Why Evolution is True is a blog written by Jerry Coyne, centered on evolution and biology but also dealing with diverse topics like politics, culture, and cats.
I was just reminded that in 2020 Hari Sridhar interviewed me about what is perhaps my most cited paper (1561 times to date, though my book Speciation with Allen Orr was cited almost six times as often), and certainly one of the few good ideas I’ve had in my life (the paper was also co-written with Orr). You can see the paper by clicking below, and there was a followup paper in 1997 with the word “revisited” tacked on the title below; that was written since new genetic-distance data had appeared.
Here’s the good idea as it came out in the interview:
HS: You were interested in Drosophila and the genetics of Drosophila right from the time of your PhD. What was the motivation for this particular piece of work?
Jerry Coyne: Well, the motivation is implicit in the paper. I was interested in the genetic basis of reproductive isolation in Drosophila. I realized that there were a lot of data out there on the genetic distances between different closely-related species of flies as measured by electrophoresis, and from reading a lot of the old literature – Patterson & Stone (1949, Univ. Texas Publ. 4920: 7-17), and The Genetics and Biology of Drosophila book series ‑that there is an immense amount of data on the crossability of flies, their sexual isolation, the sterility and viability of hybrids. And it came to me one day in Maryland – I can still remember this – that you could combine that different data using electrophoresis as the estimate of divergence time, and then the other parameters as estimates of the degree of reproductive isolation. By doing that, you could get some kind of estimate of the time course over which reproductive isolation evolves. After that, it was just a matter of compiling that data. It took a long time because it’s all in different places – papers, books and stuff. Nobody had thought to put them together before. It was just a matter of compiling the electrophoretic data with the crossability data and then seeing what came out of that. That was the motivation.
one more Q&A:
HS: At the time when you did this work, did you anticipate, at all, the kind of impact it would have on the field? Do you have a sense of what it mostly gets cited for?
JC: Yeah, it gets cited for the reason that we wrote it, actually. Well, two things. First, It gives an idea of the time course of speciation. But also, the result showing that sympatric species get reproductively isolated much more quickly, in terms of pre-zygotic isolation, than allopatric species, was unanticipated. It supports the idea that there’s either reinforcement or reproductive character displacement. I just said, well, let’s look at these data. Then we went back to all the original papers and looked at the ranges to see whether the species lived in sympatry or not. That was a lot of work too because, a lot of the time, range data is not presented as ranges.You have to look at where the flies were captured and, sort of, get an idea of whether the ranges overlapped or not. Those two aspects of the paper were important. Remember, the paper is incomplete because it leaves out a number of forms of reproductive isolation that could be very important in nature, like post-mating pre-zygotic isolation, sperm competition, ecological isolation and temporal isolation. Those aren’t included, because there’s no data. But the support for reinforcement that we showed, the high degree of pre-mating isolation between sympatric species as opposed to allopatric pairs, stimulated, stimulated, I think, work on reinforcement. Even in my own laboratory, my student, Daniel Matute, worked on reinforcement, I think, partly because of the data from this original paper. So it had a number of influences on the field. I don’t know how important it is. It’s a novel approach. It’s one that you can’t really us with most species because of the lack of crossability data. There have been a few other studies. Leonie Moyle did a similar study in tomatoes, I think, and Tamra Mendelson did a study on darters collecting information on genetic distance. The problem with darters and all other groups is that you just don’t have the ability to do laboratory crosses that you have in Drosophila. So Tammie was limited to about 12-13 species.
I’m sorry to say that I haven’t kept up diligently with other folks’ followup work, as there are more papers building on this one (e.g. here, here, and here). In general, I think, they’ve supported our main conclusions, especially the cool one that sexual isolation (mate discrimination) appears to evolve more quickly between groups that experience some period of “sympatry” (living in the same area) after speciation has begun. That in turn supports the idea of “reinforcement”: that if there is a reproductive penalty to hybridizing (e.g. producing hybrids that are sterile or weak), natural selection will build up mate discrimination so that the production of hybrids is less likely. (The idea is that you leave more of your genes to future generations when you produce healthy, conspecific hybrids, so any gene that favors mating with your own species will be favored.) And indeed, we found a strong pattern of heightened sexual isolation among species that are sympatric rather than allopatric (“geographically isolated”).
I liked the original idea of using genetic-distance data to figure out the time course of speciation (or rather, aspects of speciation: mate discrimination and hybrid sterility/inviability) because speciation is often very slow and reconstructing the process (and seeing if there are any generalizations to be made) can be done only by using proxies of divergence time, which in our case was the “genetic distance” calculated using gel electrophoresis. As I note in the interview, gel electrophoresis is pretty much dead, and DNA sequencing of fly species is the way to go.
The Berkeley News, the publicity site for the University of California at Berkeley, has a piece out announcing a new book that was published out in December (photo below). Since it was published by the venal and greedy Springer, the hardback of Speciesism in Biology and Culture will cost you only $159.
Click on the screenshot below to read about the book, which comprises nine essays rejecting humans’ view that we are the top and most important species, and that species can be ranked by their “superiority”. With that rejection I wholeheartedly agree. But there’s another theme, too: one which I think is misguided: species aren’t even real. Click below to read the article, which has a summary of the book.
Here are the book’s two themes:
In a new book, a group of scientists and philosophers places part of the blame on an attitude prevalent among scientists and the general public — the false belief that species are uniquely real, and that some species are superior to others.
To the researchers, this is analogous to racism — the fallacious belief that races exist as branches on the tree of life, and that some races are superior to others.
Now I agree that there is no hierarchy of species: we just happen to be the one that evolved a big brain with which we can control all other species, and an organ we can use to pat ourselves on the back as better than others. (A flea doesn’t have the capacity to see itself as superior to other species—but it can suck their blood!).
But the view that species are not “uniquely real” is a gross distortion. Species are far more real and discernible than human “races”, whose demarcation is somewhat subjective although even the “classic” races are not totally invented social constructs (they contain biological information).
If species weren’t real, however, there would be no problem of “the origin of species”, and nature would be a spectrum—a rainbow with no joints between its constituents. Orangutans, gorillas, and humans would all be arbitrary entities: “social constructs”. So would pigeons, starlings, robins, and cardinals. But I’m getting ahead of myself. A few more excerpts about the supposed non-reality of species:
Mishler has argued for decades against considering individual species as the most important grouping, particularly when discussing conservation. [JAC: actually, in the U.S. it is subspecies that are the units that must be conserved.] He laid out his arguments in a 2021 book, What, If Anything, Are Species? ( CRC Press), in which he proposed getting rid of taxonomic rankings altogether, including the binomial system for naming species that is used universally today. [JAC: As you know, this idea hasn’t caught on, nor will it.]
One key reason is that species distinctions are not equivalent across all branches on the tree of life. Bacteria that look identical may vary as much genetically as a dog from a cat, while some birds that live in totally different areas and look different can be nearly identical genetically. On the other hand, lineages — the sequence of organisms that have evolved from one another over millions of years — are consistent across all forms of life.
“Evidence shows that a species of amoeba does not mean the same thing as a species of fungus, animal or anything,” Swartz said. “And if species are not uniquely real, then where does that leave us? Is there anything that means the same thing across the tree of life? The answer to that question is: lineages. These are branches on the tree of life that maintain genealogical connections across time and space. They include children, or descendants, and their parents, or ancestors, on through animals broadly and their distant relatives. Lineages are branches across the tree of life.”
Throwing out the concept of species would eliminate the artificial dividing line that helps justify the belief that some species are more important. Instead, the authors maintain that humans are just one part of a genealogy connecting all living things. This interconnectedness forms an ecological web that sustains the planet and us, and that deserves to be protected equally with humans.
Mishler goes one step further, arguing that lineages should be respected — not for how they can benefit humans, but intrinsically, as part of the web of life. He detests the term “ecosystem services,” which implies that the natural world exists to service humanity.
. . .The authors point out that the standard definition of a species is a population that cannot breed with closely related populations. But Mishler said this definition is muddied by the fact that there is often wide variation within a breeding population; sometimes two separate species can and do successfully interbreed, and some species don’t breed at all.
. . .“Alan Templeton summarized it most succinctly: The trouble with species is too little sex and too much sex,” he said. “There are asexual groups that don’t do sex at all, but still have lineages. And then there are plants, like the orchid, which can just about be crossed with every other orchid, yet they’re bizarrely different from each other. So, reproductive compatibility, while a nice idea, just doesn’t work empirically.”
Species also can evolve because they get separated geographically or ecologically, not because of an inability to breed.
