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’ve finished reading Richard Dawkins’s new book Flights of Fancy: Defying gravity by Design and Evolution, and quite enjoyed it, for it incorporates many of the principles he’s emphasized in his books—the gene’s-eye view, coevolution, arms races, step-by-step adaptive evolution, evolutionary convergence, and so on—into a discussion about a single adaptation, flight. Well, not a single adaptation, since he discusses gliding animals like flying squirrels and flying fish, the true flight of animals like birds and bats, and the “flying” of pollen via their insect vectors or of seeds via dispersal mechanisms.
And the “design” in the title refers to a convergence between what natural selection has done to facilitate aerial movement and how humans through deliberate design have mimicked many of nature’s contrivances in aircraft.
One of the pleasures of the book is discovering odd but fascinating bits of flight-related natural history that show us the power and “cleverness” of natural selection. Her are two examples he discusses.
The first is the Javan cucumber, Alsomitra macrocarpa, found on the tropical islands of East Asia. It disperses its seeds from a gourd that disgorges remarkable gliding seeds with two wings that look like this:
From Wikipedia:
The seed or samara of this species is unusual in having two flat bracts extending either side of the seed to form a wing-like shape with the seed embedded along one long edge and the wings angled slightly back from it. As the seed ripens the wings dry and the long edge furthest from the seed curls slightly upwards. When ripe, the seed drops off and its aerodynamic form allows it to glide away from the tree. The wing spans some 13 cm and can glide for great distances. The seed moves through the air like a butterfly in flight — it gains height, stalls, dips and accelerates, once again producing lift, a process termed phugoid oscillation. In the past it was often found on the decks of ships at sea.
You can see that dipping and stalling in the second video below.
By the way, Richard has a great discussion about why it’s adaptive for plants to disperse their seeds. If your parents are growing in a good place, why send them many meters away to a place that might not be as good? But you can read about that yourself.
The remarkable aspects of these seeds is how far they glide, ensuring that many of them find themselves well away from their parent. Here are two videos, one from Attenborough, showing the phenomenon. These features are certainly evolved as adaptive for the plant.
The Attenborough introduction:
. . . and a video from YouTube showing the remarkable gliding power of these seeds, which resemble plantlike albatrosses in their ability to glide very far without flapping:
This next one is remarkable. I’ve taught about “pseudocopulation”, a trait that some orchids use to spread their pollen using wasps as surrogate genitalia. There are many orchids that resemble female wasps, and randy males try to copulate with them. (Some of these “wasp orchids” even produce pheromones resembling those of female wasps.)
The “intent” of the orchid (I’m speaking in evolutionary jargon, of course) is to affix its pollen sacs, or pollinia, onto the wasp. After a fruitless attempt at copulation, the wasp, with a sac of pollen stuck to its body, eventually makes its way to another orchid, tries to bonk it, and transfers the pollinia to the female’s stigma, effecting fertilization. (These wasp-mimicking orchids almost never produce nectar to lure the insects; they save their energy and just act sexy.)
One remarkable orchid that tricks wasps this way is the “hammer orchid“; or rather orchids, as there are ten species in the genus Drakaea found in western Australia. A wasp, lured by a part of the flower that looks like a female wasp (it also smells like one), lands on that bit and tries to copulate with it. (The Attenborough video below shows the similarity between flower and female wasp.) At some point, the female-mimicking part of the flower folds over, hammering the male wasp against the pollinia. The pollen sticks to its body, and fertilization is halfway there. The act is completed when the frustrated wasp tries copulating with a different orchid, and the hammer then puts the pollina in contact with the stigma.
Females of these species are wingless (see below) and the orchid has evolved (unknowingly, of course) to look like them. The resemblance is facilitated by the female wasps’ habit of hanging atop a step to lure the males, which is just what the orchid does.
Note that only males can pollinate these orchids (or any wasp-mimicking orchid), so the flower potentially loses half its pollinators. But since females can’t pollinate anyway, being wingless, it’s not that big a loss. What puzzles is me is why the wasp nevertheless persists trying to copulate with the flower. If it ever learned not to waste its time, the flower would go extinct.
Have a look. There are even more remarkable tactics that orchids use to fertilize each other, but I’ll save those for later.
Denis Noble (born 1936) is a British physiologist highly regarded for his work in that field (he has an FRS). Wikipedia notes his accomplishments:
He is one of the pioneers of systems biology and developed the first viable mathematical model of the working heart in 1960.
What the article doesn’t discuss is that Noble has spent the last period of his life attacking neo-Darwinism, asserting that its most important foundations are either wrong or overemphasized. Noble is regarded by colleagues I respect as a bit of an enthusiast, bordering on an unhealthy obsession, though he’s much admired by the “Third Way of Evolution” group who argue that neo-Darwinism either needs a serious revision or a trip to the garbage can. Noble shows us that you can be a great physiologist but a lousy evolutionary biologist.
The gene-centered view of evolution is wrong [This is connected with #2.]
Evolution is not a gradual gene-by-gene process but is macromutational.
Scientists have not been able to create new species in the lab or greenhouse, and we haven’t seen speciation occurring in nature.
Wrong, partly right but irrrelevant, wrong, almost completely wrong, and totally wrong (speciation is my own area).
And yet Noble continues to bang on about “the broken paradigm of Neo-Darwinism,” which happens to be the subtitle of his new article (below) in IAI News, usually a respectable website run by the Institute of Art and Ideas.
