Is prezygotic isolation more important than postzygotic isolation at the onset of speciation?

A simulation study casts doubt on this common statement.

“Ethological barriers to random mating constitute the largest and most important class of isolating mechanisms in animals.” If you like bold, in-your-face proclamations, you can always turn to the work of German biologist Ernst Mayr. He never shied away from strong statements, such as the one at the start of this blog post. In his book Animal Species and Evolution, he argued that speciation in animals usually begins with behavioral differences. Members from different populations prefer familiar faces and rarely interbreed. This behavior – assortative mating – generates a first genetic barrier between the populations. Later in the speciation process, selection against the occasional hybrid can arise in the form of sterility or unviability.

To put this argument in the modern jargon of speciation research: prezygotic isolation is more important than postzygotic isolation in the initial stages of speciation. If you are not familiar with these terms, I will briefly define them. Luckily, I can quote from the introduction of my PhD thesis for a concise explanation.

Prezygotic isolation mechanisms act before fertilization, whereas postzygotic isolation mechanisms act after fertilization and can be either intrinsic or extrinsic. Intrinsic postzygotic isolation mechanisms lead to sterility or unviability of the offspring, while extrinsic postzygotic isolation mechanisms encompass lower fitness of the offspring for ecological or behavioral reasons, not developmental defects.

The idea that prezygotic isolation is more important than postzygotic isolation makes intuitive sense. First, there is a reduction in gene flow between populations due to behavioral differences. Next, the populations follow different evolutionary paths and accumulate genetic incompatibilities. When the populations establish secondary contact, potential hybrids are sterile or unviable due to the many genetic mismatches. Makes sense, right? But if there is one thing I have learned during my short scientific career, it is to be careful with intuitive ideas. Evolutionary biology is often counterintuitive.

 

Simulating Clines

A recent study in the journal The American Naturalist put this idea to the test. Darren Irwin performed some simulations comparing different levels of assortative mating and postzygotic isolation (i.e. hybrid fitness). To assess the impact of these processes on speciation, he turned to cline theory. Loyal readers of this blog might be familiar with this mathematical framework, but to get everyone on the same page I will outline the main concepts of cline theory (based on a previous blog post).

Let’s say you have a white species and a black species that produce gray offspring in a hybrid zone. You observe birds along a transect and note down their plumage color. When you put the data in the graph, you will see a transition from white birds (when you were in the habitat of the white birds) through grayish birds (in the hybrid zone) to black birds (in the black bird habitat). The useful aspect of cline theory is that the shape of a cline can tell you something about the biology of the birds. For example, if the gray hybrids interbreed with their parental species, there will be a variety of backcrosses of different colors. Some more white and some more black, depending on the species they crossed with. This will result in a smooth transition from white through different (perhaps 50) shades of gray to black. In other words, a wide cline. However, if gray birds cannot find a mate, there will be mostly gray hybrids in the contact zone. This will result in a rapid transition from white to black plumage, a steep cline.

Darren Irwin applied this approach to his simulations. A wide transition with many backcrosses indicates that reproductive isolation is weak and there will be gene flow between the pure populations. A steep cline, however, points to strong reproductive isolation. What kind of cline does assortative mating produce?

An example of a cline plot. There are only white birds to the left of the hybrid zone and black ones on the right side. In the hybrid zone there are different shades of gray birds.

 

Genetic Bridge

Let’s start by comparing two simple scenarios: (1) modest selection against hybrids (10% reduction in hybrid fitness) and (2) modest assortative mating (female is 10 times more likely to pick a partner from her own species). Running the first scenario produces a steep cline because hybrids are less fit than their parents and rarely backcross. This result confirms the so-called tension zone model that was described by Nick Barton and Godfrey Hewitt in the 1980s. The narrow hybrid zone is maintained by a balance between pure individuals moving into the contact zone and selection against unfit hybrids.

The second model – with modest assortative mating – produces a much wider cline. Because there is no selection against hybrids, they can interbreed with each other and their parental species. These dynamics result in a variety of hybrids and backcrosses that form a genetic bridge between the initial parental populations. If gene flow is sufficiently high, the two populations might even merge into one large population.