A more natural grouping is by lineage — ancestor-descendant pairs connected across time — or by clade, which consists of all the descendants of a creature.
Mishler and his colleagues have argued for years that species aren’t real, but their views haven’t gained any traction in the biological community beyond those few people who already reject the reality of species. Perhaps that explains this book.
The biological species concept (BSC), used by nearly all evolutionists, including me, is based on reproduction: a species consists of a group of populations whose members can exchange genes with each other, but cannot exchange genes with members of different species—where the different species live in the same area in nature—because there are barriers that impede genetic exchange between different species.
Now the entire first chapter of our book Speciation, by Coyne and Orr, is a defense of the BSC, a discussion of its problems (no, it’s not perfect), and an argument that it’s superior to all other species concepts because it gives us a handle on why organisms in nature don’t form a spectrum (see the Appendix for a discussion of alternative species concepts, including “lineage concepts” mentioned by Mishler).
First, the question of whether species are “real” is the same as the question “is nature a continuum or lumpy?”. That is, when we look at organisms like mammals or birds or trees in one place, do we see a continuum of variation that we can partition only subjectively, or are there discrete entities that are recognized widely as distinct? And for nearly all groups of sexually reproducing organisms, nature is lumpy. You already know this if you try to identify birds or mammals or other sexually reproducing organisms in the wild. We don’t have a spectrum of birds but, in one area, you see a series of discrete types that you can easily identify. Those groups (in one area; see below) are biological species: robins, starlings, pigeons, etc. etc., and are formally recognized with Latin binomials. They are real, and you or Joe or Jill can easily slot what you see into a small number of bird groups—species. The lumpiness of nature in one area is, in fact, THE “species problem”, the problem that, despite the title of his book, Darwin didn’t answer. (He didn’t answer it because he had no knowledge of genetics and therefore no concept of reproductive barriers.) We need to explain why, in one area, we see a number of discrete forms and no intermediates (or only a few, which could be hybrids that are often sterile.)
The answer to the species question is that reproductive barriers, which are many (we have a chapter on each type in Speciation), keep species distinct by preventing any blurring that would occur with gene flow. Though hybridization between species in one area is more common than we used to think, in most groups it is rare, and if the hybrids are sterile or inviable, then they pose no problem for “blurring” species boundaries.
Now some caveats, for the BSC isn’t perfect:
a.) The BSC is meant to apply to sexually reproducing organisms because it’s based on genetic exchange between individuals or the lack thereof. In organisms like bacteria that are largely asexual, you can’t use it easily. Now whether those organisms form clumps as discrete as those seen in sexually-reproducing species isn’t clear: few people are interested in that topic, which I think is important. This issue is discussed at the end of the first chapter of Speciation.
b.) Two groups must usually live in the same place if you are to determine with certainty whether they are members of different species. If they do not form hybrids that are viable and fertile where they co-occur, they are different species. This is true, for example, of the lion and tiger, which used to co-occur in India before the lion was extirpated. They formed no hybrids in nature. (They sometimes do in zoos, but that’s because captivity can eliminate some reproductive barriers that occur in nature, like aversion to mating with other species. I call this the “prison effect”).
c.) If two similar species live in different places, it’s hard to tell if they’re different species or simply different populations of the same species. If you bring them together in the zoo or lab and they do not hybridize, or form sterile or inviable hybrids, then they are different biological species. But if they do form hybrids, even some fertile ones, the question is still unresolved, for, as I said, some true biological species hybridize in captivity but not in the wild. One can only guess in such circumstances. This kind of guessing is what biologists do when they designate very similar populations that live in different areas as “subspecies”. The “zoo or lab” tests are one-way: they can tell you that populations living in different areas are members of different species, but can’t tell you for sure that they’re members of the same species.
d.) Speciation is a process, usually occurring between geographically isolated populations of a single species. With no possibility of gene exchange, these populations begin to genetically diverge due to various processes like natural selection, sexual selection (a subset of natural selection), genetic drift, and so on. If that divergence occurs to the point that, when the different populations re-establish geographical contact, they do not exchange genes, then full speciation has occurred. But it need not occur: there are many time when populations aren’t isolated long enough to become reproductively isolated, and in that case they can re-establish contact and exchange genes. Those are not members of different species. (This re-establishment of contact is why human populations did not evolve into different species.)
Or, there could be some reproductive isolation but it’s not complete. In such cases we have to make a judgment, like calling them “incipient species” or “groups with incomplete reproductive isolation.” It turns out that there are evolutionary processes that, upon re-contact of incompletely isolated populations, can drive them, though natural selection, to evolve into different and full biological species. One such process is called “reinforcement”, and it’s been seen to work in both nature and the lab.
The upshot is that because the evolution of one species into two or more is a continuous process, there will be stages of the process in which there is some reproductive isolation but it’s not complete. (Geographically isolated populations will, if left long enough, nearly always become full species). That means that there will sometimes be problems establishing whether two populations are species or not. I like to say that spatially isolated populations become more and more “species-like” with time, and, when reproductive isolation is complete, finally attain the status of full biological species.
e.) The reality of species is also seen by common sense (the value of bird guides, for example), by the remarkable coincidence between indigenous people and outside scientists in recognizing the same groups existing in one location, and through using statistical methods to see if individuals fall into discrete phenotypic or genetic clusters. This is the very first topic we take up in our book, and provide ample evidence that clustering in one area is real, and that the same clusters are identified by both local residents and biologists from outside the area, establishing that the clustering is not simply the result of humans subjectively partitioning a continuuum of nature into discrete units.
If you think that species aren’t real, go outside for half an hour and look at birds. If you know your birds, do they form a continuum, or does each bird you see fall neatly into a group that has been recognized, described, and written up in bird guides? You already know the answer. Bird species are real, and that’s true in other groups of plants and animals.
I could go on, but if you can get hold of Speciation by Coyne and Orr, I’d suggest reading Chapter 1, which gives evidence for the reality of species. Since our book was taken over by Oxford University Press, it’s now as expensive as the one above, so try to get it from a library. Chapter 1 is, I think, accessible to the scientifically interested layperson. (The book, however, is written for professional evolutionists: grad students, advanced undergrads, or professional evolutionary biologists. I always tell my friends not to read it unless they’re willing to slog through the stuff meant for professional evolutionists.)
So yes, species are real in sexually-reproducing organisms, but there are intermediate cases because it’s a process that takes a lot of time—evolutionary time. Finding cases that are hard to decide does not negate the value of the BSC, for, in the end, it’s the genetic barriers between species that allow them to continue diverging from each other without “pollution” by genes from other populations. In other words, it is the evolution of reproductive barriers that produces the lumpiness of nature that we see in one area.
And that is the great value of the BSC: it explains why nature is lumpy, a question that wasn’t answered by evolutionists until around 1935 or so. It answers the species question, at least in sexually-reproducing organisms. The concept of genetic barriers (reproductive isolation) gives a natural explanation for nature’s lumpiness, and thus the question of “the origin of species” in sexually reproducing groups boils down to the question of “the origin of genetic barriers. And that gives us something to work with at last! How do those barriers arise, and what is their nature? As I wrote in Speciation, I don’t know of a single study on the origin of species of plants or animals in nature that is not about the origin of genetic barriers and reproductive isolation. That’s how pervasive and useful the BSC has been!
As for the “lineage concept” of species, it’s deeply confused, and you can read the Appendix of my book to understand why. Just one point here: what lineages are we talking about? Lineages of genes are different from lineages of populations, and those differ from lineages of biological species. Species concepts based on using lineages of genes, for example, always wind up in a big muddle, and have not been used to answer the question of why nature is “lumpy.” Insofar as lineages are constrained to remain separate, it’s because they’re reproductively isolated! But read the book to see more. Or look at any intro text on evolutionary biology, like this one.
In short, yes, I agree that no species is better than any other, or has any kind of natural hegemony over other species. That idea is crazy, though of course humans do kill and eat members of other species. But that doesn’t mean that we’re better than, say, cows—any more than lions are “better” than gazelles. So here I agree with the book’s authors.
But I think their view that species aren’t real is deeply misguided. It is, I think, an example of what I call “the reverse appeal to nature.” This is what I call the tendency to impose onto nature your own ideological or biological prejudices. The regular philosophical “appeal to nature” is the misguided idea that “what is natural is good”. (It’s similar but not identical to the “naturalistic fallacy,” which is “what we see in nature is what we ought to do.”)
