What is the difference between hard and soft selection?

An interesting perspective on the nature of natural selection.

You might be surprised to read that my employer – Wageningen University – houses several creationists. Their scientific work generally focuses on non-evolutionary topics, such as nitrogen deposition, but that does not prevent some colleagues from commenting on evolutionary biology. One professor, for example, said the following in an interview: “Humans not only share 96 percent of their DNA with monkeys, but also 88 percent with mice, while according to the theory of evolution humans are not descended from mice.” This statement clearly shows that he has no clue how evolution works. Humans are not directly descended from monkeys or mice, but share a common ancestor with both. And because the common ancestor of humans and monkeys is more recent than the one of humans and mice, we share more genetic material with monkeys. This “critique” on evolution actually contains supporting evidence for the fact that all life on our planet evolved. Unfortunately, this professor is blinded by religious dogma and cannot approach evolutionary questions unbiased. Any discussion is thus useless (see for example this blog post).

Another common creationist argument is that “random mutations and natural selection” are insufficient to explain the diversity of life. However, evolution is so much more than “random mutations and natural selection” (otherwise, evolutionary textbooks would be quite short). There are several other evolutionary processes, such as genetic drift and sexual selection, to consider. In addition, the term natural selection covers a whole range of possible selective mechanisms, such as directional selection or balancing selection. In fact, I recently came across an Ecology Letters paper on the concepts of hard and soft selection. An interesting framework that I had not thought about yet. So, in this blog post, I will try to explain the difference between these two types of selection.

Population Genetic Paradox

The concepts of hard and soft selection were introduced by Bruce Wallace to resolve an apparent paradox in population genetics. In the 1960s, evolutionary biologists assumed that any given environment has one optimal genotype. Any other genotypes in the population would thus have a lower fitness compared to this optimal genetic combination. This reasoning implies that populations with high levels of genetic variation will have a low mean fitness. When researchers were able to quantify genetic diversity in wild populations, they were surprised to find high levels of variation (see for example Hubby and Lewontin 1966). These observations were at odds with the proposed relationship between genetic variation and mean fitness. To resolve this paradox, Wallace introduced the concept of soft selection where the fitness of an individual is not quantified relative to an optimal genotype, but against the fitness of its conspecifics.

Bears and Hares

The theoretical explanation might make your head spin, so let’s use an example to clarify these concepts. Imagine a population of bears that are competing for a certain number of caves for hibernation. The bears are either aggressive or submissive. And obviously, aggressive bears will outcompete the submissive ones. When there are enough caves for all bears, there will be no selection. However, once there are more bears than caves, selection will occur. Now, the selection dynamics will depend on the behavioral composition of the population. In a situation with few aggressive bears, some submissive bears will be able to secure a cave and will have a higher relative fitness. But when there are more aggressive bears, the submissive bears will be outcompeted and will have a lower relative fitness. The fitness of the submissive bears thus depends on the amount of aggressive bears in the population. This is soft selection.

To understand how hard selection is different, consider another example: predation on snowshoe hares. The risk of predation depends on the coat color of the individual. Better camouflaged hares will survive better. The relative fitness of the different coat colors does not depend on the composition of the population. Badly camouflaged individuals – such as a white hare in a brownish landscape – will have a lower chance of survival regardless of the number of well-camouflaged conspecifics. This situation would result in hard selection.

Two examples to illustrate soft and hard selection. In the bear example, the fitness of the submissive individuals depends on the behavioral composition of the population. In the hare example, however, the fitness of badly camouflages (white) snowshoe hares is independent of the number of well-camouflaged individuals. From: Bell et al. (2021) Ecology Letters.

Slushy Selection

These two examples only scratch the surface when it comes to hard and soft selection. Bruce Wallace developed a population genetic model to explore the strength of selection in different scenarios. And numerous authors have expanded on these ideas. Some have argued that hard and soft selection are just two extremes on a continuum, while others have suggested that both types of selection might operate on the same traits (i.e. slushy selection). Diving into these details and exploring the ecological and evolutionary impacts of hard versus soft selection would takes us too far down the rabbit (or hare?) hole.

My goal was to introduce these concepts and show that evolution is more complex than “random mutations and natural selection”. If creationists would take to the time to really understand the intricacies of the evolutionary process, they would quickly realize how silly some of their arguments are. More importantly, they might discover how interesting and exciting evolutionary biology is.

