider. This confirms the extensive hybridization uncovered in the other genetic analyses. If you want to know more about cline theory, you can check out this blog post (also based on Barn Swallows): A Lesson in Cline Theory with Some Hybridizing Barn Swallows.

The genetic analyses uncovered a few hybrids in the migratory divides (rustica‐tytleri and rustica‐gutturalis) and many hybrids in the non-migratory hybrid zone (tytleri-gutturalis). From Scordato et al. (2020) Ecology Letters

 

Prediction 2: Migratory phenotypes are most important

If different migration strategies are connected to reproductive isolation, you would expect that migration-related phenotypes contribute more to genetic divergence between the subspecies. And indeed, in the Russian migratory divide a migration-related factor (carbon isotope values, which correspond to different wintering areas) explained almost 20% of the genetic variance. In the non-migratory contact zones, this factor only accounted for about 2% of the variance. The remainder of genetic variance was explained by geography and color.

In migratory divides (left column), a large part of genetic variance was explained by migration (yellow circle). In contact zones without migratory divides (right column), most variation was covered by geography (green circle) and color (blue circle). From Scordato et al. (2020) Ecology Letters

 

Prediction 3: Assortative mating

As explained in the beginning, migratory divides could culminate in assortative mating when individuals arrive at different times. The researchers used phenotype networks to see which characteristics best explained mating choice. These networks showed that “In the two transects with migratory divides, carbon isotope values [a proxy for migration strategy] were correlated within pairs, indicating assortative mating by overwintering habitat.”

A couple of Barn Swallows. In the migratory divides, birds choose their mates according to migration strategy. © Arend | Wikimedia Commons

 

More migratory divides…

By testing these three predictions, this study confidently shows that migratory divides can contribute to reproductive isolation. Whether this pattern holds for other migratory divides remains to be investigated. However, the Barn Swallow case is just one example in this region. Several other bird species come into secondary contact at the Tibetan Plateau and might also show migratory divides (see Irwin & Irwin 2005). There is still much to discover in these Asian mountains.

 

References

Scordato, E. S., Smith, C. C., Semenov, G. A., Liu, Y., Wilkins, M. R., Liang, W., Rubtsov, A., Sundev, G., Koyama, K., Turbek, S. P., Wunder, M. B., Stricker, R. J. & Safran, S. J. (2020). Migratory divides coincide with reproductive barriers across replicated avian hybrid zones above the Tibetan Plateau. Ecology letters, 23(2), 231-241.

 

This paper has been added to the Hirundinidae page.

Babblers provide evidence for “speciation cycles” in the Himalayan-Hengduan Mountains

How does speciation unfold in these mountain ranges?

Allopatric speciation seems to be the most common model for the origin of new bird species. However, given the widespread occurrence of hybridization (as exemplified by this website), it is possible that some species arose in the face of extensive gene flow. A recent study in the journal Molecular Phylogenetics and Evolution explores this question in the Himalayan-Hengduan Mountains where four pairs of closely related species co-occur. Did they originate in allopatry or not?

Grey-cheeked Fulvetta, one of the eight species in this study. © Robert tdc | Flickr

 

Eight Species

Fend Dong and colleagues examined the genetic make-up of eight babbler species:

• Grey-cheeked Fulvetta (Alcippe morrisonia)
• Nepal Fulvetta (Alcippe nipalensis)
• White-browed Fulvetta (Fulvetta vinipectus)
• Grey-hooded Fulvetta (Fulvetta cinereiceps)
• Red-billed Leiothrix (Leiothrix lutea)
• Silver-eared Mesia (Leiothrix argentauris)
• Yellow-throated Fulvetta (Schoeniparus cinereus)
• Rufous-winged fulvetta (Schoeniparus castaneceps)

For each species pair – you can match the species pairs by genus name – the researchers constructed phylogenetic networks and performed Isolation-with-Migration analyses.

Phylogenetic networks of four babbler species pairs show clear differentiation between the species, suggesting a history of allopaty. From: Dong et al. (2020) Molecular Phylogenetics and Evolution

Speciation Cycle

Both approaches pointed to an allopatric scenario for all species pairs. The networks clearly separated the species and in the Isolation-with-Migration analyses allopatric models were favored over models with gene flow. So, avian speciation in the Himalayan-Hengduan Mountains seems to be mainly allopatric.

An interesting, but not very surprising finding… But wait, there is more! The researchers also uncovered a positive relationship between divergence time and level of sympatry. In other words, species that diverged longer ago showed a higher degree of overlap in distribution. This could be because these older species have had more time to develop behavioral and ecological differences that allow sympatric coexistence. This observation fits the so-called speciation cycle as described in a recent Nature Ecology & Evolution paper by Jay McEntee and colleagues: “From onset to completion, the process is often viewed as a cycle with three stages, beginning with geographic isolation (allopatry), followed by secondary contact initiated at range edges and finally prolonged spatial coexistence in overlapping geographical ranges (sympatry).” These speciation cycles result in local accumulation of species numbers, possibly explaining some of the biodiversity hotspots on Earth.

 

References

Dong, F., Hung, C. M., & Yang, X. J. (2020). Secondary contact after allopatric divergence explains avian speciation and high species diversity in the Himalayan-Hengduan Mountains. Molecular Phylogenetics and Evolution, 143, 106671.

McEntee, J. P., Tobias, J. A., Sheard, C., & Burleigh, J. G. (2018). Tempo and timing of ecological trait divergence in bird speciation. Nature Ecology & Evolution, 2(7), 1120-1127.