Noble is especially excited because he sees himself in a war for the soul of biology, a soul currently occupied by the modern theory of evolution. And so, in this article (see below), Noble once again raises the specter of Lamarckian evolution: the idea—which he sees as both very important and unduly neglected—that adaptations can arise from modifications of an organism’s heredity directly by the environment. (The classic example is a giraffe stretching its neck to reach leaves on trees, and that usage elongates the neck, an environmentally-induced change that somehow worms its way into the mechanism of inheritance so that giraffes eventually evolve long necks)
Darwin himself held a form of Lamarckism, positing that cells of organism, induced by the environment, could feed “pangenes” thoughout the body in a way that could modify its inheritance. That’s why Darwin was always emphasizing “changed conditions” as a source of heritable variation. The problem with this view is that tests of environmental modification as a source of inherited variation have almost never succeeded, and even when they do, they have not created adaptations. The idea that Lamarckian inheritance is an important cause of adaptation is, to put it mildly, ridiculous. (I note here that DARWIN WAS WRONG about inheritance.)
Nevertheless, Noble persists—in the face of all the facts—to make the same tired old assertions. Click to read, and shame on IAI!
First Noble argues that environmental modifications can produce traits that become inherited, though only for a short while. This is true, but the phenomenon is rare.
Noble:
Modern physiology has vindicated Darwin’s idea. The small vesicles, called exosomes or extracellular vesicles, poured out by all cells of the body can function precisely as Darwin’s idea proposed. They have now been proven to communicate such acquired characteristics as metabolic disorders, and sexual preferences, to the germ-line via small regulatory RNA molecules. We can therefore be sure that Lamarckian use-disuse memory can be passed across generations. Weismann’s assertion that the inheritance of acquired characteristics is impossible was therefore incorrect. The debate now centres on two questions: “how often this happens and, when it does, for how many generations do the changes persist?”
The standard neo-Darwinist defense against this clear break of the Weismann Barrier has been to suggest that it only happens in unimportant circumstances and persists for very few generations. There is assumed to be no permanent transmission. The DNA continues “hard” transmission while “soft” inheritance inevitably dies away.
Yes, and soft inheritance can also be mediated by epigenetic (environmentally induced) modification of the DNA, usually by putting methyl groups (−CH3) onto the DNA bases. As Noble admits, these forms of inheritance gradually go away, with nearly all epigenetic modifications erased during the process of reproduction. That’s one reason why this kind of inheritance can’t be the basis of long-term adaptation. But wait! Noble says that short-term adaptation is of great value!
This defence fails to recognise the great virtue of “soft” inheritance, which is precisely the possibility that it can be temporary.
Yes, and so can adaptation based on genes (think of the increase in beak size of the Galápagos finches, which was reversed in a single generation when the size-inducing drought went away). If there’s substantial variation, these reversals can be fast. But Noble fails to recognize that most adaptations hang around for many generations, and those cannot be based on “Lamarckian” inheritance. It’s almost as if Noble is claiming that this form of inheritance, by some kind of group selection, has been installed in the organism to facilitate short term adaptation!
Here’s one example that people like Noble trot out when attacking modern evolutionary theory:
Consider a species under extreme environmental stress, such as the Dutch population during the starvation winter of the 1940s in the Second World War. The inherited signs of that stress have now been passed down three generations, to the great-grandchildren of the 1940s population. The chances are that it will progressively die out as the later generations experience good nutrition. And so it should!
The “Dutch famine syndrome” was caused by epigenetic modification of fetuses in utero when their mothers were starving during the Dutch “hunger winter” of 1944-1945. I’m not sure about the three generations, but I’ll let that pass. What we know for sure is that these offspring show DNA methylation changes probably due to starvation.
BUT this was not adaptive! The epigenetic changes reduced the health of their carriers, as this article shows. From its abstract (my emphasis):
This paper describes the findings from a cohort study of 2414 people born around the time of the Dutch famine. Exposure to famine during any stage of gestation was associated with glucose intolerance. We found more coronary heart disease, a more atherogenic lipid profile, disturbed blood coagulation, increased stress responsiveness and more obesity among those exposed to famine in early gestation. Women exposed to famine in early gestation also had an increased risk of breast cancer. People exposed to famine in mid gestation had more microalbuminuria and obstructive airways disease. These findings show that maternal undernutrition during gestation has important effects on health in later life, but that the effects on health depend on its timing during gestation. Especially early gestation seems to be a vulnerable period. Adequate dietary advice to women before and during pregnancy seems a promising strategy in preventing chronic diseases in future generations.
Now is that, on top of the inherited stress, an adaptive change? I think not.
It is intellectually irresponsible for Noble to suggest that the Dutch Famine syndrome has anything to do with adaptive change, much less evolution. In fact, I know of not a single adaptation that rests on epigenetic modification. I may have missed one or two, but when adaptive genetic changes in animals (including humans) are localized, they invariably are found to rest on base-pair changes in the DNA. When you map adaptations in humans, like changes in lactase persistence or adaptive skin color, you find that they are based not on methylation or episomes or micro-RNAs, but on good old-fashioned mutations that change the sequence of DNA. And so they must be, because these changes have lasted for many generations.
It’s intellectually irresponsible of Noble not to mention that, too.
Further, Noble cites the well-known phenomenon of “genetic assimilation,” in which an environmental change exposes genetic variation that can then be subject to selection, as if this were some kind of refutation of neo-Darwinism. (One hypothetical example: if you starve plants, it may, by stunting them al, hide genetic variation for height.) Noble says that studies of genetic assimilation, which are in all the textbooks, are actually discouraged by evolutionists who don’t like their non-Darwinian implications:
Given the importance of the question, why have so few attempts been made on the genetic assimilation of “soft” inheritance since Waddington’s work? The answer is that funding organisations would not be willing to support such work. If you submit a Lamarckian inheritance project to standard grant bodies, you will be almost certain to receive a firm rejection. Such is the hold of the Neo-Darwinian paradigm on innovative ideas in evolutionary biology.
