This molting strategy might speed up the evolution of flightlessness.
Numerous bird species have lost the ability to fly. Think, for example, about the many rail species that abandoned a flying lifestyle when they colonized new islands. Although the evolution of flightlessness is mostly connected with island species, there might be other factors that increase the likelihood of losing the ability to fly. In a recent study in The American Naturalist, Ryan Terrill investigated the influence of molting strategy on this evolutionary transition. Different bird species use different approaches to renew their plumage. Some species molt their wing feathers sequentially to maintain the ability to fly, while other species become flightless when they replace their wing feathers simultaneously. Because bird with a simultaneous molting strategy have to survive a period of flightlessness, they may transition more easily to a completely flightless life. In other words, simultaneous wing molt could function a pre-adaptation (or exaptation) for the loss of flight.
Rate of Evolution
To test this hypothesis, Terrill compared the evolutionary speed at which species lost the ability to fly for diverse molting strategies. Using different sets of phylogenetic trees, he consistently found “an elevated rate of evolution of flightlessness in birds with simultaneous wing molt”. This result supports the idea that this molting strategy can affect the evolutionary transition to a non-flying lifestyle. During the flightless molting period, these species have to acquire food and escape predators without flying. Morphological and behavioral adaptations to survive this period might predispose these species to evolve flightlessness at some point in their evolution. However, it is important to keep in mind that simultaneous wing molt is not a prerequisite for flightlessness, it can only speed up its evolution.
The connection between molting strategy and the evolution of flightlessness. Taxa in red undergo simultaneous wing molt whereas taxa in gray do not. About 68% of flightless species are in the 3% of birds that show simultaneous wing molt. From: Terrill (2020) The American Naturalist.
Exaptation
In the paper, Terrill uses the term “pre-adaptation” to describe the potential influence of simultaneous wing molt on the evolution of flightlessness in birds. To laypeople, this term seems to imply some predetermined purpose. As if evolution is preparing a species for an important adaptive change. This is obviously not the case. Evolution is a blind process without any foresight. Science writer Carl Zimmer put it nicely: “There’s no foresight involved, though—simply the lucky coincidence that a feature that evolved to do one thing may turn out later to do another thing even better.” To avoid this confusion, some biologists prefer the term “exaptation”, as proposed by Gould and Vrba. Whatever you want to call this interesting phenomenon, it is important to be aware that evolution does not follow a preprogrammed path. It is just the complex interplay between chance and necessity. And as evolutionary biologists, we are fortunate to work on unweaving this wonderful web of life.
References
Terrill, R. S. (2020). Simultaneous Wing Molt as a Catalyst for the Evolution of Flightlessness in Birds. The American Naturalist, 196(6), 775-784.
Researchers unravel the genetic basis of particular plumage patches.
Finding the genetic basis of plumage coloration remains one of the most exciting topics in ornithology. In the past, researchers mainly focused on the genes underlying whole-body coloration, such as the white and blue morphs in Snow Geese (Anser caerulescens). Nowadays, we can zoom in on particular patches on the body and use genomic data to pinpoint candidate genes associated with certain plumage patterns. For example, a recent study on Setophaga warblers found that a single genetic variant determined the colors of the cheek, crown and flank in these birds. This study took advantage of hybrids between Townsend’s Warbler (S. townsendi) and Hermit Warbler (S. occidentalis) which show a range of plumage combinations, making it easier to link particular genomic regions to plumage traits.
Another recent study in the Proceedings of the Royal Society B took a similar approach to detect “plumage genes” in the Yellow-shafted Flicker (Colaptes auratus auratus) and the Red-shafted Flicker (C. a. lathami), two woodpeckers differ in the coloration of several plumage patches. Extensive hybridization in North America has resulted in the full range of possible plumage combinations. Stephanie Aguillon and her colleagues used this phenotypic variation to their advantage to find the genes associated with these plumage patches.
Pigments
Before delving into the genomic analyses, we need to understand the difference between two pigments: melanin and carotenoids. The pigment melanin is produced endogenously (i.e. by the bird itself) and leads to grey, black and brown colors. Carotenoids, on the other hand, are obtained through the diet and underlie red, yellow and orange coloration. The plumage patches in the woodpeckers are determined by both pigments types.
