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.
The article goes on:
Another problem is that looking at genes rather than observable features makes it easier to find new species, leading to what some researchers have called taxonomic anarchy. For instance, a biologist can argue that a previously recognised species should really be split into two or more “new” species, as happened when genetic analysis of the African elephant led to its being separated into savannah and forest-dwelling species.
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.
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Not getting a damn thing done today, unless I throw my computer in the ocean….
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This is the one I need to get many times over. The primary thing for amateurs like me is that you remove the confusion that others cause.
I just want to say that I love these posts, even if I don’t comment on them. I don’t have anything of value to say — I’m not a scientist — but I learn a lot, which is the most important thing!
I agree with BJ above. I feel very privileged to receive on this site the sort of education that I would have to pay thousands of dollars for if I was 50 years younger.
Any chance of getting Jody Hey to respond to your comments?
I, too, love these posts. One of the reasons I love this site is that we get a wide array of content from PCC(E). We get cat posts, photography, science lessons, political discussion and if we are lucky travelling food posts. It is a nice thing in the world.
I don’t like the traveling food posts, but only because they make me so damn jeally 😛
I’m sure Jody will see the post, and will respond if he feels he needs to.
Absolutely. Posts like this are exactly what the interweb is for.
There’s a small (but vocal) group of musicologists who make a remarkably similar argument that the historical eras: medieval; renaissance;, baroque, don’t really exist because they are man-made. Well, yes, they are, but it doesn’t render such very useful categories meaningless.
They think that man-made things don’t exist? What kept me from drowning when I drove over what I thought was the Golden Gate Bridge?
“The fact that there is such a thing a twilight does not mean that we cannot distinguish between day and night.”
– Samuel Johnson, supposedly, although I was unable to find a source.
I don’t have any problem with recognising species as real and recognising shared lineages. Species exist at a specific point in time. There’s a couple of these things were people say that the borders are fuzzy, therefore something is meaningless but we recognise colours like red, blue and green even though we also know that colours are continuous and merge into one another. That continuity doesn’t stop us from recognising specific colours.
But I do disagree with the BSC. I know you like it and Dawkins also mentions it in his books but it’s really a zoologist’s way of looking at things. Frankly, the BSC is only useful in the minority of cases. The BSC is essentially meaningless for prokaryotes and, although you dismiss it here, hybridisation is a major cause of speciation in plants.
You make some good points about prokayotes and plants and I look forward to responses, but I wanted to comment about your colors analogy. While it’s true that in the EMS colors are a spectrum -nature has a way of lumping things that can otherwise be described in some ways as continuous.
For example, though the EMS is a continuum of wavelengths, the absorption spectra for individual elements is not. It is highly discrete and by understanding that fact of nature we can understand physical and chemical properties of the element. Just so with biological species – if we recognize that (many forms of life) can be lumped into reproductively isolated groups, we understand a property of that group and how it arose. Nature is often very lumpy.
Hi Jason. Of course you’re right that hybridization is a major source of new plant “lumps” (as Jerry put it) in the palpable lumpiness of nature. But that observation doesn’t invalidate the BSC.
At any one point in time, plant species (if they exist) must be reproductively isolated from other species. If they were not, then widespread mating between them would render them not recognizable as distinct “lumps”.
When hybridization occurs, leading to the formation of a hybrid population with a different combination of genomes and phenotypes, one has a new lump. If the descendants of the hybridization event go on to mate with members of both the original lumps (the geneticist call this backcrossing), then eventually the lumpiness will be smoothed out and one big species will be produced by hybridization and backcrossing between members of the two original species.
But if the descendants don’t go on to mate with members of both the original lumps, then lumpiness is enhanced and now there are three lumps or species (the two original species, plus the new hybrid species). This is what a lot of biologists think of as hybrid speciation in plants (sorry I guess you’re a botanist and I don’t mean to tell you what you already know – just explanation for other readers).
This is an important speciation mechanism, but it can only lead to recognizable species variation if the occasional hybridization event occurs in a context of overall reproductive isolation among most lineages at most times. That’s the defining feature of the BSC, and I think it applies to plants just as readily as to animals.
Apologies if I misconstrued something about your comment or your idea about how plant speciation occurs (or how plant species are identified or categorized).
I can agree with all your points there and there is a certain amount of reproductive isolation. It’s true it causes all sorts of fertility issues in animals but plants don’t need to reproduce sexually so even when there are uneven chromosomes, plants can happily go on reproducing. I can’t recall how much hybridisation happened in plants but it was quite sizeable though. In any case, I mostly brought up plants as counter to how quickly Jerry dismissed hybridisation.