Get this straight, Dr. Noble: GENETIC ASSIMILATION IS NOT LAMARCKIAN INHERITANCE! As every evolutionist with more than a handful of neurons knows, in these cases the environment exposes standard DNA mutations, allowing them to be selected in the classical neo-Darwinian fashion.
One clue that genetic assimilation depends on genetic variation and not changes in the environment becoming genetic variation is this: Conrad Waddington, who popularized the phenomenon of genetic assimilation using experiments in Drosophila, had a student repeat those experiments with an inbred strain of flies, a strain that had almost no genetic variation. Voilà: no genetic assimilation, no change in the trait. If the Lamarckian theory were correct, there should still have been changes in the character in inbred lines.
Once again we smell the odor of intellectual mendacity in Noble’s prose.
At the end, Noble beefs about how a 2016 Royal Society symposium he organized, on “New trends in evolutionary biology: biological, philosophical, and social science perspectives,” was protested by 20 other fellows of the Royal Society. He couches this as censorship against discussing new ideas, but he’s wrong. Many of the speakers at the symposium were touting ideas that had already been already refuted, and the purpose of the meeting was to show that neo-Darwinism is dead. It’s as if there were to be a Royal Society symposium on Intelligent Design (the ID people, by the way, loved that symposium). Of course one would object if someone who knows jack about evolutionary biology organizes a symposium designed to dismantle its modern form. It’s like a fox organizing a symposium on how to breed chickens.
I’m not saying that Noble has no right to weigh in on modern evolutionary biology simply because he was a physiologist. No, I’m asserting that Noble’s claims about the death of modern evolutionary biology should be ignored because there is virtually no data to support them. His claims should be ignored because he is ignorant, and willfully so. (Others have corrected him many times.)
I’m through with Noble; he says the same thing over and over again, tilting at the windmill of modern evolutionary biology with a soda straw. I probably should have ignored Noble’s mush, but the laws of physics compelled me to write. At least the readers here can be aware of his numerous errors and misstatements, even if Noble plays the same tune until he’s underground.
Of the several independent assertions that constitute Darwin’s “theory of evolution” in On The Origin of Species, Darwin regarded the idea of natural selection as his most important and original. After all, it alone explained how naturalistic processes could lead to the remarkable adaptations of animals and plants heretofore seen as some of the strongest evidence for God. And although the idea of evolution itself had been broached by others before Darwin, including his own grandfather Erasmus, natural selection seemed to be sui generis.
Well, not entirely. It was anticipated by several people, including the Scottish polymath James Hutton in 1794. But the most remarkable precursor to the idea of natural selection was published by Scottish horticulturalist and agriculturalist Patrick Matthew (1790-1874) as an appendix to his book On Naval Timber and Arboriculture (1831). Although the book was about how to build ships using wood, and what kind of wood to use, Matthew added a 28-page Appendix. In that Appendix were 29 sentences that laid out what he called “selection by the law of nature”, which bore a striking similarity to the idea made famous by Darwin 28 years later.
You can read Matthew’s appendix at Darwin Online, and some of the statements about his views of natural selection can be seen and discussed in context here,here, and here. ‘
Here are two excerpts from Matthew’s Appendix:
THERE is a law universal in nature, tending to render every reproductive being the best possibly suited to its condition that its kind, or that organized matter, is susceptible of, which appears intended to model the physical and mental or instinctive powers, to their highest perfection, and to continue them so. This law sustains the lion in his strength, the hare in her swiftness, and the fox in his wiles. As Nature, in all her modifications of life, has a power of increase far beyond what is needed to supply the place of what falls by Time’s decay, those individuals who possess not the requisite strength, swiftness, hardihood, or cunning, fall prematurely without reproducing—either a prey to their natural devourers, or sinking under disease, generally induced by want of nourishment, their place being occupied by the more perfect of their own kind, who are pressing on the means of subsistence.
. . . There is more beauty and unity of design in this continual balancing of life to circumstance, and greater conformity to those dispositions of nature which are manifest to us, than in total destruction and new creation. It is improbable that much of this diversification is owing to commixture of species nearly allied, all change by this appears very limited, and confined within the bounds of what is called Species; the progeny of the same parents, under great difference of circumstance, might, in several generations, even become distinct species, incapable of co-reproduction.
The self-regulating adaptive disposition of organized life may, in part, be traced to the extreme fecundity of Nature, who, as before stated, has, in all the varieties of her offspring, a prolific power much beyond (in many cases a thousandfold) what is necessary to fill up the vacancies caused by senile decay. As the field of existence is limited and pre-occupied, it is only the hardier, more robust, better suited to circumstance individuals, who are able to struggle forward to maturity, these inhabiting only the situations to which they have superior adaptation and greater power of occupancy than any other kind; the weaker, less circumstance-suited, being permaturely destroyed. This principle is in constant action, it regulates the colour, the figure, the capacities, and instincts; those individuals of each species, whose colour and covering are best suited to concealment or protection from enemies, or defence from vicissitude and inclemencies of climate, whose figure is best accommodated to health, strength, defence, and support; whose capacities and instincts can best regulate the physical energies to self-advantage according to circumstances—in such immense waste of primary and youthful life, those only come forward to maturity from the strict ordeal by which Nature tests their adaptation to her standard of perfection and fitness to continue their kind by reproduction.
Well yes, that has variation, differential survival, culling of most individuals in a species, speciation, and adaptation—all features of Darwin’s own theory. It’s a remarkable anticipation of Darwin’s ideas.