The researchers compared the genomes of 10 Yellow-shafted Flickers, 10 Red-shafted Flickers and 48 hybrids. A genome-wide association (GWA) analysis uncovered several genomic regions associated with the plumage patches. Some genomic regions were connected with multiple traits, whereas others were unique to one particular trait. In total, the researchers identified 112 candidate genes.
An overview of the different plumage patches under investigation (figure a). The researchers took advantage of the full range of phenotypes (figure b-c) to identify genomic regions associated with the coloration of these plumage patches (figure d). From: Aguillon et al. (2021).
Candidate Gene CYP2J19
I will not discuss all 112 candidate genes in detail, but focus on one particularly interesting gene: CYP2J19. This gene resides on chromosome 8 and was significantly associated with the coloration of the wing and tail (which form the characteristic shaft). Loyal readers of this blog might recognize the name of this gene: it also underlies the forecrown coloration in Red-fronted Tinkerbird (Pogoniulus pusillus) and Yellow-fronted Tinkerbird (P. chrysoconus), which I covered in a previous blog post. This gene seems to be one of the most important genes determining red and yellow coloration in birds.
The analyses revealed some connections between carotenoid traits and melanin genes. This is very unexpected as these pigments derive from different biochemical pathways. The researchers offer three possible explanations for this surprising finding:
The melanin genes have pleiotropic effects, meaning that they affect more than one trait. The regulatory genes associated with melanin production might also control the synthesis of carotenoids.
The coloration of some traits is a combination of both pigment types. For example, Yellow-shafted Flickers overlay carotenoids with melanin to produce a black malar stripe.
The melanin genes control the absence of melanin within the feathers. A reduction of this pigment will bring out the red or yellow colors.
More detailed genomic analyses are needed to discriminate between these explanations. Luckily, there are plenty of hybrids to work with.
References
Aguillon, S. M., Walsh, J., & Lovette, I. J. (2021). Extensive hybridization reveals multiple coloration genes underlying a complex plumage phenotype. Proceedings of the Royal Society B, 288(1943), 20201805.
Exploring the consequences of dispersal capacity and demographic stability.
Almost one in four bird species breeds in the Neotropical lowlands. But where did all this diversity come from? Most research on avian speciation in this region focused on geographical barriers, such as the formation of rivers, the rise of mountains, or the fragmentation of forest habitats during Pleistocene. Apart from these extrinsic factors, however, the ecology of birds can also play a role. In a previous blog post, for example, I described how the habitat preference of certain species determined the likelihood of crossing land bridges and colonizing islands in Southeast Asia. Forest specialists needed forested areas to disperse and could not transverse the land bridges which consisted mostly of open vegetation. Generalist species, on the other hand, did use the land bridges and travelled freely between islands.
A similar reasoning can be applied to diet. Several studies on Neotropical birds found that species that eat the reproductive parts of a plant (e.g., fruits, seeds, flowers) have greater dispersal abilities than species that feed on arthropods. This relationship between diet and dispersal can probably be explained by the seasonal availability of the food sources. In the tropics, arthropods are present year-round, so insectivores do not have to move far to forage. Fruits and flowers, however, show a seasonal pattern, forcing frugivores and nectivores to disperse more in search of food. Moreover, the dynamics of food resources will have an influence on the demographic changes of the species: stable food availability will lead to stable populations, whereas fluctuating food sources will result in fluctuating populations. This all sounds very logical, but can we detect the consequences of these processes in the genetic make-up of a bird species?
Genetic Divergence
A recent study in the journal Ecology Letters used this information to make some predictions about the level of genetic divergence in Neotropical bird species. The year-round availability of arthropods and the limited dispersal capacity of insectivores should lead to clear genetic differences between populations with this diet. The situation for frugivores and nectivores – with unstable food sources and higher dispersal rates – will occasionally result in population expansions and consequent gene flow might reduce levels of genetic differentiation. The researchers tested these predictions by estimating genetic divergence in 56 Neotropical bird species (using the mitochondrial gene ND2) and correlating this estimate with several ecological traits. Based on literature, they determined the diet, forest use (interior vs. forest edge) and vertical stratum (canopy or understory) of the species. In addition, stable isotope analyses of nitrogen were used to get an independent measure of diet for a subset of species.