References

Bell, D. A., Kovach, R. P., Robinson, Z. L., Whiteley, A. R., & Reed, T. E. (2021). The ecological causes and consequences of hard and soft selection. Ecology Letters24(7), 1505-1521.

Wallace, B. (1975). Hard and soft selection revisited. Evolution, 465-473.

Featured image: Grizzly Bear (Ursus arctos) © Gregory “Slobirdr” Smith | Wikimedia Commons

Genomic evidence for sexual traits as honest indicators of immune function in birds?

Recent study reports correlated evolution of immune and pigmentation genes.

Charles Darwin famously wrote: “The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!” He was referring to the fact that the elaborate tail of this colorful bird could not be explained by his recently published theory of natural selection. How could such a clumsy feature improve the survival chances of a male peacock? Later on, Darwin proposed a solution to this conundrum in another book The Descent of Man, and Selection in Relation to Sex: sexual selection. According to this mechanism, males compete for access to females, either directly through male-male competition (think of the antlers of male deer) or indirectly by advertising themselves with beautiful songs and extravagant feathers.

But how do females make a choice? Some authors have argued that females pick a partner based on aesthetic preferences; females just select what they “like”. Richard Prum has defended this neutral model of sexual selection in his book The Evolution of Beauty. Alternatively, the elaborate traits of males are honest signals that females use to discriminate between males with “good” and “bad” genes. One particular hypothesis – proposed by William Hamilton and Marlene Zuk – suggests that sexual traits indicate the immune function of a bird. Males that can easily fend off parasites will have plenty of energy left to develop extravagant feathers, while males infected with parasites will look drab and sickly. A recent study in the journal Frontiers in Ecology and Evolution tested this Hamilton-Zuk hypothesis with genomic data.

Purifying Selection

Shubham Jaiswal and his colleagues used a set of eleven high-quality bird genomes to gain more insights into the genomic basis of sexual selection. First, they estimated the strength of sexual selection for each species, making a distinction between pre-copulatory selection (i.e. females picking a partner) and post-copulatory selection (i.e. sperm competition). Pre-copulatory selection was estimated by scoring the degree of sexual dimorphism: the more different males and females look, the stronger sexual selection. For example, the Indian Peafowl (Pavo cristatus) scored high for this index, while the Budgerigar (Melopsittacus undulatus) had the lowest score. Post-copulatory selection could be assessed through the ratio of testis to body weight. Species with fierce sperm competition need to produce more sperm cells and will probably have bigger testes.

These two measures of sexual selection – sexual dimorphism and testis weight – were consequently correlated with several parameters of gene evolution, such as substitution rates and estimates of selection. The analyses resulted in a set of 60 candidate genes that are potentially targets of sexual selection. Interestingly, most of these genes are involved in the regulation of gene expression and some are known to coordinate the development of sexual dimorphism. Additional tests of selection indicated that the majority of these genes are subject to purifying selection, the removal of (slightly) deleterious genetic variants. This evolutionary model is in line with the Hamilton-Zuk hypothesis where males with inferior immune systems are selected against.

An overview of the genomic resources used in this study (Figure A) and an example of variation in sexual dichromatism scores (Figure B). From: Jaiswal et al. (2021) Frontiers in Ecology and Evolution.

Correlated Evolution

The observation of purifying selection on the candidate genes might make sense within the framework of the Hamilton-Zuk hypothesis. However, it is not direct evidence for this controversial idea. The most convincing piece of evidence came from a second set of analyses, namely patterns of correlated evolution between different genes. The researchers identified 228 genes that showed significant signs of correlated evolution and had well-defined functional annotations. Within this network of correlated evolution, many gene pairs were involved in immunity and feather development or pigmentation. Based on these findings, the researchers noted that:

[This] provides a “mechanistic link” or a connection between genome and phenotypic coevolution which in such cases would include plumage color and other secondary sexual characters responsible for sexual selection and honest signaling. Therefore, the Hamilton-Zuk explanation for the persistence of variation in the phenotypes of sexual selection as a consequence of the arms-race between parasite and immune genes is substantiated by this study.

However, it is important to keep in mind that these are correlations. Remember the age-old warning: correlation is not causation. These candidate genes are a promising starting point for future research, but we should not jump to conclusions. There is still much to learn about the genetic basis of sexual selection, whether it involves the immune system or not.

A network representation of gene pairs showing patterns of correlated evolution (Figure A). And a section of this network where only the immune-related, feather-related, and pigmentation-related are shown (Figure B). From: Jaiswal et al. (2021).