Even if we say the BSC is broadly valid in plants, that doesn’t solve the problem of bacteria though. Bacterial reproduction is asexual so it doesn’t make sense to talk about reproductive isolation. Plus, bacteria are happy to exchange genes all over the place. And bacteria make up the vast majority of life, whether in population size, biomass or number of species. The BSC is an zoological species definition and it might work very well there but to generalise from that is like observing the Amish and saying all human clothing doesn’t have buttons.
(I’ve tried to post this countless times, even from different browsers. If this goes through… good. If not, I’ll try again in the morning.)
Some plants are obviously random in their mating – if we can call it that – when wind spreads their pollen. Others have to manipulate insects to do that for them. These things are really interesting to me – like lichens or corals with different types of life working in a combination.
This may interest you –
“We propose that the adaptive radiation of the Mediterranean orchid genus Ophrys, comprising several hundred species, is due to coevolutionary dynamics between these plants and their pollinators”
https://onlinelibrary.wiley.com/doi/10.1111/brv.12633#.X2m5OFx9Io8.twitter
I don’t think it is correct to suggest that wind pollinated plants are random in their mating. Clearly they rely on chance in the sense that they have no control over where each pollen grain lands but the fact that the ‘lumpiness’ of grass communities is maintained in a prairie or pasture indicates that they nevertheless succeed in usually mating only with other members of the same species. The non random element of their mating is associated with the requirement for the pollen to land an a compatible stigma. They may also produce their pollen at different stages of the season or different times of day as other mechanisms which keep the species separate.
You clearly haven’t read my book where we discuss the importance of polyploidy. If you’re talking about diploid hybrid speciation, that occurs, but only rarely, and it, too, produces new species if the descendants are reproductively isolated from the parents.
Clearly you haven’t read my book, or even my answer above, which notes that of course you can’t use the BSC in asexually reproducing organisms. Then one has to see if those groups have populations as distinct as species in eukaryotes. The answer is unclear.
And yes, most plants do reproduce sexually. Frankly, I don’t think you know what you’re talking about. Most evolutionary botanists do use the BSC. And EVERYBODY, botanist or zoologist, when it comes to studying the question of “how do new species form?”, concentrate on finding out “how did reproductive isolation evolve?” That alone shows that both botanists and zoologists implicitly recognize that in both plants and animals, reproductive isolation is essential to the creation of species.
No, I have not read your book. Or at least not the one about speciation specifically. However, I think you are consistently misreading what I am writing.
My first point was not about the BSC in plants, it was about how common hybridisation is at a broad view. Reading what you wrote, makes it sound like new species arising from hybridisation is extremely rare.
E.g. “even when hybrids are formed they sometimes are sterile or don’t mate back to one parental species /…/ Yes, introgression is more common than we thought, but often it occurred in the distant past or, if it occurred more recently, is limited.”
However, hybridisation is quite common in plants: “One pattern that emerges is that hybridization is more frequent in plants where it occurs in 25% of the species, whereas it only occurs in 10% of animal species.” (https://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1008404)
To be absolutely clear, I did not say that the BSC is meaningless in plants, just that hybridisation is common.
I did say the BSC is meaningless in bacteria. This is something that you have essentially acknowledged when you say the BSC can’t be used in asexually reproducing organisms. My point is that those asexually reproducing organisms are the vast majority of life. This paper (https://www.journals.uchicago.edu/doi/pdf/10.1086/693564) estimates bacteria account for 70-90% of species (obviously not defined using the BSC). It gets even worse if we assume there are 10 million plant and animal species, then if it’s true that there are 10^12 bacterial species (estimated here: https://biologydirect.biomedcentral.com/articles/10.1186/s13062-020-00261-8), that makes the cases where the BSC is applicable to far less than 1% of species on Earth. Even if we quibble about what exactly defines a bacterial species, the BSC is still only applicable to very small fraction of what lives on this planet.
Your point is 100% correct, but Jason doesn’t seem to have understood it.
I think this is helpful wrt plants vs. animals. I think most botanists and zoologists would agree that the BSC is an idea about what species are at any point in time, and how they are different from each other (that is, species are lumps of organisms that don’t mate with each other when they occur together and could mate). And they agree that this BSC is useful in deciding what lumps in nature to give species names.
But some zoologists go too far and also conclude that the evolution of reproductive isolation is the mechanism or process (not just the concept or the criterion for naming) that leads to the formation of new species. And here they differ from many botanists, who would say that reproductive compatibility (pollen-stigma compatibility) is widespread among many plant species and is not the mechanism by which new species arise and are recognized in many plant groups. Those plant species don’t mate with each other (they are reproductively isolated, and they are palpable lumps in nature, and they are good biological species under the BSC). But those lumps did not arise via selection for reproductive incompatibility.