Statistical analyses revealed that diet was the main determining factor in predicting genetic divergene. The researchers write that “Birds species consuming plant products such as fruit, seeds and nectar have significantly less mitochondrial divergence between Belize and Panama than species consuming solely arthropods and species with mixed arthropod- and plant-based diets.” Exactly as expected based on the reasoning I explained above. Isn’t it wonderful when a hypothesis is supported by the data?
Statistical analyses indicated that frugivores have significantly lower genetic divergence than insectivores and mixed diet species. From: Miller et al. (2021) Ecology Letters.
Demography
But what about the stability of the populations: are insectivorous populations more stable than frugivores and nectivores? To test this hypothesis, the researchers calculated population growth rates using the software LAMARC. One of the outputs from this analysis – the statistic R2 – indicates population expansion. This statistic was significant for 12 of the 20 (60%) frugivore and nectivore populations, while only 6 of the 25 (23%) insectivore populations showed a significant population expansion. Similarly, the statistic g (a measure of population dynamics based on coalescent theory) pointed to more expanding populations in frugivores and nectivores compared to insectivores. These patterns are in line with the population dynamics we can expect based on the availability of the food sources.
Putting it all together, this study nicely shows how demographic fluctuations and differences in dispersal capacity associated with a particular foraging ecology have predictable effects on levels of genetic divergence. In other words, ecology matters.
References
Miller, M. J., Bermingham, E., Turner, B. L., Touchon, J. C., Johnson, A. B., & Winker, K. (2021). Demographic consequences of foraging ecology explain genetic diversification in Neotropical bird species. Ecology Letters, 24(3), 563-571.
On the Pacific archipelago of New Caledonia, you can observe an excellent example of tool use in birds. The local crow species – the New Caledonian Crow (Corvus moneduloides) – collects and manufactures tools for foraging. This behavior has resulted in several morphological adaptations, such as a straight bill to better handle the tools and large eyes to facilitate coordination during tool use. In addition, some studies reported unusually large brains in these birds, probably associated with increased cognitive abilities (although other studies could not replicate these findings). For a long time, it was thought that New Caledonian Crows were the only natural tool users in the genus Corvus. Until the Hawaiian Crow (C. hawaiiensis) showed its tool-using skills to the scientific community. These two species are not closely related, suggesting that tool use evolved convergently. An excellent opportunity to unravel the genetic basis of this behavior. A recent study in the journal Molecular Ecology sequenced the genomes of these species (along with 10 other crow species) to tackle this challenge.
Positive Selection
A large team of international researchers joined forces and used several approaches to detect genes under positive selection in the New Caledonian Crow and the Hawaiian Crow. One approach (the dN/dS ratio) relies on the ratio between nonsynonymous and synonymous substitutions. As I explained in a previous blog post, nonsynonymous substitutions lead to a change in the protein sequence (i.e. another amino acid) whereas synonymous substitutions do not due to the redundancy in the genetic code. A higher number of nonsynonymous substitutions suggests positive selection and it apparent in a dN/dS ratio larger than one. The second approach – the McDonald-Kreitman test – uses a similar calculation by comparing the amount of variation within a species to the divergence between species at sites with synonymous and nonsynonymous substitutions. Finally, the third approach focuses on selective sweeps, i.e. the situation in which a genetic variant is beneficial and increases in frequency. The resulting selection event leads to a reduction in genetic diversity in the genomic region where this beneficial variant resides. Several summary statistics, such as Tajima’s D and Fay & Wu’s H, can pick up these signatures of selection.
The first two approaches uncovered 26 genes under positive selection (12 in the dN/dS ratio test and 14 in the McDonald-Kreitman test). These genes play a role in the development of the brain, the nervous system and the eye. The selective sweep search pointed to 11 genomic regions that contained 350 genes, some of which are known to be involved in the evolution of bill morphology (e.g., CALM1). A nice set of candidate genes that require further research.
An overview of the phylogeny and distributions of the Corvus species in the study. The tool using species are highlighted in yellow. From: Dussex et al. (2021) Molecular Ecology.