The reverse appeal to nature simply stands that appeal on its head, saying “what we think is good must be what occurs in nature.” Another example of is using ideology to deny that there are two sexes in nature because you have an ideology that maintains that biological sex is a spectrum. You must thus claim that what exists in nature must be what your ideology tells you exists. This is why we see the pervasive ideological denialism of what is a palpable truth recognized by biologists. (And yes, there are only two sexes in humans and other animals.)
Perhaps the ideology behind the “species are not real” claim is that if you don’t think there should be a hierarchy of species, you can simply deny that species exist. But you don’t have to deny the existence of species to be kind to animals.
If you have questions about species or speciation, I’ll try to read the comments within a day and answer them. Or, best, consult this:
Today we have another science-and-photo lesson from Athayde Tonhasca Júnior, whose captions are indented. I still have all the readers’ photos sent before, so don’t worry—yours will show up eventually. (I’m back in Chicago, but very fatigued.)
MISANDRIST BACTERIA
Athayde Tonhasca Júnior
J.K. Rowling and allies are battling hard against the Twitter mob, rabid activists and grovelling, dishonest academics to prevent women replaced with ‘birthing people’ or ‘individuals with a cervix’, and having their spaces encroached by penis-dangling non-birthing people. But at least female Homo sapiens don’t have bacteria as their enemies.
Bacteria are single-cell organisms found practically everywhere on the planet: your body alone harbours millions of them, mostly living quietly on your skin and inside your gut. But bacteria from the genus Wolbachia* have a specific niche: they spend their lives inside the cells of insects and other arthropods and are transmitted exclusively through the female germline—the cells that pass on their genetic material to the progeny.
That’s a good strategy for those bacteria living inside a female host because they are transferred to her offspring via the eggs’ nutritious and protected cytoplasm (the gooey solution that fills each cell). But bacteria in a male host are virtually doomed: they have little chance of being transmitted because sperm cells have almost no cytoplasm. For the Wolbachia‘s perspective, male hosts are a dangerous prospective. But these bacteria deal ruthlessly and efficiently with the problem: they make males irrelevant, or just get rid of them.
For some flies, beetles, wasps, moths, mites and isopods (woodlice relatives), Wolbachia-infected males mating with uninfected females are incapable of reproducing because the bacteria interferes with the paternal chromosomes, resulting in embryonic death. Mating with females carrying the same Wolbachia strain is not affected. The consequence is that Wolbachia-free females have lower chances of reproduction, while infected females can spread the bacteria through the population. This process, known as cytoplasmic incompatibility, is the most common effect of Wolbachia.
But there’s more. For some butterflies, true bugs (Hemiptera) and isopods, the bacteria turn genetic males into infertile or functional females by inhibiting the production of hormones that trigger the development of male sexual characteristics, a process known as feminisation. Interestingly, feminisation can be ‘cured’ by antibiotics that kill Wolbachia (e.g., Narita et al., 2007. Applied and Environmental Microbiology 73: 4332–4341).
Wolbachia’s cunning has no end: they take advantage of the reproductive system of Hymenoptera (bees, wasps and ants), where fertilized eggs contain two pairs of chromosomes and develop into females, whereas non-fertilized eggs contain one copy of each chromosome and develop into males. In some parasitic wasps, the number of chromosomes doubles in infected male egg cells. So voila, those eggs develop into females that reproduce asexually, giving origin to a new generation of infected female clones that can pass on the bacteria. And just like feminisation, asexuality can be ‘cured’ by treating wasps with antibiotics or heat, which kills the bacteria. In the laboratory, antibiotic ministrations over several generations induces wasp populations to revert to a sexually reproductive mode (e.g., Stouthamer et al., 1990. Proceedings of the National Academy of Sciences 87: 2424-2427).
Last but not least, Wolbachia may resort to outright homicide: infected males of some beetles and butterflies are killed during embryonic or larval stages, resulting in populations heavily skewed towards females.
The mechanisms used by Wolbachia to manipulate their hosts remain largely unknown and speculative, even though this is a hot research topic. By whatever device, Wolbachia take control of their hosts’ reproduction for their own benefit, so they spread quickly throughout the population wherever they are introduced. [JAC: This manipulation of hosts is can be seen as an “extended phenotype” of the bacteria.]
You may think that such male-bashing shenanigans are oddities; but you would be wrong. It is estimated that 40 to 60% of all arthropod species are infected by Wolbachia. These are remarkable numbers, considering that these bacteria were unknown until 1924 when Wolbachia pipientis was first identified. And they are only one of the many microorganisms causing sexual aberrations in insects.
These facts and figures sound alarming; could Wolbachia be a risk to invertebrates? Highly distorted sex ratios could threaten populations or even whole species. But data from a variety of studies suggest that these bacteria are symbionts, that is, they have established a close and sustained relationship with their hosts. And as improbable as it sounds, these female chauvinistic bacteria can be good: there are instances of increased fertility, fitness and resistance against certain viruses in Wolbachia-infected hosts. Wolbachia may even be a factor in insect speciation (when populations evolve to become distinct species). If two populations become infected with different types of Wolbachia, males from either population may be unable to fertilize females from the other; with time, these two populations split into different species (Campbell et al., 1994. Insect Molecular Biology 2: 225-237).
The effects of these bacteria on their hosts have been used for our benefit. Viruses like dengue, Zika, chikungunya and yellow fever have hard times multiplying inside Aedes aegypti mosquitoes infected with Wolbachia. So researchers and mosquito control organisations are breeding Wolbachia-carrying mosquitoes and releasing them into areas of mosquito-borne diseases. Tests with modified mosquitoes have shown significant reductions of dengue incidence in Singapore, Brazil, and Indonesia. The potential for the management or control of other pests is enormous.
Considering the prevalence of Wolbachia, one would not expect pollinators to go unscathed. Indeed, honey bees, bumble bees, several solitary bees, wasps and hoverflies harbour the bacteria. Data for these groups are still scarce, but 66% of Germany’s native bees may be infected (Gerth et al., 2011. Systematics and Biodiversity 9: 319-327). We have the vaguest understanding about the implications of Wolbachia infections for pollinators, but we can assume they are susceptible to the same effects found in other invertebrates, i.e., cytoplasmic incompatibility, feminization, induced parthenogenesis and male killing. So many characteristics of our pollinating species such as sex ratios, biology, ecology, behaviour, distribution and phylogeny (their evolutionary history) could have been shaped or at least influenced by some bacteria whose workings we are just beginning to understand. Wolbachia is a good example of the vast area of known unknowns in the field of natural sciences.
I have to brag a bit in the title because if you say a paper is an “oldie,” you have to also say “it’s a goodie”. But I think this one is—it’s the first of two papers I wrote with my then-grad-student Allen Orr on the time course of speciation in Drosophila. And it’s one of the few good ideas I’ve ever had. I don’t know how often it’s been cited—I don’t look up stuff like that—but it has been influential in inspiring others to do related work. I’m writing about this paper because I recently revisited it in an interview (see below).
Here’s a very brief summary of what we did. I realized one day, when I was at the University of Maryland, that there existed a tremendous amount of data about the sexual isolation and hybrid sterility/inviability of various Drosophila (fruit fly) species tested in the lab. There also existed, separately, a large amount of data on the “genetic distance” between these species as judged from gel electrophoresis. This genetic difference is a rough measure of the times since the species diverged. The more similar the electrophoretic profiles, the younger the species. (The actual real-time calibration of the distance is hard, as Drosophila has no fossil record, but we did our best.)
You could, I realized, take various pairs of species, see how much reproductive isolation they had between them—how much mating discrimination and whether the hybrids were viable and fertile—and correlate that with the genetic distance between members of each pair. If you plotted genetic distance against the degree of genetic isolation, you could get a “time course” of speciation, seeing which forms of isolation evolved earliest, what rate they evolved at, whether it would make a difference if the species lived in the same or different areas, and so on.
Of course there are lots of issues here, one being that measures of reproductive divergence between various pairs of species aren’t evolutionarily independent, so we had to do phylogenetic corrections. Further, sexual isolation and sterility/inviability are only two of the reproductive barriers that separate species, and we had to neglect types of genetic isolation that could operate in nature but couldn’t be measured in the lab (e.g., different preferences for food or microhabitats).
The results, though, were surprisingly clean and enlightening. For example, we found that sexual isolation—but not hybrid inviability—evolves ten times faster between species now found in the same area than those now found in different areas. This result, which has held up in repeats of our work, suggests that natural selection “reinforces”, or strengthens, mate discrimination between species when they live in the same place. That’s probably because there is a genetic penalty to be paid, in the form of hybrid problems, if you actually mate with the “wrong” species; and you only have that kind of selection operating in species that live in the same area, and have a chance to produce hybrids.