At least I hope that’s a fair characterization of the difference. It’s an extension of the species concept into an explanation or account of the speciation process, one that applies less well to some taxa like plants. Maybe a botanist can show me where I got that wrong.
If you read Speciation, you’ll see that pollen-stigma compatibility is “not the mechanism by which new species are arise and recognized,” and nobody ever said that this is the only form of reproductive isolation that biologists deem responsible for plant speciation. There are many forms of premating isolation in plants, like flowering time differences, ecological preference, pollinator preference, and so on, that can contribute to reproductive isolation in plants.
I don’t think anybody who’s a sentient evolutionist claims that “selection for reproductive incompatibility” is the only, or even the main method of speciation in plants. Finally, there’s no “selection for reproductive incompatibility itself”. That would be maladaptive. Reproductive incompatibility would generally be a byproduct of divergent selection in isolated populations, and we also talk about that in Speciation. I fear you’re attacking a view that nobody holds.
Although the BSC does not apply to microbes that do not mate, microbiologists do tend to find “lumpiness” in nature. Arguments that the tree of life is not valid for Bacteria and Archaea because of horizontal gene transfer are in my opinion exaggerated. Although the use of one gene – small subunit ribosomal RNA – can be criticised other genes involved in information transfer tend to show the same patterns in phylogenetic trees, suggesting there is a genuine “tree of cells” which we can approach if not obtain directly. And every time we isolate a culture which is a green sulfur bacterium or a endospore-forming thermophile, and sequence the 16S rRNA we get the expected result every single time.
Do you have data showing that “the BSC is useful in a minority of cases,” including plants. If you’re talking about allopolyploidy, we talks about that in the book, and yes, it involves hybridization but it is only one contributor to reproductive isolation and, contrary to what you say, allopolyploids are new plant species BECAUSE THEY ARE REPRODUCTIVELY ISOLATED (through hybrid sterility) from their parental species.
And many botanists like Loren Rieseberg disagree with you about the BSC not being applicable in plants.
I view species definitions in plants (the group I work with) as a hierarchy of definitions. I view the BSC as the first definition, the standard we want to use.
Plants, however, have a diversity of ways of reproducing and as sessile organisms in a variable world they have more to gain from hybridization than do animals that can move to pursue their preferred habitats if necessary. So plants often don’t cooperate with our preferred definition. Therefore, botanists find several species definitions useful for different situations, with the last on the list little more than “because I said so.”
However, in all these cases, we’re comparing the plants we’re working on with plants for which the BSC works and trying to achieve something like the same level of variation per “lump” if we possible. We’re still using the BSC as the standard.
Is this “there’s no such things as species” idea becoming more common amongst the younger set? I know there’s always been a tug o’ war between lumpers and splitters but this seems different. I couldn’t help but wonder if this is part of the new ideological bent that has led the left to declare no differences between men and women, no such thing as biological sex, everything is a spectrum and so on. After all, if men and women are identical with no such thing as male and female and one’s gender can change merely by the power of saying so, then why would they treat the rest of biology any different?
And if (a big IF) they were correct, then why can’t I see bald eagles and ruby-throated hummingbirds banging in my back yard and producing giant hovering nectar-eaters with 6 ft wingspans that use their needle-shaped beaks to spear fish out of the lake in the winter? And where’s our Humanzee or human centipede? 🤔
Of course I’m being cynical again. But it does seem like a bizarre form of argument that mirrors the Ken Hams of the world. One group says there’s no actual species because everything mixes and blends together and the other claims everything is a distinct “kind“ and no changes ever make it another “kind”.
Or am I missing something here in their species argument?
Yes, cultural & social influences are strong -but I hope only cyclical… 🙁
“Sadistic cladistics”. I just finished lecturing on methods of drawing phylogenetic trees. Synapomorphies, plesiomorphic traits, polytomies … all that fun terminology. And dammit, I missed using “sadistic cladistics”!
There is always next year.
I suppose, as our host does, that the New Scientist article is largely a contrivance to generate artificial buzz and interest. But one suspects that their worry about the species concept could also be related to the now conventional woke rejection of the discrete, binary nature of sex. If “gender” is a continuum, as they say incessantly, then why not make the same kind of claim for species? Why, if we can pick any personal pronoun we please, why not be, or at least blend into, a cool species? I pick Panthera uncia for me.
[At the same time, devotees of “Whiteness Studies” and associated doctrines insist that white and non-white are as binary as can be. This might be a logical contradiction, but worrying about logic is so uncool.]
The problem with the BSC is, it is limited to organisms using sexual reproduction, at least occasionally. It is not the case of the bdelloid rotifers, which didn’t use sexual reproduction since tens of millions of years. They are nevertheless divided into three orders, four families and about 450 “lumps”. Individuals can be ascribed to a given lump according to morphological, ecological and genetic criteria. If these lumps are not species, what are they?