Reference Genomes
Despite the detection of several genes under putative positive selection, the researchers remain careful and write that “genetic changes associated with tool use in crows appear subtle.” Most positively selected genes seem to be involved in morphological changes in the beak and the eye (as indicated above). It is possible that other important adaptations – such as cognition – are not due to changes in protein-coding genes, but rather related to differences in gene expression (see for example this recent study). Moreover, some adaptive changes might be underpinned by several genes with small effects, making them harder to detect.
The analyses also revealed an important methodological issue. The use of different reference genomes – either the New Caledonian Crow or the Hooded Crow (C. cornix) – resulted in the detection of different candidate genes under selection. This finding highlights the danger of using a single reference genome in population genomic analyses. Indeed, another recent paper in Molecular Ecology showed how the choice of a reference genome can significantly impact analyses on demography and genetic diversity. Hence, the researchers of the crow study provide some important advice: “Ideally, reference genomes should be assembled for each species under consideration along with population genomic data to also account for within-species variation.” A few years ago, such an approach would seem unaffordable. However, the genomic resources for avian studies are accumulating rapidly. And so are the tools to use them.
References
Dussex, N., Kutschera, V. E., Wiberg, R. A. W., Parker, D. J., Hunt, G. R., Gray, R. D., Rutherford, K., Abe, H., Fleischer, R. C., Ritchie, M. G., Rutz, C., Wolf, J. B. W. & Gemmell, N. J. (2021). A genome‐wide investigation of adaptive signatures in protein‐coding genes related to tool behaviour in New Caledonian and Hawaiian crows. Molecular Ecology, 30(4), 973-986.
We have a pretty good idea when and where the Chicken (Gallus gallus) was domesticated, although this history has been muddled by hybridization with several wild species. For other domesticated bird species, things are not so clear. Take the Helmeted Guineafowl (Numida meleagris) for example. Charles Darwin proposed that this species was domesticated in East Africa, whereas some archeological finds suggests West Africa. Luckily, we can turn to genomic data to resolve this issue. By reconstructing the evolutionary history of the Helmeted Guineafowl and characterizing the genetic make-up of different domestic populations, we can hone in on its most likely origin of domestication. A recent study in the journal Genome Biology and Evolution used a high quality reference genome to tackle this challenge.
Gene Flow
Based on 89 domestic and 34 wild Helmeted Guineafowl genomes, Quan-Kuan Shen and colleagues determined the phylogenetic relationships between different populations across the world. It turned out that the domestic clades were most closely related to birds from Nigeria. However, this result could be influenced by recent gene flow. Perhaps the Helmeted Guineafowl were domesticated in the east (Sudan and Kenya) and later interbred with western birds in Nigeria. To account for the possible effects of gene flow, the researchers tested several demographic scenarios. The most likely model indicated a single domestication event in West Africa about 5500 years ago. Apparently, Darwin was wrong about the Helmeted Guineafowl.
A phylogenetic analysis of more than 100 genomes revealed that domestic birds are most closely related to wild birds from Nigeria in western Africa. From: Shen et al. (2021) Genome Biology and Evolution.
Italian Breeds
Estimates of genetic diversity revealed that all domestic populations lost a significant amount of genetic variation. A common consequence of domestication with strong human-induced selection pressures. The lowest levels of genetic diversity were observed in the Italian populations which comprise two breeds: Camosciata and Selvatica. The first breed is known for its peculiar white plumage. The researchers screened the genome of this breed for signatures of selection to pinpoint promising candidate genes underlying this particular phenotype. Their analyses uncovered several genes, including TYR which is known to account for white plumage in chickens. A more detailed look at this gene revealed a mutation at position 218 (a tryptophan became a glycine) that was fixed in the Camosciata breed. This gene is linked to several other genes under selection (e.g., PGR and MMP13) and might thus influence multiple traits in these Italian birds.
An overview of genes under selection in the Italian breeds Selvatica and Camosciata. The white plumage in the Camosciata breed can be traced back to a mutation in the TYR-gene (indicated with a star in the alignment). From: Shen et al. (2021) Genome Biology and Evolution.