Here’s a graph from the second of our paper of papers showing two plots of the degree of sexual isolation between pairs of species (y axis) against their electrophoretic genetic distance (a measure of the divergence time between members of each pair). “Allopatric” taxa are pairs of species that are geographically isolated at present, while “sympatric” taxa are pairs of species that live in the same general area. (These data are phylogenetically corrected.) You can see that the degree of sexual isolation appears much earlier (at lower genetic distances) when the taxa live in the same area. This is a very striking result that is highly statistically significant. It suggests that natural selection operates on species living in the same place to “reinforce” their sexual isolation. You don’t see this difference for hybrid sterility or inviability, which are not expected to be reinforced by selection.
I digress, but it’s nice to think about this good old work. Allen came on board the project at the beginning, and we spent several years collecting the data (which was scattered all over the literature), calculating statistics when only raw data were given, and analyzing the data. Thus the paper didn’t come out (in Evolution) until 1989, three years after we’d moved to Chicago.
Then electrophoretic data and reproductive-isolation data continued to accumulate, so in 1997 we published an update of the 1989 paper. The additional data confirmed the patterns we’d seen before. And now, since nobody does electrophoresis any more, and estimates of genetic divergence come from DNA sequences, we can’t do this analysis further. (DNA-sequence data does not exist for most of the species we used.) Similar work has been done in fish and tomatoes, and at least two researchers have redone our analyses in flies using different techniques (the conclusions remain good).
The references to our two papers are given at the bottom, along with the links to them (free access).
This long introduction just wrote itself, when what I really want to do is call your attention to an interview I did about that first paper with Hari Sridhar at his site Reflections on Papers Past. Hari, a a post-doctoral researcher at the National Centre for Biological Sciences, Bengaluru (formerly Bangalore), India, has been interviewing scientists about well known papers in ecology and evolution since 2016. He was kind enough to interview me about the first Coyne and Orr paper, and you can see the interview by clicking on the link below. I haven’t read the final version, which is a transcript of an audio conversation, so be aware that it’s spoken language. I did read a draft and corrected a few phrases that were unintelligible over the phone.
If you’re interested in papers in ecology and evolution, you might have a wander round Hari’s site; there are lots of interesting papers and interviews, many with people I know.
Click below to see the interview.
I want to add that although Allen was my grad student during much of the time we wrote these papers, it was a total collaboration. As with all my students, I don’t micromanage their work or ever tell them what research to do. Allen was interested in the project from the beginning, and contributed tons of work and many ideas to the two papers. And our collaboration continued in what I consider my most important scientific accomplishment, the book Speciation (Coyne and Orr, 2004; note that the book is now expensive but was about $50 when it first came out).
Here are Allen and I at the Evolution meetings in Portland in 2010. Allen was president of the Society for the Study of Evolution, and I was an incoming President, so he briefed me about the job. We had a great time in Portland, as that was before the city went nuts.
And as a measure of the fame of our work, you can’t get bigger than this. My collaboration with Allen was featured in the 2001 movie “Evolution” (a dreadful film!), as a scrawled reference on the blackboard behind two of the stars, David Duchovny and Orlando Jones. See below. It says “Read Coyne and Orr. ‘Drosophila’ pp. xx8-450”. Note that the page numbers don’t correspond to either paper that we wrote, though it may refer to the book. But even in the book those pages don’t correspond to anything that would be a reading assignment.
Another ex-student of mine, Mohamed Noor, called me up and said he’d seen the movie and noticed a reference to our paper on the blackboard. I didn’t believe him, so I had to go see the movie myself. Sure enough, we were in there! Someone later sent me a screenshot (below).
I would call that real fame! Pity they got the page numbers wrong. I’ve always wondered who wrote that on the board and how they knew about our work.
Once again the magazines are hyping Big New Changes in Evolutionary Theory. This time, though, it’s the respected TheEconomist, which has a policy of not showing the authors’ names. They should have, for some authors should be given an education about their subject, or at least be held accountable for errors. I am surprised that the website has such a long article, though I don’t often read The Economist, so I was unaware that they did long-form science.
Unfortunately, this is not good long-form science because it distorts and exaggerates the evidence for the role of hybridization in speciation.
First, note the subtitle of the new article below (thanks to many readers for sending it to me). “The origin of species is more complex than Darwin envisaged.” That’s not even wrong. Darwin didn’t advance much of a theory of speciation in The Origin, as he had little idea of what a species was. And what he did say about speciation in that book was, as I note on the first page of my and Allen Orr’s book Speciation, “muddled or wrong.” Nobody touts Darwin as an expert on speciation, despite the title of his great book.
The modern theory of speciation began coalescing in the 1930s and 1940s with the works of Theodosius Dobzhansky and Ernst Mayr, supplemented by Ledyard Stebbins, whose big contribution was to show that in plants, a form of hybrid speciation called “allopolyploidy,” was important in forming new species. All of these authors took speciation to mean the origin of reproductive isolating barriers impeding gene exchange between separate species, with the barriers generally arising by selection causing evolutionary divergence between geographically isolated populations. If substantial barriers arose as a byproduct of that evolutionary divergence, then speciation had occurred.
That view hasn’t changed much, although our view of how species arise has been a bit refined. But it surely hasn’t been “upturned,” as the article implies: what we know about speciation still rests on a scaffold erected 80 years ago. Yes, of course Darwin’s view of speciation has been completely revised, but we’ve known that for eight decades. It’s like saying, “How genetics has upturned the theory of inheritance,” with the subtitle, “The way heredity works is more complex than Darwin envisaged.”
And the major framework of Darwinism, the five-fold thesis that organisms evolved, that they did so slowly rather than instantly (and in a replacement rather than an individual transformation way), that there is a branching tree of life that began with one ancestral species, that all species, living or dead, have common ancestors, and that the mechanism for creating adaptations (the “match” between an organism, its way of life, and the environment) is due to natural selection—none of these five propositions are affected by the discovery that, as the article notes, hybridization is more common than we used to think. Yes, hybridization is more common than we used to think. What is not true—or at least is unevidenced and probably untrue—is that hybridization is a major cause of speciation in animals. (It is in plants.) But have a read below; The Economist piece is free:
The article was written to show the increasing prevalence of gene exchange between “species” in nature, and, more important, to emphasize that this gene exchange has been instrumental in creating new species. That is, there is a non-Darwinian form of speciation that involves not bifurcation of family trees, but exchange between branches of family trees, leading to new species. That is, here’s how we get new species, and one can say that it is indeed does not lie within the bifurcating-tree framework of Darwinism as limned above (figure from the Evolution paper below):
There are two ways this can happen. First, species can produce full hybrids and then the hybrid genome, forming a full population of hybrid individuals, can sort itself out into a new species—that is, a new group of populations that are reproductively isolated from the parental species. And this itself can take two forms. The first, allopolyploidy in plants, involves two plant species with different chromosome numbers (or arrangements) hybridizing, and that hybrid then doubles its number of chromosomes, forming an “allopolyploid” population that will be reproductively isolated from the two parental species. (Hybrids with the parents will produce sterile individuals with three sets of chromosomes.) In my book Speciation with Allen Orr, we show that this kind of speciation is pretty common, accounting for up to 4% of speciation events in flowering plants and 7% in ferns.
The second form of full-hybrid speciation is called “homoploid hybrid speciation”, and involves not an increase in chromosome number, but a normal diploid hybrid forming a population of hybrids, which then evolves into a population reproductively isolated from both parents. The Economist claims this is fairly common. But data I show below suggest it isn’t.
Finally, there is a third way that hybridization can contribute to speciation. That is through introgression: the occasional infusion of genes between species that could be used in adaptation and speciation. (To be part of speciation, those infused genes have to contribute to reproductive isolation between the new “part hybrid” species and its ancestors.) Thus we don’t get an entire population of full hybrids evolving into a new species; rather, speciation occurs in a population that’s taken up a handful of genes from another species through occasional hybrids. This introgression has happened between modern Homo sapiens and the Neandertals on the one hand and the Denisovans on the other, but it hasn’t lead to new species. That form of introgression is roughly equivalent to mutation on a larger scale, introducing genetic variation that can be (and was, in the case of Neanderthals) used to adapt to environmental changes.