Does this mean that Matthew deserves credit for the idea of natural selection? Only as an anticipation of Darwin’s far more thorough explication (Darwin, by the way, never read Matthews’ Appendix). Matthew deserves no more credit for natural selection as a popular idea than does Erasmus Darwin for evolution. Matthew’s ideas weren’t adopted, were almost never cited, had no influence in biology, and Matthew never realized until after The Origin was published (and sold out the printing in a single day) that he once had within his grasp The Big Idea that explained the design-like features of nature.
Nevertheless, several people have tried to diminish Darwin’s idea by pointing out that Matthew had it first—and that Darwin plagiarized it. The latest attempt is by Mike Sutton in this book published two months ago (click on image to go to Amazon link):
I haven’t read it, but according to Geoff Cole, a cognitive scientist at the Centre for Brain Science at the University of Essex, who reviewed the book in the latest issue of Evolution (click below for free access), Sutton’s book is a real hit job on Darwin.
The title of Sutton’s book clearly asserts that Darwin took credit for Matthew’s theory, and it’s true that once Patrick Matthew had read The Origin, he argued for his own precedence, even though Darwin had never seen the “incriminating” sentences above. Sutton also claims that Matthew’s idea had real priority because Naval Timber was cited by others before 1859, but as Cole notes in a very critical but polite review, those citations were almost all to the book itself, not to the ideas in the Appendix.
Cole also notes Sutton’s ridiculous accusations of Darwin’s “plagiarism”:
What is most uncomfortable about Sutton’s thesis is his treatment and personal attack on Darwin. He suggests that Darwin ”was a plagiarist who lied repeatedly” and undertook “deliberate, knowing fraud”. Indeed, “the biggest science fraud in history”; fraud that Darwin supposedly hoped “nobody would notice”. Sutton also expresses suspicion about the chronic illness Darwin was known to suffer; a subject that many historians have written about (e.g., Hayman, 2009). From every single account of Darwin and how he went about his life, these “lies” are the complete opposite of what we know about the man. I have lost count of the number of times I have seen a scholar write that a particular event “is testament to his honesty”. As Browne (1985) stated, “By the time Descent of Man was published in 1871 reviewers were falling over themselves to congratulate Darwin’s “unassailable integrity and candour, and his “wonderful thoroughness and truthfulness” (Browne, 1985, p.257 & 258).
Every serious historian who’s studied Darwin’s life knows that he was neither a plagiarist nor a liar, although he did, understandably, want to preserve credit for his own ideas. After Matthew wrote a claim of his priority in The Gardner’s Chronicle in 1859, Darwin not only published an acknowledgement of Matthew’s precedence in the same magazine, but also inserted this long acknowledgment of Matthew’s work into the 3rd edition of On the Origin of Species:
In 1831 Mr. Patrick Matthew published his work on ‘Naval Timber and Arboriculture,’ in which he gives precisely the same view on the origin of species as that (presently to be alluded to) propounded by Mr. Wallace and myself in the ‘Linnean Journal,’ and as that enlarged on in the present volume. Unfortunately the view was given by Mr. Matthew very briefly in scattered passages in an Appendix to a work on a different subject, so that it remained unnoticed until Mr. Matthew himself drew attention to it in the ‘Gardener’s Chronicle,’ on April 7th, 1860. The differences of Mr. Matthew’s view from mine are not of much importance: he seems to consider that the world was nearly depopulated at successive periods, and then re-stocked; and he gives, as an alternative, that new forms may be generated “without the presence of any mould or germ of former aggregates.” I am not sure that I understand some passages; but it seems that he attributes much influence to the direct action of the conditions of life. He clearly saw, however, the full force of the principle of natural selection. In answer to a letter of mine (published in Gard. Chron., April 13th), fully acknowledging that Mr. Matthew had anticipated me, he with generous candour wrote a letter (Gard. Chron. May 12th) containing the following passage:—”To me the conception of this law of Nature came intuitively as a self-evident fact, almost without an effort of concentrated thought. Mr. Darwin here seems to have more merit in the discovery than I have had; to me it did not appear a discovery. He seems to have worked it out by inductive reason, slowly and with due caution to have made his way synthetically from fact to fact onwards; while with me it was by a general glance at the scheme of Nature that I estimated this select production of species as an à priori recognisable fact—an axiom requiring only to be pointed out to be admitted by unprejudiced minds of sufficient grasp.”
Cole explains patiently why Darwin should get nearly all the credit for the idea of natural selection. A few excerpts from Cole’s excellent review:
Who then should be credited with discovering the process by which evolution occurs? Matthew, Hutton, Maupertuis, Wells? Or anyone else who also chipped in? The answer is simple. Charles Darwin.
. . . A necessary condition of insight is that the knowledge must be reflected upon and placed within the appropriate context. Unless a person fully recognises what they have said, done, or found, no formal insight has occurred. There is no priority.
. . . I suspect Matthew was annoyed with himself, as I was with myself, for not realising the importance of what he had written. That may have been why he dedicated so much of his later efforts on his priority claim. If he had realised he would surely have submitted an academic paper outlining his theory; a paper that was only about the theory. Given fear of religious establishment, this could have initially been anonymously penned. He may have even published a book on the origin of all life forms and how the development of every single species can be explained. He would have also repeatedly used his phrase “the process of natural selection”, a phrase Sutton places great emphasis on, as opposed to the one time he did so in Naval Timber. As it was, there was no paper or book. There was no in-depth development of ideas about evolution and how it relates to divergence, heredity, the geological record, geographic distribution, classification, morphology, and embryology. No lengthy discussion of how there are problems and “difficulties” with his own theory. There was not 30 years of methodical work in which he used his theory to explain aspects of cross-pollination and movement in plants, not to mention work on human psychology, sexual behaviour, and emotions. There were no lengthy and numerous discussions with colleagues about his theory and when he should go public.