References
Shen, Q. K., Peng, M. S., Adeola, A. C., Kui, L., Duan, S., Miao, Y. W., … & Zhang, Y. P. (2021). Genomic Analyses of Unveil Helmeted Guinea Fowl (Numida meleagris) Domestication in West Africa. Genome Biology and Evolution, 13(6), evab090.
Mapping patterns of gene flow across numerous islands.
The study of island biogeography is full of fascinating concepts and questions. For example, I have covered the “paradox of the great speciator” in previous blog posts, mainly using white-eyes (genus Zosterops) as an example. This paradox relates to the observation that some lineages have the ability to disperse over many islands, but still manage to speciate despite their impressive colonization capacity. Why does regular gene flow not counteract the formation of new species? You can read some possible solutions to this paradox here. But in this blog post, I want to focus on another, related concept: “supertramps”. This term was coined by Jared Diamond and refers to vagile species that repeatedly colonize remote islands, but do not speciate due to high levels of gene flow.
When members of supertramps colonize an island, they will not evolve specialized traits because continuous gene flow will impede local adaptation. This might result in a seemingly random pattern of these traits across islands. Ernst Mayr noticed such a pattern in the body size of the Polynesian Wattled Honeyeater (Foulehaio carunculatus) and proposed two possible explanations. Perhaps the body size distribution follows Bergmann’s Rule with species of smaller size residing in warmer regions. Or the small island populations are subject to genetic drift, leading to random changes in body size. A recent study in the journal Molecular Phylogenetics and Evolution revisited the ideas of Mayr and used a genomic approach to reconstruct the evolution of the Wattled Honeyeater species complex.
Gene Flow
Phylogenetic analyses of more than 4000 ultraconserved elements revealed nine distinct lineages within the Polynesian Wattled Honeyeater. Estimates of gene flow pointed to some genetic connectivity between adjacent islands, but not more distant ones. These genetic patterns are not as pronounced as you would expect for a supertramp species. It seems that these birds do not disperse over water that frequently. The current levels of genetic differentiation across the archipelago reflect a balance between dispersal capacity (helping the birds reach remote islands) and sedentary behavior (allowing for some genetic differences to evolve). This particular balance is just one possible combination on a continuum from extreme supertramps to sedentary species.
Map depicting gene flow between different populations. Solid black arrows between island or archipelago groups indicate statistically supported gene flow by multiple analyses. The dotted line was only supported by one analysis (the RAxML tree). The gray arrow indicates a marginally supported gene flow event between Lau and Samoa. From: Mapel et al. (2021) Molecular Phylogenetics and Evolution.
Island Rule
But what about the body size conundrum that occupied Ernst Mayr? The researchers detected no phylogenetic signal in the distribution of body sizes, which supports the explanation that the random variation in this trait is mainly due to genetic drift. However, the analyses also revealed that body size tends to increase with decreasing island area. This pattern is consistent with the island rule, which predicts that smaller species might increase in size on small islands due to lack of competition with other species. A nice observation that opens many avenues for further research.
References
Mapel, X. M., Gyllenhaal, E. F., Modak, T. H., DeCicco, L. H., Naikatini, A., Utzurrum, R. B., Seamon, J. O., Cibois, A., Thibault, J.-C., Sorenson, M. D., Moyle, R. G., Barrow, L. N. & Andersen, M. J. (2021). Inter-and intra-archipelago dynamics of population structure and gene flow in a Polynesian bird. Molecular phylogenetics and evolution, 156, 107034.
A closer look at captive hybrids between the Chiloé Wigeon and the Philippine Duck.
The Avian Hybrids Project officially started in 2015 with the publication of a short paper in Ibis. My main goal was to gather the scientific literature on avian hybridization in one place. Later on, I started publishing blog posts on a wide range of bird-related topics, summarizing many scientific publications. The blog post you are currently reading is number 400. An important milestone for the Avian Hybrids Project. To celebrate this achievement, I decided to cover one of my own recent papers that was recently published in the journal Ecology and Evolution. In this paper, Jan Harteman and I describe some interesting hybrids between the Chiloé Wigeon (Mareca sibilatrix) and the Philippine Duck (Anas luzonica).