This form of “introgressive speciation”, too, is much rarer than The Economist says, and for the same reasons why homoploid hybrid speciation is rare: we simply don’t have many examples of parental genes in hybrids actually causing the reproductive isolation themselves, though they may cause new morphologies and traits in hybrids that can speciate by more conventional means. (That is, natural selection operates on hybrid or introgressed populations, producing reproductive isolation as a byproduct of the adaptive change, so that genes in the original parents aren’t the cause of reproductive barriers.)
This paper on birds in Evolution, published in 2014, sets out the criteria for homoploid hybrid speciation. Nearly all the examples cited in The Economist piece were already published by then:
Here are the authors’ criteria for homoploid hybrid speciation, the case most emphasized by The Economist:
. . . we define hybrid speciation as a speciation event in which hybridization is crucial in the establishment of reproductive isolation. Although we agree with previous reviews on the definition, we focus this piece on establishing standards for the genetic and phenotypic evidence required to demonstrate that homoploid hybrid speciation has occurred. To demonstrate that hybrid speciation has occurred given this definition, three criteria must be satisfied: (1) reproductive isolation of hybrid lineages from the parental species, (2) evidence of hybridization in the genome, and (3) evidence that this reproductive isolation is a consequence of hybridization. By contrast, a large number of empirical studies have simply used genetic evidence of hybridization (Criterion 2) as support for hybrid speciation (see below).
In our discussion, we evaluate the strength of evidence for homoploid hybrid speciation in studies published in the last decade against these three criteria. We argue that much of the evidence presented in proposed cases of homoploid hybrid speciation does not provide strong support for the hypothesis of hybrid speciation. In addition, we outline the evidence required to support hybrid speciation and suggest promising directions for future studies.
The criteria, though stringent, seem quite reasonable to apply to claims of hybrid speciation. I won’t go through all the analysis, but just present this graph of how many cases fit each of the authors’ three criteria. Plants are in dark green, fungi in lighter green, and animals in very light green. Note the y axis is number of studies, and it goes up to only fifty. The last column are the cases that fit all three criteria, that is, cases that might well represent homoploid hybrid speciation. Note how low the bar is!
For meeting all the authors’ criteria for homoploid hybrid speciation (last column), we have three cases in plants (all involve the superb work of Loren Rieseberg’s group on sunflowers in the genus Helianthus), none in fungi, and exactly one in animals, the hybrid butterfly species Heliconius heurippa.
The “Big Bird” case of a hybrid species of Galápagos finch that starts out The Economist‘s article isn’t considered, but it, too, is a bit problematic, as it involves a very few finches that show some reproductive isolation but are in a population of very recent origin that hasn’t been examined in about a decade, as I recall. If that small population persists and remains reproductively isolated from the two other finches on the island, we’ll have one more case in animals. That would make a total of five cases of homoploid hybrid speciation among all three groups examined, and that’s a very small number. It’s certainly not enough cases to say that this kind of hybrid speciation has been at all common, much less ubiquitous. Now it may be that there are more cases that we simply don’t know about, but until we find them, we’re not justified in saying that they’re common, much less that “evolutionary theory is upturned.”
A related paper published in 2018 examines claimed cases of hybrid speciation in birds. Click on the screenshot to read it:
Author Jente Ottenburghs examines seven purported cases of hybrid speciation in birds, some of which are mentioned in the Economist piece. He uses two criteria for whether bird speciation is homoploid hybrid speciation, the same type considered by Schumer et al. above. That is, these are cases in which a hybrid population is supposed to have evolved into new species. And, like Schumer et al., he judges the evidence on whether the reproductive isolation comes directly as a result of the hybridization, which is true hybrid speciation, or whether the hybrid population evolves reproductive isolation by more conventional processes, like natural selection acting on new mutations to create evolutionary divergence—with the byproduct of reproductive isolation—in geographically separated populations.
Ottenburghs finds only one convincing case of homoploid hybrid speciation in birds: the “Big Bird” case of incipient speciation in Galápagos finches. He finds just three cases in which species form after hybridization but the reproductive isolation is not the direct consequence of hybridization (Audubon’s Warbler, likely to be renamed by the Woke), the Golden-crowned Manakin, and the Italian Sparrow. So even here, in a group where hybrid speciation is supposed to be common, we have fairly convincing evidence in only four cases.
The upshot. Considering eukaryotes—I’m not dealing with bacterial “species” here, a complicated issue discussed in Speciation—we have at most five cases of true homoploid hybrid speciation, and then a few more cases of speciation after hybridization in which the reproductive isolation evolves by conventional Darwinian means. There is a difference between hybrid speciation and speciation that occurs via neo-Darwinian processes in a population of hybrids.
In other words, we don’t have near enough data to “overturn evolutionary theory”, or to say that new species often don’t form by a branching process. As far as we know, Darwin’s bifurcating tree is still good for nearly all eukaryotes.
Nor do we think that “Darwin’s concept of speciation as a slow and gradual process” is overturned by homoploid hybrid speciation, which, if it evolves via normal processes of selection and drift, could still be very slow. Yes, polyploidy is quick (a new species arises in three generations), and is not something Darwin considered. But allopolyploidy (the hybrid form of polyploid speciation) has been recognized as an important form of speciation in plants since at least 1950, when Ledyard Stebbins published Variation and Evolution in Plants, drawing plants into the Modern Evolutionary Synthesis. Well, that was 70 years ago. Allopolyploidy is important, but it’s old news.
But one thing we know now that we didn’t know before, and I think I’ve emphasized this, is that introgression—genes moving into species from other species—is more common than we used to think. This comes from the molecular innovations that have made such introgressions detectable. But that doesn’t mean that hybrid speciation is more common than we used to think. In fact, it may well be less common than we used to think.
It seems to me that The Economist‘s policy of not naming the authors of its pieces is not a good one for stuff like this, for the author, whoever he or she is, is represented as giving scientific facts and conclusions. If they’re exaggerated or misrepresented, the author should be held accountable.
Yesterday I began “deconstructing” (as the cool kids say) the claims in the new issue of New Scientist, below, stating that evolutionary theory needs a reboot. I don’t intend to go through all 13 “novelties” that supposedly call for an “Extended Evolutionary Synthesis”, but I’ll tackle just a few this week, for “unpacking” (as the cool kids say) all the errors and distortions of the entire article would wear me out. And the rag magazine probably enjoys these posts as all they care about are clicks, not scientific accuracy.
Yesterday I criticized the magazine’s claim that “genetic plasticity”—the observation that the expression of genes and the traits they produce depend on the internal and external environment—is something novel that was just discovered recently, and that it refutes the widespread idea of genetic determinism. Well, this kind of plasticity isn’t new (it’s been around for a century), it doesn’t refute “genetic determinism” construed in some ways, and almost no biologists accept the form of genetic determinism that New Scientist claims is widespread. Today we take up an area I know something more about: speciation.
Point 5 of their article is the assertion, in caps, “SPECIES DON’T REALLY EXIST.” That will be news to the many of us who already see Homo sapiens as a species that’s different from gorillas, orangs, and the two chimp species. It will also surprise those of us who can instantly recognize a local bird as a robin, a starling, a pigeon, a mallard, and so on. Field guides, after all, would be useless if species weren’t distinct.
For, as Ernst Mayr and Theodosius Dobzhansky recognized in the 1930s, nature is not a continuum in which one form blends imperceptibly into another. Rather, nature is “lumpy” if inspected in a single area, and the lumps correspond to species. (This and the other issues below are all discussed in the first chapter and Appendix of my book Speciation with H. Allen Orr.)
The issue is then not to define species a priori, forcing the lumps in nature into the Procrustean bed of that definition, but rather to conceptialize species: describe in words what they represent. In the first paragraph, then, author Colin Barras gets it wrong:
FOR most of history, we have had little trouble defining species. There was a general assumption that a finite number of distinct forms of life had existed unchanged since creation, each sitting in a clearly defined pigeonhole: human, housefly, hawthorn and so on. Within the past few centuries, and particularly after Darwin, evolutionary theory has emerged as a more satisfactory way to explain how species came into existence. Yet in doing so, it has made species far harder to define.
Well, the issue isn’t how to define species but to find out how to recognize them. And yes, evolutionary theory since the 1930s has provided not only a good criterion for recognition, but also a good explanation of how species come into existence: how the process of speciation works. The explanation is, contra New Scientist, intimately connected with how we conceptualize species, for if we don’t know what these discrete units of nature are, how can we possibly understand how they came into being? Yes, there are lots of new species “definitions” that have arisen in the last several decades, but only one has stood the test of time, and is recognized as an accurate conceptualization of nature’s lumpiness by evolutionists. It’s called the “biological species concept” (BSC), and is roughly this:
A species is a group of populations whose individuals have the ability to exchange genes with other members of the group where they coexist in nature. In contrast, individuals belonging to different species cannot exchange genes in nature: they are reproductively isolated from each other.