In fact, Sutton acts like a creationist, arguing that generations of evolutionary biologists have realized that Matthew should really get credit for the idea; but we have, because of our mindless adulation of Darwin, kept that quiet:
Essentially, Sutton has to explain why generations of evolutionary biologists and the like have never come to the same conclusion as himself. The usual explanation is that we are all involved in a “cover up” (p. 5) or part of the “Darwin Industry”, as Sutton calls it, in which a “loosely affiliated in-group of scientists, historians of science, other writers, publishers, editors, and journals, share a common goal to protect the perception of Charles Darwin as a genius science hero” (p. 10). But how This article is protected by copyright. All rights reserved. about this for an alternative explanation? Those generations of biologists have independently decided that there is nothing to see here, that Darwin should be honoured with discovering evolution. Furthermore, if a few sentences in which natural selection is referenced warrants priority, as Sutton seems to believe, then why pick out Patrick Matthew? Why not his predecessors, Hutton, Wells, or Maupertuis? In fact, shouldn’t Matthew be accused of plagiarism, having failed to acknowledge the fact that his ”own original child” was described at least 30 years before by various others?
Sutton’s book is his latest, in his decade-long, attempt to undermine Darwin’s priority. As all others before, this one will fail.
Of that there’s no doubt. Matthew’s independent musings about natural selection are a remarkable coincidence, but he didn’t make much of them, didn’t examine them further, and certainly didn’t try to integrate them into a grand theory of organic evolution. But judge for yourself: I hope you’ve read The Origin, so just peruse Matthew’s brief discussion and then ask yourself whether Matthew should get the lion’s share of the credit for the idea of natural selection.
************
One brief correction of Cole’s fine review: on its first page it describes Darwin as being “the ship’s naturalist” on the voyage of the Beagle. That’s a common misconception, for an “official” naturalist—the ship’s surgeon Robert McCormick—had already been designated. Darwin sailed on the Beagle using his own money, and his position was as both a “self funded naturalist” and also the “captain’s companion”. He was taken aboard largely to provide gentlemanly company for Captain FitzRoy, with whom he dined and conversed. Darwin’s researches and collections during the voyage were done on his own volition and enthusiasm.
The classic tale of “gene-culture coevolution” in humans—the notion that cultural changes in behavior changed the selection pressures that impinged on us—is the evolution of “lactase persistence” (LP) over the past four thousand years. LP is a trait that allows you to consume, as an adult, lots of milk or dairy products without suffering the side effects of indigestion, flatulence, or diarrhea.
Young children are able to tolerate milk while nursing, of course, but after weaning many of them no longer tolerate milk—they are lactose intolerant (LI). The ability to digest lactose goes away after weaning because the gene producing the necessary enzyme gets turned off.
The gain of LP, which enables you to drink milk and eat dairy products into adulthood without ill effect, rests on single mutations in the control region of the gene producing lactase, an enzyme that breaks down the milk sugar lactose. These mutations have arisen independently several times, but only after humans began “pastoral” activities: drinking milk from domesticated sheep, goats, and cows. And the mutations act to keep lactase turned on even after weaning. (Why humans turn off the gene after weaning isn’t known, but presumably involved the metabolic cost of producing an enzyme that wasn’t used in our ancestors, who didn’t drink milk after weaning until about about 10,000 years ago—when farming and animal domestication began.)
Based on analysis of fossil DNA, the LP mutations began spreading through Europe (starting from what is now Turkey) about 4000 years ago. And so the classic story—one that I taught my evolution classes—is that humans began drinking milk from captive herds, and that gave an advantage to retaining the ability to digest milk even after weaning. Ergo, natural selection for the nutritional benefits of milk led to the spread of LP mutations, as their carriers may have had better health (ergo more offspring) than individuals who turn off the enzyme at weaning).
This leads to the “coevolution” that is the classic evolutionary tale: a change in human behavior (raising animals for milk) led to selection for the persistence of the milk-digesting enzyme, and thus to genetic evolution. The “coevolution” part is the speculation that being able to digest milk without side effects would cause humans to raise even more dairy animals and drink even more milk, intensifying the selection for LP, and so the gene for LP would keep increasing in frequency.
A new paper in Nature, which is being touted all over social media, argues against this classic story, suggesting that it’s more complex than previously envisioned. Although the new results are touted as overturning the earlier story, they really don’t. There is still human genetic evolution promoted by a change in culture, and there’s still a reproductive advantage in drinking milk.
The new part of the story is simply that that reproductive advantage comes not constantly (as previously envisioned), but only during times of famine and disease, when those who couldn’t digest lactose were at a severe disadvantage because the diarrhea caused by lactose intolerance would contribute to the death of diseased or malnourished individuals. This is a twist on the main story, but doesn’t overturn it completely. There’s still the connection between culture and human evolution, and there’s still a reproductive advantage to LP that leads to natural selection and genetic evolution of our species. What’s different is how and when the selection acts (see “the upshot” at the bottom).
Click the title screenshot below to read, or you can download the pdf here. The full reference is at the bottom, and Nature deemed this worthy of two News and Views pieces in the same issue: (here and here).
First, the authors show the spread of dairy use in the figure below (the redder the color, the more milk usage over time in Eurasia. This was estimated from looking at the frequency of pot shards that had milk residue (click to enlarge). By 1500 BC, milk use was widespread.