Plumage Patterns
Let’s start from the beginning. In the spring of 2020 Jan contacted me with an intriguing story. A female Chiloé Wigeon had mated with a male Philippine Duck, producing four hybrids. He asked me if these hybrids would be interesting to study in more detail. We decided to let them grow up to see how their plumage patterns would develop. And we were in for a surprise! Chiloé Wigeon and Philippine Duck are both sexually monochromatic (i.e., both sexes look alike). But the hybrids showed clear sexual dichromatism. The males exhibited the iridescent green head pattern of the Chiloé Wigeon, whereas the females developed the dark crown and eye stripe of the Philippine Duck.
This observation triggered me to dive into the literature and learn more about the genetic and developmental mechanisms of sexual mono- and dichromatism in ducks. It turns out that the showy male plumage is the default state in both sexes, while the production of estrogen culminates in the development of cryptic female-type plumage. The plumage patterns in the hybrids were probably the outcome of different levels of estrogen production. Chiloé Wigeon and Philippine Duck belong to different branches on the evolutionary tree, diverging about 13 million years ago. It is thus likely that sexual monochromatism arose independently in these species, with different modifier genes controlling the production of estrogen. More detailed genetic analyses will be needed to identify these genes.
In addition, the hybrids formed two pairs, of which one pair produced a clutch of six unfertilized eggs. It is difficult to pinpoint the exact reason for infertility of these eggs. One of the sexes (or both) might be sterile, or fertilization was unsuccessful due to genetic incompatibilities between sperm and egg cells. Another interesting question to explore further.
This study takes me back to the basis of the Avian Hybrids Project: documenting hybridization in birds. In the current scientific climate of big grants and fancy genomic tools, it is easy to forget about the simple description of interesting observations. Other notable examples include the pairing between between a Cerulean Warbler and a Black-throated Blue Warbler in Indiana (see this blog post) or the reports of several fairywren hybrids (see this blog post). The disdain for such descriptive studies by some researchers was nicely illustrated by one of the reviewers who wrote that “there is not really a whole lot here.” Luckily, the other reviewer and the editor were enthusiastic about our work, resulting in a swift acceptance.
In addition, this paper highlights the importance of reporting avian hybrids in the scientific literature. If Jan had not contacted me, these hybrids might have gone unnoticed. Getting an overview of all known avian hybrids is a daunting task. Eugene McCarthy has produced a nice overview of hybridization in birds with his “Handbook of Avian Hybrids of the World”. However, several hybrid records in this book are unreliable and require further investigation (a quest that I have recently started with by checking the reliability of tinamou hybrids). The description of hybrids in peer-reviewed papers is sorely needed to obtain a better overview of the incidence of hybridization in birds. And who knows? Maybe some hybrids might even provide some crucial insights into fundamental scientific questions, such as the evolution of sexual mono- and dichromatism in ducks.
References
Ottenburghs, J. & Harteman, J. (2021) Sexually dichromatic hybrids between two monochromatic duck species, the Chiloé wigeon and the Philippine duck. Ecology and Evolution.
A guest post by Cody Porter on a recent study in The American Naturalist.
How do new bird species form? Clearly, there are many dimensions to this question, but a nearly universal requirement for bird speciation is geographic isolation. Most avian sister species, especially those that diverged recently, have largely or entirely non-overlapping geographic ranges. Furthermore, sister species of birds rarely co-occur in small, circumscribed areas like oceanic islands. This and other lines of evidence suggest that most bird populations need to be physically separated for long periods of time for speciation to occur. It is probably no accident that Ernst Mayr, who vehemently argued against the possibility of sympatric speciation for most of his career, was an ornithologist.
In some respects, this is a strange phenomenon. Classic theoretical models indicate that there are two basic requirements for sympatric speciation: ecological divergence and assortative mating. There is a wealth of evidence for both criteria in birds, including groups in the early stages of divergence (Darwin’s finches are an obvious example, but far from the only one). So why do all lines of evidence point to allopatric speciation as being responsible for the diversity of Darwin’s finches and most other birds and vertebrates?