The outcomes of simulating hybrid zone dynamics. A reduction in hybrid fitness (i.e. postzygotic isolation) results in a steep cline (in green), while modest assortative mating produces a wide cline (in purple). Adapted from: Irwin (2020) The American Naturalist

 

Search Costs

These initial findings indicate that modest assortative mating cannot jumpstart speciation. However, further analyses uncovered some conditions in which assortative mating leads to a narrow, steep cline, namely (1) when assortative mating is encoded by a single locus, (2) when assortative mating is very strong, and (3) when the cost of searching for a mate is high.

The first condition – assortative mating is encoded by a single locus – is not very realistic. Genomic studies have shown that reproductive isolation is mostly encoded by multiple genes (but see this case of single gene speciation in snails). The other two conditions – strong assortative mating and high mate searching cost – are actually a form of postzygotic isolation because they impact the mating chances of hybrids. Indeed, if assortative mating of the parental populations is strong, the hybrids won’t be able to find a mate.

The effect of search time is more complex. In the simulations, search time was modelled as the cost a female pays when she rejects a potential mate. A high search cost thus leads to females that quickly choose a mate. This is at the disadvantage of hybrids because they are rare in the population. In addition, hybrid females in the center of the hybrid zone will most likely mate with other hybrids. This lowers the likelihood of backcrossing with the parental populations and the consequent formation of a genetic bridge.

Results of simulations with and without mate search costs. Each graph shows the relationship between assortative mating (on the x-axis) and hybrid fitness (on the y-axis). The colors indicate the width of the cline (on the left) and the probability of a bimodal hybrid zone (i.e. two distinct populations, on the right). Including mate search costs leads to narrower clines (more yellow in the left figure) and a higher probability of distinct populations (more yellow in the right figure).

 

Postzygotic Isolation

These simulations suggest that assortative mating is less important in speciation than we think. But it’s just a modelling exercise, some of you might say. How realistic are these results? Well, several studies have tried to estimate the strength of assortative mating in wild populations. The highest estimate by Christophe Randler – based on 58 avian hybrid zone studies – points to  an assortative mating strength of 2.6. This is considerable lower than the modest strength of 10 used in the simulations. Of course, this strength might vary between hybrid zones, but it seems that assortative mating is rarely strong enough to keep populations separate.

This study thus indicates that assortative mating on its own cannot prevent populations from merging, some form of postzygotic isolation is needed (e.g., lower mating chances for hybrids). The intuitive idea that “prezygotic isolation is more important than postzygotic isolation” does not hold here. Why did this statement become so popular? The main reason is probably the focus on extreme forms of postzygotic isolation (i.e. hybrid sterility and unviability) that take long to evolve. We should not forget other postzygotic isolation mechanisms, such as sexual selection against hybrids. This insight opens up new research opportunities, nicely summarized at the end of the paper: “While assortative mating, unless perfect or very nearly so, is ineffective on its own in maintaining isolation of two species, the effects of sexual selection and sexual signals on postzygotic isolation are likely strong and worthy of renewed research focus.”

 

References

Irwin, D. E. (2020). Assortative mating in hybrid zones is remarkably ineffective in promoting speciation. The American Naturalist, 195(6), E150-E167.

Featured image © Zorba the Geek | CC-BY-SA-2.0 Wikimedia Commons

 

A common species with a complex history: The evolutionary story of the Great Tit

Genetic study uncovers five distinct groups within this widely distributed songbird.

One of most studied bird species is a taxonomic mess. The widely distributed Great Tit complex (Parus major) has been divided into 43 subspecies and ornithologists are still debating where to draw species limits between all these subspecies. In the Handbook of Birds of the World, you will find only one species, while the IOC World Bird List recognizes three distinct species: Great Tit (P. major), Japanese Tit (P. minor) and Cinereous Tit (P. cinereus). These taxonomic disputes are interesting to follow, but I prefer to focus on the evolutionary history of these birds (as I have explained in previous blog posts, such as here and here). However, an unstable taxonomy often indicates that something interesting is going on. A recent study in the Journal of Biogeography took a closer look at the Great Tit species complex and uncovered some peculiar patterns.

A Great Tit in Italy © Banellino | Wikmedia Commons

 

Five Groups

The researchers collected no less than 340 samples from 67 geographic populations. They sequences several genes: the mitochondrial cytochrome b (Cytb) and NADH dehydrogenase subunit 2 (ND2) genes, as well as the nuclear β-fibrinogen intron 5 (Fib5) and the transforming growth factor beta 2 intron 5 (TGFB2).