References

Jaiswal, S. K., Gupta, A., Shafer, A., PK, V. P., Vijay, N., & Sharma, V. K. (2021). Genomic Insights Into the Molecular Basis of Sexual Selection in Birds. Frontiers in Ecology and Evolution, 2.

Featured image: Indian Peafowl (Pavo cristatus) © Gabriel Castaldini | Wikimedia Commons

Social bonds and migratory behavior determine the likelihood of hybridization in birds

What behavioral mechanisms underlie these associations?

Sometimes the title of a paper says it all: “Sociality and migration predict hybridization across birds.” In a recent study in the Proceedings of the Royal Society B, Gavin Leighton and his colleagues correlated the incidence of hybridization across birds with several life history traits. These analyses revealed that bird species with long-lasting social bonds are less likely to hybridize. In addition, a migratory lifestyle increases the likelihood of hybridization. Some interesting findings that require more exploration. What behavioral mechanisms could be driving these associations between sociality, migration and hybridization?

Pair Bonds and Migration

Species with no pair bonds will look for a new partner every year, increasing the opportunities for making the “wrong” choice and hybridize. Species with long-lasting social bonds, on the other hand, will often pair for several mating seasons. This reproductive strategy can lead to substantial fitness costs if an individual mates with the “wrong” species. The resulting hybrids might be sterile or unviable, culminating in a significant waste of reproductive effort. Such maladaptive hybridization can lead to increased selection for proper species recognition. An interesting exception to this result – which might prove the rule – concerns my own study system: geese. Most goose species tend to form long-lasting pair bonds, but still engage in regular hybridization. The production of hybrids has been attributed to several behavioral mechanisms, such as forced extra-pair copulations, interspecific brood parasitism and vagrant individuals (see this paper for an overview).

The last mechanism related to goose hybridization – vagrant individuals – leads into the next main finding of the study: migratory species are more likely to hybridize. To again focus geese, migratory goose species might have a higher chance of reaching new habitats where they interbreed with local species. For example, North American Snow Geese (Anser caerulescens) are occasionally observed in Europe during migration and hybrids between Snow Goose and several European species have indeed been reported. An additional explanation relates to mate choice in sedentary species: these birds might have extended periods of mate searching and pair formation, lowering the probability of choosing a partner from another species.

Phylogenetic distribution of hybridization (grey, inner circle), social bond length (green, middle circle), and migratory status (blue/yellow, outer circle). From: Leighton et al. (2021).

Future Research

Clearly, there are multiple mechanisms to explain the associations uncovered in this study. The details might differ across the avian phylogeny, but the broad-scale importance of sociality and migration seems justified. This macroevolutionary perspective opens new exciting avenues for further research, disentangling how these life history traits (and probably many other factors) interact to determine the probability of hybridization.

A final note on the dataset. The researchers assembled a dataset of bird hybrids using the Handbook of Avian Hybrids of the World, indicating that they took “a liberal approach and included any reported hybridization where an individual was identified by sight as a putative hybrid.” I completely understand this pragmatic choice as it is nearly impossible to assess the reliability of each hybrid combination mentioned in the Handbook of Avian Hybrids of the World. However, I have become more skeptical about the list of bird hybrids that Eugene McCarthy has produced (see for example this blog post). I do not think that the inclusion of less reliable hybrids has affected the analyses in the present study, but I do encourage ornithologists to be careful in using this book in their future work. Always check the original source before including a dubious hybrid record.

References

Leighton, G. M., Lu, L. J., Holop, E., Dobler, J., & Ligon, R. A. (2021). Sociality and migration predict hybridization across birds. Proceedings of the Royal Society B288(1947), 20201946.

Featured image: Greylag Goose (Anser anser) x Canada Goose (Branta canadensis) © Dirk Ottenburghs

The benefits and drawbacks of living on an island

If you were a bird, would you want to live on an island?

Island populations have played a key role in the development of evolutionary theory. I guess everyone is familiar with the scientific work on Darwin’s Finches – including groundbreaking work by Charles Darwin himself, David Lack, and more recently Peter and Rosemary Grant. Other examples include convergent evolution of anole lizards on tropical islands and the adaptive radiation of honeycreepers on Hawaii. Evolution on islands can take surprising turns. Just think of the extinct giant swan (Cygnus falconeri) on Sicily and Malta or the flightless Kakapo (Strigops habroptilus) of New Zealand. Studying evolution on islands is certainly worthwhile, but what about actually living on island? Do organisms that colonize island environments do better or worse than their mainland counterparts? The answer to that question depends on the aspect you focus on, as nicely illustrated by some recent studies in Biology Letters and Current Biology.