Bills and PineCones
An important caveat to the aforementioned models is that most assume reproduction coincides with periods of strong performance tradeoffs associated with alternative resources. If this occurs, ecologically divergent populations may become “automatically” reproductively isolated from each other. For example, populations might breed in different habitats where alternative food resources occur that each is adapted to. This occurs in some organisms such as some herbivorous insects, where sympatric host races reproduce on different host plants (e.g., apple and hawthorn races of Rhagoletis pomonella). Perhaps unsurprisingly, the strongest evidence for sympatric speciation generally comes from herbivorous insects. By contrast, most birds (indeed, most vertebrates) time reproduction to coincide with abundant food resources that multiple species can easily exploit. For example, during the breeding season, different species of Darwin’s finches converge in their use of arthropods — a very different situation than in the nonbreeding season, when each specializes on a different subset of resources. These patterns suggest that weak tradeoffs and resulting ecological convergence during the breeding season may preclude sympatric speciation in many organisms. But can we test this hypothesis directly?
This was the central goal of my dissertation research on the Red Crossbill (Loxia curvirostra) complex. Previous work has shown that bill morphology of sympatric ecotypes has diversified in response to tradeoffs in the ability to efficiently feed on different conifers. For example, the western hemlock crossbill has a tiny bill that allows it to rapidly remove seeds from the small cones of Western Hemlock (Tsuga heterophylla). On the other hand, the large bill of ponderosa pine crossbills are unwieldy when feeding on hemlock, but is great for prying open the large, woody cones of Ponderosa Pine (Pinus ponderosa), which western hemlock crossbills cannot feed on. During most breeding seasons, sympatric ecotypes primarily feed on their respective “key conifers” that impose such tradeoffs. Every few years or so, however, conifers with easily accessible seeds (e.g., Engelmann Spruce, Picea engelmanni) produce large cone crops that multiple ecotypes feed on during breeding. We thus have multiple ecotypes breeding in sympatry in some years when there are strong feeding tradeoffs and other years when there are weak feeding tradeoffs: exactly the conditions necessary to test those classic theoretical models. Moreover, because most crossbills are nomadic, birds are constantly dispersing long distances and opportunistically breeding during different resource conditions in different locations. Thus, we have something akin to a natural experiment in crossbills.
This video initially shows two western hemlock crossbills feeding on western hemlock cones. The bird on the right consumed three seeds in 10.78 seconds. The larger, lone bird later in the clip is a ponderosa pine crossbill feeding on western hemlock cones. This bird consumed just one seed in 10.48 seconds, meaning its feeding rate is ~66% lower than that of the western hemlock crossbills.
Reproductive Isolation
My colleagues and I collected data on two sympatric ecotypes (Lodgepole Pine and Ponderosa Pine crossbills) that co-occur throughout the Rocky Mountains. Based on data from 10 breeding seasons, we found striking support for those classic sympatric speciation models. When feeding tradeoffs are strong, reproductive isolation between ecotypes is nearly complete, which is necessary for sympatric speciation. Specifically, the two ecotypes largely breed in different forest types where their respective conifers dominate and are thus unlikely to encounter each other as potential mates. Moreover, the few “migrants” that do attempt to breed in the “wrong” habitat type struggle to reproduce, further reducing opportunities for mating between ecotypes. We even found that birds are more likely to mate assortatively when tradeoffs are strong. Playback experiments and lots of data on flock composition indicate that this arises because crossbills are more likely to flock assortatively when tradeoffs are stronger. Because crossbills choose mates from within flocks, stronger assortative flocking leads to stronger assortative mating. Interestingly, previous work has shown that crossbills gain feeding efficiency benefits by assortative flocking only when tradeoffs are strong, thus explaining the flocking data. When feeding tradeoffs are weak, all of these barriers to gene flow are much weaker – not nearly strong enough for sympatric speciation to occur. We even found that resource availability (for example, how abundant food resources are) affects reproductive isolation, with higher availability leading to lower isolation. This latter result suggests important extensions to those classic theoretical models.
These data may help explain why some groups, like herbivorous insects, are more prone to speciating in sympatry than others, such as most birds. It is worth noting that there may be some crossbill lineages that have undergone sympatric speciation (e.g., the Cassia Crossbill, L. sinesciuris). What’s interesting is that these crossbills specialize on extremely stable seed resources that do not fluctuate from year-to-year. Thus, unlike most crossbills, these lineages never experience periods of abundant food and weak tradeoffs. This likely explains why barriers to gene flow in such systems are remarkably strong and stable from year-to-year, which in turn may explain why these lineages are genomically divergent from sympatric ecotypes.