Analyses of the mitochondrial genes revealed five main groups. The Northern and Western Eurasia group contains individuals from western Europe, the Iberian Peninsula and North Africa, as well as populations from across the north part of Eurasia all the way to the Russian Far East. Taxonomically, this large group corresponds to the nominate major. The remaining four groups are scattered across Asia. The Central Asia group consists of individuals from Uzbekistan, Tajikistan and western China. The Eastern Asia group includes birds from East Asia and Southeast Asia, including China, South Korea, Japan, the Indochinese Peninsula, Malaysia and Java. The Eastern Himalaya group spans the eastern portion of the Himalayas and corresponds to the subspecies tibetanus and subtibetanus. Finally; the Southern Asia group houses individuals from Sri Lanka, the Indian Subcontinent, and parts of Afghanistan. The contrast between Western Europe (one group) and Asia (four groups) probably reflects the topographic complexity around the Himalayas with a diverse selection of habitats, providing ample opportunity for populations to diversify.

Distribution of the five main mitochondrial groups within the Great Tit species complex. From: Song et al. (2020) Journal of Biogeography

 

Taxonomic Grey Zone

Interestingly, analyses of the nuclear genes could not resolve the phylogenetic relationships between the populations. This suggests that the mitochondrial groups diverged recently – about 1.5 million years ago according to their calculations – and that the nuclear variation has not been divided over these groups yet (a phenomenon known as incomplete lineage sorting). Moreover, some populations might still be connected by occasional gene flow. For example, birds of the major and minor groups are known to hybridize in the Amur River area in the Russian Far East and China.

This pattern of clear mitochondrial groups and lack of nuclear population structure is common in Eurasian bird species that originated in the last few million years. In my own work, I have found similar patterns in the Bean Goose complex, where Taiga (Anser fabalis fabalis) and Tundra Bean Goose (A. f. serrirostris) started diverging about 2.5 million years ago and occasionally interbreed. These cases provide evolutionary biologists with the exciting opportunity to study speciation in action, but complicate taxonomic decisions. Because speciation is still ongoing, these populations end up in a taxonomic grey zone, often resulting in subjective decisions and a proliferation of subspecies.

A Japanese Tit in Osaka © Laitche | Wikimedia Commons

 

An Afterthought

While reading this paper, I realized that the authors used four very common genetic markers (the mitochondrial Cytb and ND2, and the nuclear Fib5 and TGFB2). If you browse through the ornithological literature on phylogenetics and taxonomy, you will often come across these markers. This suggests that a large part of our knowledge of avian evolutionary history is based on a tiny fraction of their genomes. Although these findings are sound and insightful, who knows how much we are missing?

My feeling that there is still much to discover with genomic data was strengthened when I read the book “Who We Are and How We Got Here” by David Reich. This book provides an overview of the recent progress in research on human evolution using genomics and ancient DNA. The findings are mind-blowing, especially the discovery of so many ghost populations that went extinct but left their genetic signatures in present-day populations (a fascinating concept that I recently covered in this BioEssays paper). It makes you wonder how many Great Tit ghost populations have wandered across Eurasia.

 

References

Song, G. et al. (2020). Great journey of Great Tits (Parus major group): Origin, diversification and historical demographics of a broadly distributed bird lineage. Journal of Biogeography.

Featured image © Francis C. Franklin | CC-BY-SA-3.0 Wikimedia Commons

 

This paper has been added to the Paridae page.

How and when did Frogmouths cross Wallace’s Line?

Analyses of mitochondrial genomes might provide the answer.

Evolutionary biologists love to quote Charles Darwin in their papers. I could also not resist the temptation in a review paper on hybridization in geese. But there are so many other brilliant naturalists with insightful and beautiful quotes. Here is Alfred Russel Wallace in a paper from 1855.

“The facts proved by geology are briefly these: that during an immense, but unknown period, the surface of the earth has undergone successive changes; land has sunk beneath the ocean, while fresh land has risen up from it; mountain chains have been elevated; islands have been formed into continents, and continents submerged till they have become islands; and these changes have taken place, not once merely, but perhaps hundreds, perhaps thousands of times.”

I choose this sentence for a reason, because today’s blog post revolves around biogeography and – more importantly – Wallace’s Line. If you would travel from Borneo to Sulawesi, you will notice a significant change in animal diversity. In Borneo, the animals are related to Asian species, while on Sulawesi you will also encounter Australian animals. The line separating the biogeographical biotas of Asia and Australia was drawn by Alfred Russell Wallace. Most bird species did venture across this line, but there are some exceptions. A recent study in the journal Biology Letters figured out when frogmouths (family Podargidae) crossed the line.