Adult Survival

Let’s start by looking at the individual level. Using a dataset of no less than 697 bird species, Guy Beauchamp investigated whether adult birds survive better on islands. He first needed to correct for certain confounding factors. Birds with a larger body size tend to have higher survival rates. And due to the well-known trade-off between reproduction and survival, a lower clutch size is often associated with better survival. Finally, species with a cooperative breeding system survive better as adults. After controlling for these factors, Beauchamp still found that “birds living on islands showed higher apparent survival than their mainland counterparts.” The exact mechanisms behind the higher survival rate on islands remain to be determined but might be related to the absence of predators and parasites on islands. So, from an individual perspective, it would be a good idea to settle on an island.

Apparent survival of adult birds is higher on islands (blue lines) compared to the mainland (red line). This pattern holds across latitudes and on both hemispheres. From: Beauchamp (2021).

Slightly Deleterious Mutations

Adult survival is thus higher on islands. But the situation is quite different on a population level. Thibault Leroy and his colleagues compared the effective population size (Ne) – a measure of genetic diversity – between songbirds on islands and on the mainland. Based on whole-genome sequences of almost 300 individuals, representing 25 species, they found that “island species exhibit significantly lower mean Ne than continental species over the last one million years.” This reduction in effective population size has serious consequences for evolutionary dynamics on islands. According to the nearly neutral theory of molecular evolution, small populations will accumulate slightly deleterious mutations due to the stronger effect of genetic drift. And indeed, the researchers found more slightly deleterious mutations in island species compared to mainland species. Hence, on a population level, island birds are genetically less healthy than their mainland relatives.

Island species (filled circles) have smaller effective population sizes and more slightly deleterious alleles (estimated by the πNS ratio) compared to their mainland counterparts. From: Leroy et al. (2021).

Paradox

These two studies appear to contradict each other. How can island birds live longer when the populations accumulate slightly deleterious mutations? The answer to this paradox is probably related to the interaction between the deleterious mutations and the island environment. As you might have guessed from the term, these mutations are only slightly deleterious. Individuals might experience some inconvenience from these mutations, but they are not deadly and they often do not impact individual survival rates that much. In addition, the absence of certain predators and parasites on islands leads to weaker selective pressures, potentially allowing island birds to live longer.

However, a sudden change in the environment or the introduction of a predator can quickly increase selective pressures on the island population. The lower level of genetic diversity and the higher mutational load of island populations might then prevent rapid adaptation, potentially resulting in extinction. Indeed, numerous island species have perished in recent times. You could say that life on an island is great until the environment changes. And unfortunately, the environment is currently changing rapidly due to human activities.

References

Beauchamp, G. (2021). Do avian species survive better on islands?. Biology Letters17(1), 20200643.

Leroy, T., Rousselle, M., Tilak, M. K., Caizergues, A. E., Scornavacca, C., Recuerda, M., … & Nabholz, B. (2021). Island songbirds as windows into evolution in small populations. Current Biology31(6), 1303-1310.

Featured image: Blue Chaffinch (Fringilla teydea) © Bartkauz | Wikimedia Commons

Body size and phylogenetic history constrain the evolution of bird song

Although there seems to be a minor role for sexual selection as well.

Different bird species produce sounds across a wide range of frequencies. From the low-frequency booming calls of the Eurasian Bittern (Botaurus stellaris) to the high-pitched sounds of the Common Firecrest (Regulus ignicapilla). Ornithologists have formulated several hypotheses to explain the evolution of these frequency differences. The Acoustic Adaptation Hypothesis, for instance, suggests that acoustic signals are adapted to maximize the effectiveness of their transmission in a particular environment. Because low-frequency sounds travel better in dense vegetation than high-frequency sounds, bird species in forests are expected to produce calls at lower frequencies because of physical constraints. The Morphological and Phylogenetic Constraint Hypothesis, however, states that body size and/or evolutionary history limit the range of frequencies that a species can reach. Larger species tend to produce lower-frequency sounds. Finally, the Sexual Selection Hypothesis posits that frequency might act as an indicator of an individual’s size, dominance or fighting ability. Low-frequency sounds give the impression of a larger individual and might thus be selected for in the context of male-male competition.