Thus the key to understanding why you have no trouble telling birds or insects apart in one plot of land is because they remain genetically distinct from one another, with the reproductive barriers (mate discrimination, hybrid inviability, and so on) preserving the differences that accumulate within each species as it adapts to its environment. In other words the species is the thing that evolves. Now of course some populations of a single species can evolve differently from others, and some species show a limited amount of gene exchange with other species: I deal with these complications in my book, which I urge you to consult for further information.
Things go really haywire in the next paragraph:
There are several aspects to the problem. One is that if we accept the idea of species evolving from other species, then we must allow that an ancestral species can gradually morph into one or more descendants. We would still like to place organisms in discrete categories, but doing so is difficult if species blur into one another through time. “As we have come to terms with evolution, it has highlighted a problem with the machinery in our heads we use for classifying,” says Frank Zachos at the Natural History Museum of Vienna in Austria.
Change in a single lineage over time is a non-problem. Of course lineages slowly transform over time, as ours did. If we evolved, for example, from Homo erectus, it becomes a purely arbitrary matter when to give the later segment of that lineage the name Homo sapiens. Everyone recognizes that this is a matter of naming, not of making a crucial and meaningful biological decision. As for the splitting of one species into several, which occurs via (usually) gradual differentiation of geographically isolated populations to the point where they can’t interbreed, it’s also arbitrary when you call the descend moieties “different species”. We know that when no gene exchange can occur, good biological species have come about, but at intermediate stages of the process, I prefer to say that populations are “becoming more and more species-like.” What New Scientist sees as problems here have been dealt with amply in the last 80 years.
Here’s another non-problem:
For Jody Hey at Temple University in Philadelphia, the more important problem is that biologists often have two objectives in mind when they define species: one is the traditional desire to divide nature into easily recognisable packages; the second is to explain, in evolutionary terms, how those species came into existence. “Humans have conflicting motivations towards species,” he says.
Some researchers argue that these two objectives can never be achieved simultaneously. Down the decades, biologists have come up with a few dozen clever ways to define species. Some make it easy to classify the organisms we encounter – by their physical appearance, for example – but tell us little about the evolutionary process itself (see “Sadistic cladistics”, page 49). Other definitions get to the heart of how species come to exist, but can be difficult to use in the real world.
But other researchers, including me and other evolutionists, do think these objectives can be achieved simultaneously. Are we mentioned, and our reasons given? Nope.
I’m a friend of Jody’s, and he’s a terrific scientist (and a reader here), but I disagree with him on this issue. If you read Speciation, you’ll see that the BSC in fact fuses these two objectives. You first conceptualize species as units of nature that have limited or no gene exchange between them where they co-occur. Then the second problem arises immediately, and comes with a built-in research program: “how do the reproductive barriers arise in the first place?” That is the problem of speciation, and the problem that Darwin, despite the title of his 1859 book, couldn’t solve, for he had no notion of species as reproductively isolated units. In fact, the two objectives have already been achieved simultaneously by evolutionists who accept the BSC. Somehow Colin Barras seems to have missed this. No species concept other than the BSC can explain the palpable lumpiness of nature, and also how it comes about.
The third issue, which comes up often, is that gene exchange between apparently distinct species occurs more often than we used to think. (We know this because we have DNA-based ways of detecting such exchange—”introgression”—that we didn’t have a few decades ago. So here’s the supposed problem of “hybrid bonanza”:
In principle, advances in genetic sequencing could have helped by indicating how genetically distinct different groups of organisms are and how long ago lineages diverged. But sequencing has arguably made the problem worse by revealing that interbreeding – more technically, introgression – between closely related “species” is common across the tree of life. “It does seem to be the rule, not the exception,” says Michael Arnold at the University of Georgia in Athens. Indeed, evidence of introgression stretches right to our front door: our ancestors interbred with various ancient hominins that might, in the eyes of some, count as distinct species.
Well, interbreeding is not ubiquitous (humans and orangs, for instance, don’t exchange genes with any other species), and even when hybrids are formed they sometimes are sterile or don’t mate back to one parental species, necessary for introgression. Hybrid ducks, for example, can be fertile, but introgression is limited because the hybrids look weird and aren’t seen as acceptable mates. Yes, introgression is more common than we thought, but often it occurred in the distant past or, if it occurred more recently, is limited. Yes, we had gene exchange with Neandertals and Denisovans, and it appears to have been more than rare, so I tend to see these groups as subspecies of H. sapiens rather than separate species. When there’s that kind of gene exchange, the problem becomes a judgment call. But this problem hasn’t persisted: now all H. sapiens belong to the same species, and there’s no question of an other species of hominin existing now.
In fact, if gene exchange were pervasive and ubiquitous, we couldn’t make family trees of plants and animals very easily: the gene exchange would blur out the twigs. But it hasn’t.
This is a non-problem as well. If you insist on calling geographically isolated populations, like giraffes, as “different species” if they have a certain amount of genetic or morphological differentiation, then that’s also a judgment call, for one can never be sure what degree of genetic differences (usually judged by DNA differences) would correspond to reproductive isolation. If you don’t care about reproductive isolation, then you have no threshhold degree of genetic difference that is biologically meaningful.
The one sure criterion for species delimitation is this: “do the forms interbreed fairly extensively where they co-occur in nature?” If yes, then they’re members of the same species. If not, they’re members of different species. (One other way to demarcate separate species is that if you cross the forms in captivity and the hybrids are completely sterile or inviable, they are separate species, for hybrids would also be sterile and inviable in nature. But if two forms hybridize in zoos and produce fertile offspring, as lions and tigers sometimes do, then it’s a judgment call. In fact, lions and tigers co-occurred in the Middle East in historical times and there are no records of hybrids. Hybridization is an artifact of captivity, as it breaks down the reproductive barriers that kept these cats isolated in nature. Lions and tigers are different species because they don’t exchange genes where they cooccurred in nature.)
The giraffes, living in different parts of Africa, can’t be tested this way because they don’t co-occur, so calling them four different species on the basis of DNA differentiation is a purely subjective exercise (see my post on the giraffes here).
There is one way that looking at genes can help us find new species that aren’t “subjectively described.” This is when you find what seems to be a single species in one area, but then genetic analysis shows that there are actually two forms that each are “fixed” for a different set of genes or chromosome patterns. This is prima facie evidence of non-interbreeding, and we have what biologists call sibling species. Two of the species I worked with, Drosophila pseudoobscura and D. persimilis, for example, were originally thought to be a single species (you can’t tell them apart by looking at them), but research showed that each group is “fixed” for a different set of chromosome arrangements, and you don’t find both arrangements in any individual, so there are no hybrids.
The last bit:
To help add more rigour to the business of defining new species, earlier this year Zachos and other biologists proposed establishing the first single authoritative list of the world’s species. “Species” itself will remain a slippery concept, but at least we could all agree on where to draw the lines.
No, we won’t all agree on where to draw the lines. The giraffe is merely one out of many, many cases in which biologists will quibble about which populations are different species.
To summarize, New Scientist is wrong: species do exist, regardless of some introgression, and we understand not just what they represent—that is, why nature is lumpy rather than continuous—but also how the lumps come to be.
Rosemary Grant, along with her partner Peter Grant at Princeton, have done pathbreaking work on speciation, particularly in the finches of the Galápagos islands. (They’re a close team, and even share one Wikipedia page). Their work, for example, has revealed unexpected levels of hybridization between what were considered “good” species, and of course the duo, along with their students, are responsible for one of the classic demonstrations of natural selection in action: an evolutionary increase in beak size in Geospiza fortis following a drought that decimated small plants, leading to starvation of smaller finches with beaks that couldn’t handle bigger and harder seeds. Their work on the finches is described in the Pulitzer-Prize-winning book, The Beak of the Finch: A Story of Evolution in Our Time by Jonathan Weiner (1994).
At any rate, Rosemary is giving an online talk on speciation today, as announced by the tweet below.
Coming Wednesday Rosemary Grant will be talking about speciation!
The talk will take place between 5-6 pm British standard time, 11am-12 noon Chicago time, and 12 noon-1 pm. Eastern Daylight Time. It’s sure to be enlightening, and I’m pretty sure it will be accessible to non-biologists. And you can access it by clicking on the link below, which will take you to YouTube directly.