We took advantage of nomadism and opportunistic breeding by two crossbill ecotypes to determine how feeding tradeoffs affect reproductive isolation. A) Both ecotypes breed in forests of mixed lodgepole and ponderosa pines. The dotted curve represents the relationship between performance and bill size on lodgepole pine and the solid curve represents the relationship on ponderosa pine. In mixed pine forests, tradeoffs are large. B) In forests of spruce the two ecotypes have more similar feeding abilities, reflecting weak tradeoffs. The locations where these crossbills breed vary yearly, because of spatiotemporal variation in cone crops that crossbills track with nomadic movements.
This blog post was written by Cody Porter. Check out more of his research on his personal website. Want to write a guest post for the Avian Hybrids Project? Get in touch with me!
References
Porter, C.K. & Benkman, C.W. (2021) Performance tradeoffs and resource availability drive variation in reproductive isolation between sympatrically diverging crossbills. The American Naturalist.
A recent study uncovers four genetic lineages in this widespread species.
During my postdoc in Sweden, I visited the Färnebofjärden National Park, hoping to catch a glimpse of the elusive White-backed Woodpecker (Dendrocopos leucotos). This large woodpecker has a wide distribution, stretching from western Europe to Japan. It has been divided into a dozen morphological subspecies. One of these subspecies – the Amami Woodpecker (owstoni) – is sometimes treated as a separate species, based on its dark plumage. However, morphology is just one aspect in the classification of (sub)species. Are these subspecies also genetically distinct? A recent study in the journal Zoologica Scripta attempted to answer this question by sequencing the DNA of 70 individuals across the range of the White-backed Woodpecker.
Distribution of the White-backed Woodpecker. Colored dots represent sampling locations of different subspecies. From: Pons et al. (2021) Zoologica Scripta.
Four Lineages
The genetic analyses – using three nuclear genes and the mitochondrial gene COI – revealed four distinct lineages. The first split in the evolutionary history of the White-backed Woodpecker occurred about 0.8 million years ago and gave rise to a Chinese lineage that contains two subspecies (tangi and insularis). A third subspecies that was not sampled in this study (fokhiensis) likely belongs to this group. The second split (ca. 0.5 million years ago) separated a southern group (represented by the subspecies lilfordi) from a northern cluster. Later on, this northern cluster was subdivided into a Eurasian (leucotos and uralensis) and a Japanese lineage (namiyei, subcirris, stejnegeri and owstoni). These patterns reveal that the Amami Woodpecker (owstoni) is not genetically differentiated from the other three Japanese subspecies, questioning its species status.
A haplotype network illustrating the four main lineages within the White-backed Woodpecker. From: Pons et al. (2021) Zoologica Scripta.
Genetic Diversity
I can imagine that this bombardment of Latin names can be confusing. So, let’s take a step back and look at the genetic patterns in some of these lineages. In the haplotype network above, you can see one line connecting the Japanese group (in yellow) with the Northern Eurasian lineage. This suggests that Japan was colonized only once by the White-backed Woodpecker, after which it diverged into the different subspecies. On the Eurasian mainland, there is one main haplotype (the big circle in the middle) that can found in Russia and Mongolia. Low genetic diversity across a large range points to a recent population expansion. Most likely, the Eurasian populations can be traced back to a glacial refugium. In contrast, the southern lineage (lilfordi) shows higher levels of genetic diversity. The researchers attribute this pattern to the fragmented distribution of these birds where each mountain population (Pyrenees, Abruzzi and Caucasia) has its own private haplotype. These findings do not only provide more insights into the evolutionary history of the White-backed Woodpecker, but can also guide conservation efforts to preserve high levels of genetic diversity.
References
Pons, J. M., Campión, D., Chiozzi, G., Ettwein, A., Grangé, J. L., Kajtoch, Ł., Mazgajski, T. D., Rakovic, M., Winkler, H. & Fuchs, J. (2021). Phylogeography of a widespread Palaearctic forest bird species: The White‐backed Woodpecker (Aves, Picidae). Zoologica Scripta, 50(2), 155-172.
Avian Hybrids 