A camouflaged Sunda Frogmouth (Batrachostomus comutus) in Indonesia © Melindra12 | Wikimedia Commons

 

Mitochondrial DNA

Frogmouths are a species-poor group of birds, represented by 13 extant species. Interestingly, these species can be found on both sides of Wallace’s Line. On the west of the line, you can spot several Batrachostomus species, but you will have to visit the east side of the line to see species of the genus Podargus. The peculiar geographic distribution raises an obvious question: when did the frogmouths cross Wallace’s Line?

To solve this mystery, Paul Oliver and his colleagues sequenced the complete mitochondrial genomes of the Sunda frogmouth (Batrachostomus cornutus), the Solomons frogmouth (Rigidipenna inexpectata) and all three Podargus species. Analyses of these sequences indicated that the initial divergence of frogmouths across Wallace’s Line probably occurred between 44 and 27 million years ago. But how did they get there?

Analyses of the mitochondrial genomes resolved the phylogenetic relationships (figure b) and provided an estimate of when these species diverged (figure c). From: Oliver et al. (2020) Biology Letters

 

Stepping stones

Geological studies of this time period (the mid-Oligocene) suggest that there were no islands directly between Asia and Australia that could have functioned as stepping stones. So, how did the frogmouths manage to cross Wallace’s line and reach Australia? The researchers suspect that the birds used a southwestern detour via the Pacific island arcs to exit the Asian region. There, they persisted for some time before colonizing Australia. This scenario is supported by the distribution of the Solomons frogmouth, which resides on the northern Solomon Islands. Perhaps other animals have followed the same route?

 

References

Oliver, P. M., Heiniger, H., Hugall, A. F., Joseph, L., & Mitchell, K. J. (2020). Oligocene divergence of frogmouth birds (Podargidae) across Wallace’s Line. Biology Letters, 16(5), 20200040.

Featured image on top: A tawny frogmouth (Podargus strigoides) in Australia © Benjamint444 | Wikimedia Commons

Inferring introgression: Genomic study on hybridizing Darwin’s Finches highlights the importance of field observations

Explaining patterns of introgression required a thorough knowledge of the study system.

Yesterday I assisted in a field course on habitat analysis for ecologists. The students would visit an field site and explore different aspects of the ecosystem. In my section, we would walk through forest plot and try to identify common Dutch tree species. Due to the Corona-measures, most students had learned about these species in an online course, without hands-on experience in the field. And it showed. Some students struggled to identify the species at first. But once they knew which traits to focus on, they managed to identify most species correctly. This experience highlights the importance of fieldwork.

During my postdoc in Sweden, most of my colleagues worked on the genetics of the black-and-white flycatcher system: pied flycatcher (Ficedula hypoleuca) and collared flycatcher (F. albicollis). To my surprise, some colleagues had not seen these species in the wild and seemed uninterested in the natural history of these beautiful birds. They preferred to focus on abstract genetic concepts (which is also interesting). But how can you interpret the genetic data when you don’t know the ecology of the species? A recent paper in the journal Nature Ecology & Evolution illustrates the importance of knowing the ins and outs of your study system.

The pointed beak of the cactus finch (Geospiza scandens) © Mike’s Birds | Wikimedia Commons

 

Geospiza Finches

When I say “Peter and Rosemary Grant”, you will probably say “Darwin’s Finches”. Indeed, the Grants are known for their long-term study of these small passerines on the Galapagos Islands. On the island of Daphne Major, they documented hybridization between medium ground finch (Geospiza fortis) and cactus finch (G. scandens). Their meticulous study revealed that these species are converging morphologically: the long beaks of G. scandens became blunter and the robust beaks of G. fortis became more pointed. The change of beak morphology was greater in G. scandens, suggesting that genes are primarily flowing from G. fortis into G. scandens.

A recent genomic study confirmed this suggestion and went one step further. Sangeet Lamichhaney and his colleagues – including the Grants – compared the patterns of genetic exchange (i.e. introgression) for different parts of the genome. The genetic analyses pointed to extensive introgression of the autosomes (i.e. any chromosome that is not a sex chromosome) and the mitochondrial DNA, but not of the Z-chromosome.