Caption (from Nature): Interpolated time slices of the frequency of dairy fat residues in potsherds (colour hue) and confidence in the estimate (colour saturation) using two-dimensional kernel density estimation. Bandwidth and saturation parameters were optimized using cross-validation. Circles indicate the observed frequencies at site-phase locations. The broad southeast to northeast cline of colour saturation at the beginning of the Neolithic period illustrates a sampling bias towards earliest evidence of milk use. Substantial heterogeneity in milk exploitation is evident across mainland Europe. By contrast, the British Isles and western France maintain a gradual decline across 7,000 years after first evidence of milk about 5500 BC. Note that interpolation can colour some areas (particularly islands) for which no data are present.
One reason the authors doubt the classical story is that while dairying and milk-drinking by adults began about 10,000 years ago, the gene for LP (determined from sequencing “fossil DNA”) didn’t spread widely until about 4,000 years ago. Why is that? The mutation for LP is dominant, which means it could have spread widely very quickly, as even carriers of one copy would have a reproductive advantage. This temporal disparity is what led the authors to propose their alternative hypotheses for the spread of the LP alleles (there are several).
Further, when the authors tried to correlate the frequencies of the LP allele with the frequency of milk use (the classical explanation), they found no correlation—that pattern was indistinguishable from a general rise in frequency over Europe regardless of milk use.
One other set of data led to the new hypothesis. That is the observation that LI people in both Britain and China can still drink lots of milk without suffering any measurable health or reproductive effects (milk drinking has recently proliferated in China). Of course, things are different now from 4000 years ago, but one of the differences led to the authors’ two hypotheses: the spread of the LP allele was promoted especially strongly in prehistoric times by the prevalence of famine and of disease—with the latter coming often from animals, either domesticated or those that hang around settlements. (As the authors note: “about 61% of known and about 75% of emerging human infectious disease today come from animals”).
So the authors erected two hypotheses, the crisis mechanism and the chronic mechanism. I’ll let them describe the hypotheses that they tested (my emphases)
Given the widespread prehistoric exploitation of milk shown here and its relatively benign effects in healthy LNP individuals today, we propose two related mechanisms for the evolution of LP. First, as postulated in ref. 24, the detrimental health consequences of high-lactose food consumption by LNP individuals would be acutely manifested during famines, leading to high but episodic selection favouring LP. This is because lactose-induced diarrhoea can shift from an inconvenient to a fatal condition in severely malnourished individuals and high-lactose (unfermented) milk products are more likely to be consumed when other food sources have been exhausted. This we name the ‘crisis mechanism’, which predicts that LP selection pressures would have been greater during times of subsistence instability. A second mechanism relates to the increased pathogen loads—especially zoonoses—associated with farming and increased population density and mobility. Mortality and morbidity due to pathogen exposure would have been amplified by the otherwise minor health effects of LNP in individuals consuming milk—particularly diarrhoea—due to fluid loss and other gut disturbances, leading to enhanced selection for LP We name this the ‘chronic mechanism’, which predicts that LP selection pressures would have increased with greater pathogen exposure.
In other words, the reproductive advantage of having the LP allele came from the reproductive disadvantage (through death) of lactose-intolerant people during times of famine and disease.
They tested the two hypotheses by correlating indices of famine and of disease deduced from archeological and paleontological evidence:
Crisis mechanism: “Subsistence instability”, or famine, was assessed by prehistoric fluctuations in population size, which, the authors say, is correlated with the likelihood of famine (they provide no evidence for the latter supposition). But the correlation gives a significantly better fit to the pattern of LP allele frequency than just assuming uniform selection over time and space.
Chronic mechanism: The authors hypothesized that the frequency of disease would correlate with the likelihood of “zoonoses” (diseases caught from animals), which itself would correlate with temporal variation in settlement densities. These data, which to me would be correlated with “prehistoric fluctuations in population size” above, also explained LP allele frequencies better than an assumption of uniform selection.
Of course, there’s no reason (and the authors say this) that both mechanisms couldn’t operate together. Curiously, though, indices of the density of domestic animals did not support the “chronic mechanism” though measurements of the proportion of wild animals around humans did. This implies that, if the “chronic mechanism” is correct, people were getting sick not from their horses, dogs, cattle, or sheep, but from wild animals (perhaps from eating them).
Other hypotheses that the authors mention but didn’t test include “drinking milk as a relatively pathogen-free fluid”, allowing “earlier weaning and thus increased fertility.” I would add that if diseases are causal here, they could come not from being around animals, but having drunk contaminated water, giving an advantage to those who prefer milk. But there’s no way of assessing that from the archaeological record.
The upshot: On the last page of the paper the authors say that they’ve debunked the prevailing narrative:
The prevailing narrative for the coevolution of dairying and LP has been a virtuous circle mechanism in which LP frequency increased through the nutritional benefits and avoidance of negative health costs of milk consumption, facilitating an increasing reliance on milk that further drove LP selection. Our findings suggest a different picture. Milk consumption did not gradually grow throughout the European Neolithic period from initially low levels but rather was widespread at the outset in an almost entirely LNP population. We show that the scale of prehistoric milk use does not help to explain European LP allele frequency trajectories and thus it also cannot account for selection intensities. Furthermore, we show that LP status has little impact on modern milk consumption, mortality or fecundity and milk consumption has little or no detrimental health impact on contemporary healthy LNP individuals.
Instead, they say that they find support for the increase of LP alleles through both famine or pathogen exposure.
Well, the data are the data, and their indices comport better with those data than does the classical hypothesis—the “prevailing narrative.” I’m still not convinced that their proxies for famine or disease are actually correlated with famine and disease themselves, but other researchers will undoubtedly dig into that.