There are many reasons why we want to know how often distinct species hybridize, i.e., form individuals resulting from the mating of a male from one species with a female from a different species. For one thing, if this kind of mixing was very frequent, it would be hard to recognize distinct species as the hybrids would form a continuum between the parents. This isn’t a problem, as species (most notably in birds, as documented in my book Speciation with Allen Orr) remain pretty distinct. But if species remain distinct despite even pervasive hybridization, as seems to happen in some groups like ducks, this is evidence that the hybrids themselves are not mating back to the parental species and blurring species boundaries. And if that’s the case, then we can ask why hybrids are effectively sterile. This could be because they are physiologically sterile, like mules, or “behaviorally sterile”: hybrids could be capable of having offspring but might have the wrong appearance or behavior to attract mates, and so would remain unmated.
But hybridization can have other evolutionary effects. It can, for example, act as a form of “mutation”: if the hybrids are fertile, there’s a chance of genes being moved from one species to another, which could then be acted on by natural selection. This phenomenon, called “adaptive introgression”, is fairly well documented in our own species: “modern” Homo sapiens, for instance, shows several genes that came from Denisovans or Neanderthals (some, like me, consider these subspecies), and were probably driven to high frequency by natural selection. This paper gives a lot of examples of adaptive introgression between more well-demarcated species.
Birds are especially good candidates for estimating rates of hybridization, as they are widely observed, there’s a whole “citizen science” project (“eBird”) in which birders and bird lovers send in records of millions of birds, and hybrids are often easily recognized.
But up to now we had very little idea of how often hybridization occurs. The only estimate, and it’s not a great or systematic one, was one made by Ernst Mayr in 1963, who claimed that he observed only about one hybrid among every 60,000 museum skins he examined: a hybridization rate of 0.00167% (0.0000167). But this low value was a purely off-the-cuff “guesstimate”.
Now, in a new paper in Evolution, three researchers used eBird data to get a better estimate of how often bird species produce hybrids. You can access the paper for free by clicking on the link below, the pdf is here, and the reference is at the bottom. It turns out that hybridization, as estimated by citizen scientists, remains low—about the same order of magnitude as Mayr’s earlier estimate.
First, how do you know when a bird is a hybrid? It’s usually done by observing a weird bird that combines the morphological characteristics of two species who have the opportunity to hybridize. (It could be verified genetically, but that’s hard to do with wild birds.) However, the intermediacy, which is rare, is usually verified by experts, which is how hybrids find their way into the eBird database.
There’s a whole site devoted to bird hybrids, and that is its two-word name. I’ll show three pictures of putative hybrids from the site. Here, for example, is a picture from Bird Hybrids of a hybrid between a mallard and a black duck:
American Black Duck x Mallard hybrid, Ottawa River, Ottawa (Ontario, Canada), 7th April 2016 – copyright Gordon Johnston (photo ID: 2787)
Greater White-fronted Goose x Canada Goose hybrid (same bird as in photo ID 1844 above; with Richardson’s Canada Goose hutchinsii), location not given (probably Colorado, USA), 25th December 2013 – copyright Cathy Sheeter (photo ID: 1845)
There are several ways to calculate hybridization rates. The simplest is just the number of hybrid birds found divided by the total number of birds observed. But bird species vary in their numbers, and if a numerous species hybridizes a lot, that could give you an overestimate of how often members of all species produce hybrids. To correct for that, you can calculate a per species hybridization rate: the total number of species that have produced hybrids divided by the total number of species observed. (You can also do this for hybridization rates between bird orders or bird families.) This of course will be higher, because a species is counted as hybridizing even if it produces only a single hybrid. Alternatively, you can take each species, calculate its hybridization rate with all other species, and then average that across all species to get an estimate of how often an average bird species produces hybrids. (That was not done in this paper.)
There are of course errors introduced by using observations from eBird. One is the assumption that hybrids are recognized as hybrids just as often as pure-species birds are recognized as pure. That’s not unlikely given that hybrids are often distinct, but it may lead to underestimates if very similar-looking species, like warblers, produce hybrids that, because the parents are similar, aren’t easily seen as hybrids. Conversely, hybridization may be overestimated because birders may report this exciting event more than once, so the same hybrid is counted multiple times. Time of year also matters, as hybrids are best recognized when the parental species are in their breeding plumage: during the mating season. I won’t go into detail about how the authors dealt with these issues, except to mention that they used location information to rule out hybrids that were reported multiple times. You can read the paper for the caveats and biases.
The methods: the authors used eBird observations reported between January 1, 2010 and December 31, 2018: nine years of data. Observations were restricted to the contiguous U.S. and submissions were scrutinized and vetted by experts.
Here are the results, all of which confirm Mayr in suggesting that bird hybridization is rare.
1.) The overall hybridization rate was calculated from 212,875 hybrids reported among 334,770,194 birds, or a rate of 0.064%. That is forty times the rate that Mayr reported.
2.) The corrected hybridization rate eliminating possible multiple sightings of the same hybrid: 0.076%, about the same as the uncorrected rate.
3.) The hybridization rate eliminating species that were very prone to hybridization. Eliminating the 10 most frequently hybridizing species, mostly ducks and gulls, which hybridize like gangbusters, brought the hybridization rate down to 0.009%, about 5 times higher than Mayr’s rate. It turns out that the families Anatidae (ducks) and Laridae (gulls) contributed 83% of all bird hybrids in the dataset.
4.) The species hybridization rate: 242 species were implicated in forming hybrids out of 1146 species available giving a frequency of 21% of species forming hybrids at all.
5.) The order hybridization rate. There are 25 orders of birds in the U.S.; 16 of these had species involved in at least one hybridization event, giving a rate of 64%. Here’s a plot of the wide variation in hybridization rate among bird orders. You can see that the two orders Anseriformes, which contains ducks, geese, and swans, and Charadriiformes, which includes gulls, comprise the bulk of hybridization among birds.
6.) The family hybridization rate. there are 95 bird families in the U.S., and 35 had species involved in hybridization, giving a rate of 37%.
The upshot. Correcting for multiple reports of hybrids and eliminating the sluttiest species of birds, the researchers got a rate of about 5 hybrids per 60,000 species: five times higher than that of Mayr—but Mayr’s value was unreliable to begin with. This still means, however that bird hybrids are rare. Here are the data shown graphically in the paper, with the hybridization rate going down as the sluttiest species are removed (oy, the mallards!):
Are there any implications beyond this? Yes. First, there are observations by my colleague Trevor Price and his associate Michelle Bouvier that crosses in captivity between members of different genera and even families can produce viable hybrids, at least in the “lab”. The absence of such hybrids from nature means either that prezygotic isolation (mating discrimination and other impediments to gene flow that operate before copulation) is very strong, or that viability of hybrids in nature is much lower than observed in captivity. (It’s probably a combination of both factors, but I suspect that mate discrimination is quite strong in the wild, and can be overcome by forcibly confining birds in captivity.)
Second, the genetic data from birds, particularly mitochondrial DNA, shows that species remain quite distinct, with species-specific DNA sequences as judged from their “bar codes”. This shows that despite even the low rates of hybridization, hybrids are not putting foreign genes into other species very often.
h/t: Luana
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Justyn, N.M., Callaghan, C.T. and Hill, G.E. (2020), Birds rarely hybridize: A citizen science approach to estimating rates of hybridization in the wild. Evolution. Accepted Author Manuscript. doi:10.1111/evo.13943
In the past week I’ve written two posts about what I think will be the last “research paper” I write, i.e., a paper in which I actually did work at the bench (pushing flies). I’ve covered the paper at some length because I think the experiment is cool, because the results were conclusive, and because it’s an experiment that many evolutionary geneticists have wanted to do in their careers, but couldn’t because it would take too long to get results within a single grant-funding cycle. It’s the kind of experiment that you do out of pure curiosity: to find out what happens.
Here’s the paper again; clicking the screenshot takes you to it (if it doesn’t, ask for a pdf):
Part 1 was a description of the study: its aims and methods, which corresponds roughly to the “Introduction” and “Methods and Materials” section of the published paper (and of most published science papers). In that post I described how we made “hybrid swarms” of two pairs of sister species, each pair comprising one widespread Drosophila species living on a continent and an endemic sister species restricted to an oceanic island (D. simulans/D. mauritiana and D. yakuba/D. santomea). Each swarm began with 50% of the DNA, organelles, and cytoplasm from the two parental species, and we made 8 replicate swarms for each pair.