Explaining Variation

To discriminate between these possible explanations, a recent study in the journal Ecology Letters took a closer look at the songs of more than 5000 songbirds. The researchers used the public database xeno-canto to collect information on the highest frequencies in different bird songs. Next, they correlated these peak frequencies with several evolutionary and ecological variables. The analyses revealed that evolutionary history (43-56%) and body mass (10-15%) explained the largest proportion of variation in peak song frequency. Heavier birds sang at lower frequencies. After accounting for these two factors, there was also a significant association with sexual size dimorphism (although the explained variation was very low: 1-3%). Peak frequencies were lower in species where males are larger than females, suggesting strong sexual selection. Song frequency might thus be a proxy for the competitive ability of males.

These patterns indicate that phylogenetic history and body size constrain the range of frequencies that a species can produce. Within the consequently limited range of frequencies, sexual selection appears to influence the peak frequency in particular species.

Most of the variation in song peak frequency was explained by body mass. Sexual size dimorphism accounted for a small fraction of the remaining variation, while there was no clear effect of tree cover. From: Mikula et al. (2021) Ecology Letters.

No Acoustic Adaptation?

Interestingly, there was no support for the Acoustic Adaptation Hypothesis. Although this hypothesis makes intuitive sense, there was no clear effect of tree cover (a proxy for habitat density) in the large dataset. The researchers propose several explanations for this negative result:

  • The analyses lacked information about bird behavior or habitat at the recording location. These factors could influence the produced frequency.
  • The estimates of habitat density were quite crude. Better measurements might pick up subtle signals.
  • Other biotic and abiotic factors that might influence background noise were not included in the analyses.

More research is thus needed to explore how important acoustic adaptation is in the evolution of bird song. Nonetheless, phylogenetic constraints seem to restrict the available evolutionary pathways (similar to genomic constraints in hybrid species). Evolution is a powerful process, but that doesn’t mean that anything is possible.

References

Mikula, P., Valcu, M., Brumm, H., Bulla, M., Forstmeier, W., Petrusková, T., Kempenaers, B. & Albrecht, T. (2021). A global analysis of song frequency in passerines provides no support for the acoustic adaptation hypothesis but suggests a role for sexual selection. Ecology letters24(3), 477-486.

Featured image: European Robin (Erithacus rubecula) © Charles J. Sharpe | Wikimedia Commons

Rampant introgression across the evolutionary tree of suboscine birds

An extensive genomic study detects numerous cases of interspecific gene flow.

Introgression – the exchange of genetic material through hybridization and backcrossing – seems to be an integral part of avian evolution. In 2017, I published a review paper on this phenomenon, trying to provide an overview of the known cases and the methods to detect introgression. At the time, my search uncovered 165 genetic studies that were published between 1987 and 2017. Most of these papers focused on members of the Passeriformes (songbirds), Galliformes (gamefowl), Anseriformes (waterfowl) and Charadriiformes (gulls, waders and auks), bird orders that display high levels of hybridization. However, I did not attempt to quantify the incidence of introgression, because this would probably be a gross underestimate. Numerous bird species have not been studied with genetic tools yet. Such as quantification would involve a large genomic data set, comprising many species. And that is exactly the approach of a study that recently appeared in the journal Evolution Letters.

D-statistics

Sonal Singhal and her colleagues focused on the suboscines, a clade of passerine birds that comprises more than 1000 species, most of which can be found in South America. Using a genomic dataset of 2389 loci (consisting of ultraconserved elements and exons) for 1306 species, the researchers looked for signatures of introgression. They relied on one of my favorite methods: the D-statistic (also known as the ABBA-BABA test). I have explained this approach so many times that I regularly dream about it (and no, the dream does not involve ABBA-songs). Here is the short explanation from my review:

The D-statistic considers ancestral (‘A’) and derived (‘B’) alleles across the genomes of four taxa. Under the scenario of incomplete lineage sorting without gene flow, two particular allelic patterns ‘ABBA’ and ‘BABA’ should occur equally frequent. An excess of either ABBA or BABA, resulting in a D-statistic that is significantly different from zero, is indicative of gene flow between two taxa.