What I want to emphasize is that if the work of Evershed et al. is accurate, it still does not overturn the story of gene-culture “coevolution”. The “coevolution” is still there, the fact that a change in human culture influenced our evolution is still there, and the fact that drinking milk conferred higher reproductive fitness is still there. What has changed is only the nature of selection. Granted, that’s a significant expansion in understanding the story, but to listen to the media—social or otherwise—you’d think that the “classical narrative” is completely wrong. It isn’t. It’s still correct in the main, but the way selection acted may be different from what we used to think. The media love “evolution scenarios are wrong” tales, and that seems to be the cast of at least some stuff I’ve seen in the news and on social media.
I’ll make this short and sweet. A team of biologists from China have found, examining a fine-grained layer of fossils dated about 164 million years ago, a species of water boatman (“true bugs” in the order Hemiptera) that provide the oldest evidence for parental care in insects. The care is given by females, who attach their eggs to their second pair of legs. The curious thing is that in all the specimens examined, females attach the eggs to only their left middle leg: a rare example of “directional asymmetry”.
You can read about it by clicking below or downloading the pdf here. The reference is at the bottom of the post.
Parental care is not that rare in today’s insects and other arthropods; you can see some examples in modern insects here. It’s also been seen in fossil insects, with the earliest cases described in the paper:
Among Mesozoic insects, the only two direct fossil evidence cases of brooding ethology are provided by the Early Cretaceous cockroach Piniblattella yixianensis with its oothecae enclosing eggs for protection and brood care; and the mid-Cretaceous scale insect Wathondara kotejai, which preserves eggs within a wax ovisac attached to the body of an adult female.
An ootheca is an egg mass, usually enclosed in a hardened shell, as in this modern cockroach (photo below). I assume the mother in the fossil species would stay with the mass, otherwise I can’t see this as an example of “brood care”:
Here’s a picture from Wikipedia labeled “cockroach (Periplaneta americana) with ootheca”:
An “ovisac” is similar: a capsule containing eggs. In the case of the scale insect above, that’s clearly brood care because the ovisac was attached to the body. The Cretaceous period lasted from 145 to 66 million years ago; and oldest of these two insects having brood care dates to about 126 million years ago.
Now, from the Haiffanggou Formation at the Xiayingzi quarry, a formation in NE China with lots of ancient mammals, dinosaurs, and insects, they’ve discovered the water boatman Krataviella popovi. Fu et al examined 157 specimens, 30 of which were females carrying eggs on the middle segment of their LEFT foreleg. Note the directionality of this asymmetry. If it were random, the chance that 30 specimens would all have eggs on the left side would be 9.3 X 10-10.
The age of this formation is 163.5 million years, so the brood care in these boatmen precedes the previous ‘record’ by about 38 million years. It’s not a unique phenomenon in insects, but it’s the earliest example of that phenomenon.
Here are two photos of females carrying eggs (red arrows), both from the paper and both on their left side. The preservation is remarkable, with some of the specimens prepared using only a sharp knife:
Figure 2 [excerpt]. Brooding in Karataviella popovi. (a) General habitus of egg-carrying specimen (NIGP177390). (b) Details of egg (NIGP177447). (c) General habitus of egg-carrying specimen (NIGP177445). Scale bars: 2 mm in (a,c,d); 1 mm in (f–h), 500 µm in (b,e)Females and males can be identified independently of egg-carrying, so this is clearly a female trait. Modern water boatmen often attach their eggs directly to the substrate with a kind of biological glue, and then leave, so there is no brood care. The authors hypothesize that the females in these fossil specimens were still using some kind of adhesive, but that it was used this way:
Since water boatmen eggs cannot adhere to new surfaces after being detached from their original place of deposition, this suggests that the females first secreted mucous substance and then laid eggs onto their own left mesotibia by specific bending movements of the abdomen, and then carried the brood until hatching. The unoccupied right mesotibia might have been used to maintain balance when swimming and feeding.
What seems unusual to me is the directionality of the trait: it’s only found on the left middle leg, never the right one. This is called “directional asymmetry”. (If eggs were laid randomly on the left or right legs, it would be called “fluctuating asymmetry”.)
Directional asymmetry has fascinated me because, if it’s an evolved trait, it means that genes producing the directional trait “know” which side of the body they’re on. How can that be? If an ancestor already had biological or genetic gradients from top to bottom and front to back, it still means that a point on the right and left side with equal positions on these other two gradients would experience the same environment. So how do genes determine which side their cells are on so those genes can be activated differentially? I’ve talked about this before, and you can read about it here, here, and here. It’s a fascinating issue that’s not fully resolved. (Of course, once a genetic directional asymmetry is in place, it can be used as a developmental key for the evolution of further asymmetries. We ourselves have a fair number of such asymmetries.)
One solution, which just pushes the question back a bit, is to posit that the females have directionally asymmetrical ovipositors, and it’s simply easier to lay eggs on your left leg than on your right. But if the ovipositors and genitals are symmetrical (the authors don’t say), then it would probably be a directional behavioral asymmetry, with females behaviorally evolved to lay eggs on only one side. I don’t see the advantage of that, but of course behaviors can be directionally asymmetrical and conditioned by genes, like handedness in humans. It’s still interesting to me that one of the earliest cases of directional asymmetry known isn’t discussed by the authors except to mention it. Their own big message is that this is the earliest case of brood care seen in insects, not that it’s directional.