Jean-François Flot et al., Genomic Evidence for ameiotic evolution in the Bdelloid rotifer adineta vaga », Nature, 2013, p. 453-457.
I address this question in my book. You clearly can’t use the BSC with asexual organisms. The question then becomes, “are they lumpy too”. The answer isn’t as clear as you imply, especially in groups that have a mixture of sexual and asexual reproduction. At any rate, most eukaryotic species on the planet reproduce sexually.
Unfortunately I lent my copy of “Speciation” to a student some years ago and he never came back … But I don’t think one can limit the thinking about speciation to sexually reproducing organisms. Bacteria are devilishly important in evolutionary terms! And they are “lumped” too – luckily for us, so we can cure infectious diseases.
Well, Jacques, I don’t think that any single species concept will cover all species (this is in my book). If you think one does, could you suggest a species concept that covers both sexually and asexually reproducing organisms?
“could you suggest a species concept that covers both sexually and asexually reproducing organisms?”
Of course no – plenty of people tried without convincing results. However, for practical purposes, be it for pinning butterflies in museum drawers, for identifying and fighting a nasty pathogen, for collecting mushrooms in the forest or for choosing the fish of our next meal, we trust the genetic homogeneity of a species rather than its limits. And the (relative) genetic homogeneity is what is shared by sexual and asexual species. A schematic comparison:
In BSC, species are defined mostly by their limits: the presence of post- or prezygotic barriers which hinder genetic exchanges with other gene pools. The internal homogeneity of the species is due to the recombinations provided by the sexual reproduction inside the gene pool. Speciation occurs when two diverging populations (most often following a geographic isolation) are selected against the production of hybrids and, ultimately, for a divergent exploitation of ressources.
In asexual organisms, genetic exchanges don’t exist, a species is a mere collection of clones, thus no question of barriers. The internal homogeneity is maintained mostly by pruning selection, getting rid of deviant clones. Speciation occurs when two diverging populations (most often following a geographic isolation) are selected for a divergent exploitation of ressources.
But isn’t a term that is being touted as a general rule about species but that only applies to tiny fraction of species, not a particularly useful one?
A tiny fraction of species? Again, you’re just making stuff up. The BSC applies broadly to plants and animals, and not necessarily to asexually reproducing organisms.
If you think the BSC hasn’t been useful in understanding species and speciation in plants and animals, you’re sorely mistaken.
It’s a very useful concept! And even when we divide asexual plants into species, we use the BSC as a guideline for how to do it (how much variation per “lump”).
But only useful for plants and animals; they’re the minority.
The vast majority of RESEARCH is done on sexually reproducing species. You’ve made your point, one that I made myself in Speciation. But the proportion of species on earth is not the gauge of what biologists consider the most interesting work.
There is increasing evidence for meiosis and sex in bdelloid rotifers. This preprint has not been peer reviewed yet but it summarizes new evidence and recently peer reviewed & published evidence. I find it convincing.
https://www.biorxiv.org/content/10.1101/2020.08.06.239590v2.full
I forgot: sub !
Whew. I think I followed most of that.
Thanks.
I get the impression that there is confusion based on the gradual transitions and sometimes subtle species boundaries. It allows a writer to dredge up supposed conflict and resolution, the framework of “good writing”. It’s similar to the situation with planets and dwarf planets. Who’s to say were that boundary reliably lies. It’s easier on the mind to see nature as discrete chunks rather than lumps. But, you can’t fool mother nature. She is what she is: messy. And that’s the best we can do.
Interesting post, especially because this morning I finished reading Chapter 11, Selection versus Drift, in Speciation by Coyne and Orr. Spoiler alert: Selection plays a much larger role in speciation than does drift. p.410. I still have much to try to understand, with my interest in plant speciation.
Nice analysis – thanks.
Did you see the article about human chromosomes replacing Neanderthal? https://scitechdaily.com/neanderthal-and-denisovan-y-chromosomes-sequenced-surprise-when-compared-to-modern-human-dna/
I know I said this before but it seems to me there are divisions between synchronic species – that is a living breeding population – & diachronic species – that is one in the fossil record or the dead population. The former biologists study, the latter palaeontologists. They sometimes seem to have different views & philosophies, for example Gould v Dawkins etc.
The European wisent has aurochs ancestry . Presumably there was a bottleneck at some point. That does not make them the same species though. But it means that the aurochs & bison genetics was still close enough for an Successful breeding at least once. I suppose whale hybrids & bear hybrids – polar bears have some brown bear genes – show that stressed populations choose some breeding opportunity over none.
But plants – they seem a lot more complicated regarding this – could you write about that?!
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The lumpiness of life seems about right… enjoyed the comments and post.