The researchers calculated this statistic for numerous trios of species. They found that 49 out of 130 trios (38%) had a significant D-statistic. The highest five D-statistics were detected in Dendrocolaptes woodcreepers, Smithornis broadbills, Neopelma manakins, Grallaria antpittas, and Phlegopsis antbirds. I have described hybridization in some of these taxa on their family pages at Avian Hybrids Project (you can click the links if you want to learn more).

An example of the ABBA-BABA test for a trio of species (plus an outgroup). From: Singhal et al. (2021) Evolution Letters.

An Underestimate?

To understand the mechanisms underlying the introgression patterns, the researchers correlated the D-statistics with several environmental variables. These analyses revealed some expected relationships. For example, introgression was most common between species that were geographically close and that experienced higher rates of climate change (which moves species around and might lead to secondary contact). In addition, introgression occurred more between species at lower latitudes. It could be that tropical regions provide more opportunities for introgressive hybridization due to the higher species richness in these areas. It would be interesting to repeat these analyses with more species-specific traits to better understand the drivers of introgression in suboscine birds.

Although an introgression incidence of 38% seems quite high, I think it is probably even an underestimate. The analyses only included trios of closely related species and might thus have missed introgression between more distantly related species. Moreover, the D-statistic does detect gene flow between sister species. It only reports only gene flow between the third species and one of the first two in the phylogenetic tree. The researchers did also not take ghost introgression into account: the exchange of genetic material with unsampled or extinct lineages (see my recent review on this topic for more details). And finally, the analyses involved only a subset of the genome. Who knows how many introgressed regions are hiding in the rest of the genome?

An overview of significant D-statistics (highlighted in orange) across the phylogeny of the suboscines. From: Singhal et al. (2021) Evolution Letters.

References

Singhal, S., Derryberry, G. E., Bravo, G. A., Derryberry, E. P., Brumfield, R. T., & Harvey, M. G. (2021). The dynamics of introgression across an avian radiation. Evolution letters5(6), 568-581.

Featured image: Green-and-black Fruiteater (Pipreola riefferii) © Francesco Veronesi | Wikimedia Commons

Does sexual selection promote speciation?

Male-male competition might be a driving factor of speciation in birds.

When I write that “sexual selection speeds up speciation”, you might nod your head in agreement. It sounds logical that sexual selection on certain traits (e.g., song or plumage color) can contribute to reproductive isolation, culminating in the origin of new species. However, several studies that approximated the strength of sexual selection as the degree of sexual dichromatism (i.e. the differences in color between males and females) found no supporting evidence for this hypothesis (see Huang & Rabosky 2014 and Cooney et al. 2017). There could be numerous explanations why these studies failed to detect a connection between sexual selection and speciation rates. Perhaps sexual dichromatism is not a reliable proxy for the strength of sexual selection? Or maybe certain environmental factors influence the relationship between sexual selection and diversification? Indeed, theoretical models suggest that sexual selection can speed up local adaptation in variable environments. A recent study in the journal Evolution revisited this evolutionary conundrum and investigated the link between sexual selection and speciation rates, taking into account environmental factors.


Measures of Sexual Selection

Justin Cally and his colleagues collected data on more than 5800 passerine species. They estimated sexual selection in two ways: (1) the degree of sexual dichromatism and (2) an index of male-biased sexual selection. This index is associated with information on sexual size dimorphism, social polygyny (i.e. male with multiple females) and the lack of parental care. For readers interested in the technical details, the index corresponds to the first principal component of a phylogenetic PCA that is correlated with the three features listed above. The two measures of sexual selection were then correlated with speciation rates across the phylogeny of the passerines. In addition, the researchers accounted for the possible effects of several ecological and environmental variables, such the seasonality of temperature and rainfall.

An overview of the parameters used in this study: the researchers tested for a relationship between diversification rate (in green) and two measures of sexual selection: sexual dichromatism (in brown) and male-biased sexual selection (in grey). From: Calley et al. (2021) Evolution.

Male-male competition

The analyses revealed that “the composite index of male-biased sexual selection, but not measures of sexual dichromatism, is correlated with the rate of speciation in passerine birds.” These findings are in line with previous studies that found no relationship between sexual dichromatism and diversification. The authors of the current study argue that this measure is not a good proxy for sexual selection, because males and females can evolve different plumage patterns for other reasons, such as adaptation to distinct ecological niches (see for example the Sulawesi Babbler, Pellorneum celebense).