The genetic analyses indicated introgression of the autosomes and of mtDNA, as shown by the position of SLB (G. scandens with blunt beak). On the Z-chromosome, this group of birds clusters with the other G. scandens samples. (Red = G. scandens, Blue = G. fortis). From Lamichhaney et al. (2020) Nature Ecology and Evolution

 

If you would show this result to my genetics-focused colleague in Sweden, she might attribute it to genetic incompatibilities on the sex chromosomes. And indeed, numerous other studies have found strong selection on sex-linked genes, contributing in reproductive isolation (check out this review on sex chromosomes and speciation). In this case, however, the ecology of the species is important. The field observations provided some crucial insights.

All female finches, including hybrid daughters, preferentially mate with males that sing the same song as their fathers’ song: mate choice is based on the imprinting of offspring on the parental morphology and song. The net result of this pattern of mating is the introgression of mtDNA and autosomal genes but few Z chromosomes from G. fortis to G. scandens. Hybrid females from these matings carry a G. scandens Z chromosome and cannot introgress any G. fortis Z chromosome. Hybrid sons, being relatively small, are at a disadvantage in competition with G. scandens males for high-quality territories and mates.

So, the reduced introgression on the Z-chromosome is not due to genetic incompatibilities, but can be explained by the behavior of the birds. The moral of this story: go into the field before you get into the lab.

 

References

Lamichhaney, S., Han, F., Webster, M. T., Grant, B. R., Grant, P. R., & Andersson, L. (2020). Female-biased gene flow between two species of Darwin’s finches. Nature Ecology & Evolution, 1-8.

 

This paper has been added to the Thraupidae page.

Uncovering cryptic diversity in the lesser short-toed lark species complex

Genetic analyses reveal several distinct lineages within this morphologically uniform bird group.

“Few groups of birds show the same level of disagreement between taxonomy based on morphology and phylogenetic relationships as inferred from DNA sequences.” This statement refers to the bird family Alaudidae: the larks. These brownish birds mostly reside in open habitat where strong selection for camouflage resulted in little morphological differences. Such bird groups can be a taxonomists’ nightmare, especially if the researcher solely focuses on morphological characteristics to pigeon-hole the specimens. Luckily, molecular data can provide some clarity. A recent study in the journal Zoologica Scripta investigated one species complex within the lark family. Does molecular data work where morphology fails?

The short-toed lark, nicely camouflaged in its natural habitat. © Juan Emilio | Wikimedia Commons

 

No Consensus

An international team of researchers took a closer look at the lesser short-toed lark (Alaudala rufescens) species complex. Due to the morphological similarities, the taxonomy of these birds has been heavily debated and up to 16 different taxa have been recognized. Some authors treated all taxa as a single species, while others split it into two species: the lesser short-toed lark and the Asian short-toed lark (A. cheleensis). Because morphological analyses did not lead a consensus, the researchers turned to molecular data. They sequenced one mitochondrial and eleven nuclear genes.

The genetic analyses revealed four distinct lineages within the species complex that separated between 1.6 and 3.2 million years ago. The authors indicate that the complex could be separated into at least four species, namely the heinei clade, the raytal clade, the rufescens clade and the cheleensis + leucophaea clade. The latter two subspecies are grouped together because they could not be confidently separated by the nuclear genes. This cryptic diversity has been masked by the slight plumage differentiation within complex.

Genetic analyses revealed five separate groups that probably represent four distinct species. From: Ghorbani et al. (2020) Zoological Scripta

 

Evolutionary Questions

This study highlights the power of molecular data to uncover cryptic diversity and inform taxonomic decisions. Although it is important to create a solid taxonomic framework, the debates about which taxa should be classified as species or subspecies are secondary to more interesting evolutionary questions. As I have argued before, instead of focusing on minor morphological differences to draw arbitrary lines between “species”, we should try to understand how this diversity originated. In case of these larks, we can unravel the genetic basis of their impressive camouflage. Do the same genes underlie this diversity or are different genes under selection in different taxa. Such questions are more exciting than deciding to call something a species or not. The birds themselves couldn’t care less…

 

References

Ghorbani, F., Aliabadian, M., Zhang, R., Irestedt, M., Hao, Y., Sundev, G., Lei, F., Ma, M., Olsson, U. & Alström, P. (2020). Densely sampled phylogenetic analyses of the Lesser Short‐toed Lark (Alaudala rufescens) – Sand Lark (A. raytal) species complex (Aves, Passeriformes) reveal cryptic diversity. Zoologica Scripta.