The object was to simply find out what happened when we “mogrelized” two species into a gemisch and then let the population go, without any experimenter-imposed selection over 20 generations. Would the mixture evolve back to one of the parental species? If so, which one, and was that reversion repeated in all eight replicates? Or would we simply get a mixture that persisted over time, without much happening? Alternatively, could we even get a newspecies—a “diploid hybrid species” that was reproductively incompatible with its two parental species? These questions bear on the compatibility of two species’ genes in a single mixture: are the genes of a species “coadapted“, that is, do they work well together but can’t tolerate the presence of genes from another species? Or is there broad tolerance for genes from a close relative?
In Part 2, posted yesterday, I described what happened (the “results” section of the paper). In short, the results were conclusive and replicable: all the swarms in both replicates reverted back to a parental species—in both cases the “dominant” mainland species (D. simulans in one case and D. yakuba in the other). This reversion was seen in several morphological traits that distinguish males from each of the parental species, in the mating behavior of the flies, in their reproductive relationships (fertility) when crossed to the two parental species, and in the sequences of the DNA itself. Since all the trait and behavioral differences are surely based on differences in DNA, all the results are mutually supportive. Nevertheless, foreign DNA was not completely eliminated from the swarms after 20 generations, but that’s only a year in the lab and it might have been further weeded out by selection over longer stretches of time.
So what does this all mean? I’ll use subheadings here.
Selection caused each of the hybrid swarms to revert to a constitution pretty much that of one parental species. In no case did we get anything like a new hybrid species: in every case the swarm reverted to looking and acting like the mainland-species parent, and its DNA sequence was on average about 93-94% that of the dominant species, with the rest of the genome showing some DNA from the minor species.
This means that some form of selection eliminated the island-species’ genes from the mix over time. We don’t know what kind of selection that is (see below), but surely a lot of it had to do with incompatibilities between the species that lead to hybrid sterility and mate discrimination. These will be eliminated no matter what environment you test the flies in, as such selection is largely independent of the environment. It just involves getting rid of genes that produce malfunctioning hybrids. And the elimination of those genes will also eliminate any DNA linked to those genes, whether it be neutral or also bad. This is the “hitchhiking effect” caused by the fact that genes sit next to each other on chromosomes, and if you eliminate one section quickly it will eliminate the adjacent genes from the population as well. What surprised me is how quick the reversion was, and how repeatable it was: not just that all the replicates reverted to the same parent, but because the sections of DNA that “allowed” foreign genes tended to be the same among replicates.
This leads to the next question:
Why did the populations always revert to the mainland species? There are several possibilities here, and the short answer is that we just don’t know. Here are all the alternatives (several of these could, of course, operate together).
a. The island species, comprising smaller populations, might be more inbred than the mainland species. If that’s the case, the island species might have had high frequencies of deleterious genes, as such genes tend to reach higher frequencies in smaller populations. (This is why small, inbred groups of humans, like the Old Order Amish and the Dunkers, show a high frequency of genetic disorders.) In such a case the “healthier” genes from the less inbred mainland species would replace these deleterious genes. I consider this unlikely because the island species are still present, at least today, in appreciable numbers, which would make them less prone to “inbreeding depression”. Further, that depression would have to have affected nearly every area of the island species’ genomes.
b. Mainland species are more ecologically generalized than island species, as the latter live in a restricted environment while the former roam over many diverse habitats. (D. santomea, for example, is restricted to the high-altitude mist/rain forest of Saõ Tomé while D. yakuba can live in open forests, grasslands, and savannas). This means that mainland species may have genomes that comprise “jack-of-all trades” genes, and thus would be more likely to replace the island species’ genes in a foreign laboratory environment. Several ecologists have proposed that island endemics are often more narrowly adapted than are their mainland relatives.
c. The mainland species was more fit in the particular environmental conditions we used (standard cornmeal/agar/yeast medium, rearing at 24°C, 12 hour light-dark cycles, and high humidity), but in other conditions the island species’ genes could have been more fit. For example, D. santomea prefers cooler temperatures than D. yakuba, and perhaps if we left the swarms at, say 18° C, the swarm would have reverted to D. santomea. This remains to be studied.
d. The genomes of the island species may simply contain more genes that cause hybrid incompatibilities than do the mainland species. This would lead to the more rapid elimination of “island” genes, and thus reversion to the mainland species. There is a bit of evidence for this in one of these pairs, but not for the other.
Why were there parallel regions of the genome that retained “foreign” genes from the island species? As I mentioned last time, some regions of the genome, like the middle of the right arm of the second chromosome in the D. santomea/D. yakuba swarm, and the tip of the left arm of the third chromosome in the D. mauritiana/D. simulans swarm, more readily retained genes from the island species, though not at high frequencies. We don’t know why this is, but it indicates a parallelism in the selective forces acting on the different replicates. The retained regions could have contained “neutral” genes from the island species, or island-species genes might even have been subject to positive selection in these locations, driving them towards high frequencies. Since the island-species genes didn’t really attain high frequencies (we saw no cases of regions that had two copies of island-species genes), I suspect these are just regions that don’t have a lot of genes that affect the fitness of the hybrids.
Further, the X chromosome had the lowest retention of foreign-species genes. This is in line with our previous observations that X chromosomes have more genes causing hybrid problems, for because they are present in only one copy in males, and can also express both dominant and recessive genes that cause hybrid problems.
Are there parallels in the amount and nature of introgression (gene admixture) between what we saw in the lab and what occurs in nature? We can answer this question in only one pair: D. yakuba/D. santomea. That’s because these species co-occur in a narrow “hybrid zone” at about 1000 m high on São Tomé, and hybrids are formed naturally. We can thus see how much foreign genome goes from one species to the other in nature.
The answer to that is: very little. There are only a few regions of each species that contain small amounts of genes from the other species, suggesting that even in nature foreign genes are not tolerated well. But the regions of the genome where we see some gene “pollution” in nature aren’t the same as the ones that show it in our lab experiment. (In nature, low amounts of gene exchange have occurred pretty evenly across the genome, while we find them concentrated in particular areas in the lab.) This could reflect either the fact that selection wasn’t finished in our swarm, or that the nature of selection in the wild differs from that in the lab, so that different foreign genes would be tolerated.
A few caveats. Our study does not show that the formation of a new species after hybridization is impossible: after all, we used only four species of Drosophila (hybrid species are unknown in that genus), and new hybrid species are known in other groups, like butterflies and sunflowers, though for diploid species they aren’t common. (Some people say they are common, but the evidence for that is very weak.)
Further, we used only two strains in each hybridization: one from each species. Though the strains weren’t inbred, it’s possible that if we started with other genetic material from these species, we’d get different results. We were constrained to use only a few strains because we had to get their DNA sequences to be able to determine which DNA in the swarms came from which species. That would have been much harder if we used more genetically heterogenous starting material. We also had to eliminate chromosome inversions in these strains whose presence would have impeded gene exchange, so were constrained to use these “homosequential” strains.
And, as I mentioned above, even with these strains the results may have differed had we reared them under different lab conditions—say using different food or different temperature. Such studies should be done, but I doubt they will be given the difficulty of doing our own research using four species, one strain of each, and under a single laboratory condition.
But the consistency of the results in our study shows that, at least under our experimental conditions, not much foreign DNA is tolerated in even closely related species.
Here are the nine species in the D. melanogaster subgroup (a male from each). D. santomea (e) is in the middle, notable for its derived lack of pigmentation. And so endeth this series of posts.
Two days ago I analyzed an article about hybrid parrots that had just appeared in the Washington Post. It was grossly misleading in assuming that two parrots of different “species” (they weren’t—one was a hybrid) had mated and produced, lo, a parrot of another “new species” (also wrong). I tweeted my correction to the Washington Post, but, to be sure they saw it, I also contacted the author of the post and her editor through another editor, pointing them to my correction.
In the meantime, I made a bet with a reader (you know who you are!) that they would not correct the errors. The reader said that they would.
I figured I’d let two days go by before looking for a correction or update, and that is now. And there is no correction, as you can see by clicking on the screenshot below.
Now granted, the story was by a local-issues journalist with no apparent scientific training, but it still contains scientific claims—claims that are wrong. And their responsibility is to correct them. As it is now, many readers think that a hybrid is the same thing as a new species, even though a single individual cannot be a new species (later there were two, but of course both were hybrids in an aviary).
What’s heartening is that many of the article’s 265 comments so far point out to reporter Vargas that the parrot chicks are not a new species but simply hybrids, and that breeders regularly produce hybrid parrots that they call “hybrids” and not “new species.” But even all those comments on top of a post by a petulant biologist won’t force the Post to admit its errors. FAKE NEWS, FOLKS!
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