A detailed look at the positive effect of the male-biased sexual selection index indicated that sexual size dimorphism was the main driver. Differences in size mostly evolve due to male-male competition in which males compete for access to females. Because body size is often associated with other morphological and ecological traits, it could speed up the divergence between young evolutionary lineages. In fact, studies on mammals suggested that the evolution of large body size allowed for the diversification of ecological strategies, leading to higher speciation rates. Similar processes might have occurred in some bird groups. These patterns highlight the potential role of male-male competition in generating diversity. Sexual selection can thus promote speciation, but maybe not in the way we might have expected.


References

Cally, J. G., Stuart‐Fox, D., Holman, L., Dale, J., & Medina, I. (2021). Male‐biased sexual selection, but not sexual dichromatism, predicts speciation in birds. Evolution75(4), 931-944.

Featured image: Superb Fairywren (Malurus cyaneus) © Benjamint444 | Wikimedia Commons

Simultaneous wing molt as a pre-adaptation for loss of flight

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.

Featured image: Titicaca Grebe (Rollandia microptera) © Tsirtalis | Wikimedia Commons

Ecology matters: Diet predicts genetic divergence in Neotropical birds

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 Letters24(3), 563-571.

Featured image: Blue-gray Tanager (Thraupis episcopus) © Mike’s Birds | Wikimedia Commons

How the Pleistocene glacial cycles drove the evolution of Arctic shorebirds

An extensive study of 69 species highlights the role of glacial and interglacial periods.

During my PhD on the evolution of geese (see this blog post for a summary), I came across the work of Pieter Ploeger. In 1968, he published an extensive overview on the distribution of arctic ducks and geese during the last ice age. Using a diverse set of data, he tried to pinpoint the areas where these birds resided during the last glacial maximum. The results of this exercise made intuitive sense, but were difficult to test at the time. The development of Species Distribution Models (SDMs) has allowed researchers to reconstruct the past distribution of species and test biogeographical hypotheses, such as the ones put forward by Ploeger. A recent study in the Journal of Biogeography used this approach to investigate how the glacial cycles during the Pleistocene affected the distribution and consequent evolution of arctic shorebirds.

Four Scenarios

Angel Arcones and his colleagues focused on 69 shorebird species and tested four scenarios that could explain the current morphological and genetic patterns. The first scenario (scenario A) assumes that the observed variation might predate the Pleistocene and the glacial cycles had thus no effect on the distribution of these species. The other three scenarios do entail an effect of the Pleistocene glacial cycles, but differ in timing. Populations could become isolated during the warmer interglacial periods (scenario B), the colder glacial periods (scenario C), or both (scenario D). To discriminate between these possibilities, the researchers used Species Distribution Models to determine the distribution of these shorebirds during the last glacial maximum (about 20,000 years ago).

The researchers made a distinction between species that show little morphological or genetic variation (i.e. monotypic species) and species that do. The results revealed that most of the monotypic species (over 65%) did not experience range fragmentation during the last glacial maximum. The more variable species, on the other hand, did show signatures of range fragmentation (62%). The most likely scenarios underlying these fragmentated distributions were roughly equally represented. These patterns confirm the idea that the Pleistocene glacial patterns have shaped the current morphological and genetic patterns in several arctic-nesting species.

The percentage of distributions of the monotypic shorebird species (orange) and species with subspecies (blue) that are explained by the four scenarios. From: Arcones et al. (2021) Journal of Biogeography.

Palearctic and Nearctic

A more detailed look at the results showed that the patterns differ between regions. In Beringia and the eastern Palearctic, climatic conditions were more stable and this area remained largely ice-free. Species residing in this region were thus less likely to be fragmented during the ice ages. The situation is drastically different in the western Palearctic and the Nearctic. Here, huge ice sheets extended to lower latitudes, pushing bird populations into several refugia in the south. The climatic differences between these regions need to be taken into account when reconstructing the evolutionary history of the local bird species.

All in all, this study highlights the importance of considering both the effects of glacial and interglacial periods in the evolution of shorebirds. And it emphasizes the significant climatic differences between biogeographical regions.

An overview of the different biogeographical regions. The Palearctic (red) and Nearctic (green) are most relevant here. © Carol | Wikimedia Commons.

References

Arcones, A., Ponti, R., Ferrer, X., & Vieites, D. R. (2021). Pleistocene glacial cycles as drivers of allopatric differentiation in Arctic shorebirds. Journal of Biogeography48(4), 747-759.

Featured image: Red Knot (Calidris canutus) © Hans Hillewaert | Wikimedia Commons