Finally, what is the advantage of evolving this kind of brood care? I’m sure you can think of answers: having your eggs with you protects them from predators, and also aerates the eggs as the beetle moves through the water. Or, as the authors note:
Karataviella adopted a strikingly similar brooding (egg-carrying) strategy to most marine and freshwater shrimps, lobsters and kin (Pleocyemata), where the females attach eggs to their pleopods using a sticky substance, allowing them to actively and intermittently adjust the position of the eggs in water or air, together with the movement during swimming that generates currents, to ensure ventilation and moistening of the eggs. Moreover, in K. popovi and some pleocyematans, a firm but elastic egg stalk is present and may contribute to the aeration of the eggs by facilitating regular shaking motion. Therefore, we speculate that the particular brooding behaviour of K. popovi effectively addresses the problems that large eggs experience relating to hypoxia, drowning and desiccation, resulting in enhanced offspring survival.
To close, here’s a drawing from the paper labeled “Ecological reconstruction of Karataviella popovi and anostracans in the Middle–Late Jurassic Daohugou biota.” What’s weird here is that all the egg masses are shown on the RIGHT mesotibia, and water boatmen do not swim upside down. Go figure.
Two days ago, for a small project, I compiled a list of ten ways that biology (and evolutionary biology in particular) has been distorted by ideology. These distortions usually come from the “progressive” (really “regressive”) Left, but the Right contributes, too. What’s important is that biological facts are being hidden or distorted in the service of people’s personal ideologies and politics. Ideologues find some lines of biological research, or conclusions from that research, uncomfortable—even deeming it dangerous—and think they’re doing a service by this kind of distortion and censorship. They aren’t.
Besides a few additional suggestions from readers in the comments, we have two new forms of distortion suggested by Richard Dawkins in a tweet:
Jerry Coyne has a fine list of 10 ideological distortions of biology. I can think of only 2 more: the recurrent ideologically-based yearnings to revive Lamarckism, and to revive group selection. Please read his list. Can you add to it?https://t.co/JQxDhQZQfx
I appreciate the call out! The “revival of Lamarckism”, I think, is the current view that epigenetic modifications of the genome, induced by the environment, can be inherited, and can constitute adaptations. There are no good examples: most of the alterations aren’t adaptive, and none of them last beyond a few generations. The ideology motivating this view is presumably a “Darwin-was-wrong” view, and perhaps the political notion that organisms are malleable by environmental change—though this form of change gets inscribed in the genes. (Another method is the “plasticity” hypothesis of Mary Jane West-Eberhard. but even my smartest colleagues can’t figure out how to interpret that theory.)
Group selection, of which we have no good examples in humans and only a few in other species (see the last chapter of my book Speciation with Allen Orr), may reflect another form of ideological “anti-Darwinism”, or perhaps a drive to explain how humans can become altruistic and kind via “selfish genes”. (But as Dawkins has explained repeatedly, apparent altruism, and certainly cooperation, can evolve via individual selection, and Steve Pinker has explained why group selection for human traits is cumbersome and unlikely.)
However, the promotion of group selection by Ed Wilson, the latest big revival of the idea, wasn’t so much in the service of an ideology but of ambition—Wilson wanted to be remembered for having his own Big Theory of human behavior, and group selection was it. His last books and talks pushed the idea that, in fact, almost every aspect of human behavior had evolved via group selection. (This isn’t just my interpretation, but one made by several of Wilson’s colleagues and friends.)
Now, this new article in Quillette, by a person using a pseudonym (no, I don’t know who it is), represents another substantial attempt to distort biology in the interest of ideology. The author documents at length how a whole group of Wikipedia articles, involving human behavior, intelligence, race, and other traits have been edited or even removed because the claims adduced weren’t comforting to the “progressive” Left. (And yes, the editing is all in the direction of expunging things that contradict wokeness). I haven’t checked the claims, which involves going through the editing history of many Wikipedia articles (the discussion is all on public view), but I direct you to the article to show you how censorious the woke editors have been.
Click to read:
The claims, if true, contradict Wikipedia‘s avowed aim of presenting the latest well-supported ideas from reputable sources; instead, they’ve cut out new and reputable sources in favor of older sources that buttress the ideologues’s claims, and have often replaced the claims of scholars with those of journalists. The aim is to effect “social justice”, not to give information.
“Tezuka” gives five examples of Wikipedia-tampering that he’s followed in depth; these are the areas covered:
1.) Recent evolution in our own species.
2.) Differences in average IQs among countries
3.) The “Flynn effect”: the observation that over the last century, IQs have risen gradually: about three points per decade
4.) The intelligence of Ashkenazi Jews, well known for being academic overachievers.
5.) Race and intelligence: the controversy about the terms “no evidence” versus “no direct evidence”
I recommend you read the article, as here I will focus on only one area: “Ashkenazigate”. This kerfuffle resulted in the removal of the entire Wikipedia article on Ashkenazi Jews and intelligence and the mention of that association on the entire site. I was especially curious about this one, as 23 and Me tells me this is my own genetic constitution:
So what happened? The author first explains why the topic deserves an article:
Although they comprise only about 0.2 percent of the world’s population, the Jewish people account for a large portion of its top achievers in domains of intellectual success. For example, they have won between a fifth and a quarter of the world’s Nobel prizes, and comprise over half of its chess champions. Ashkenazi Jews are particularly noted for their high achievement, including their high average performance on IQ tests. In his textbook IQ and Human Intelligence (Oxford University Press, 2011), Nicholas Mackintosh gives the following summary:
[I]t has long been known that Ashkenazi Jews have an unusually high average IQ (see Chapter 1); some of them also have the misfortune to suffer from a number of diseases, such as Tay Sachs disease, caused by the possession of two copies of particular recessive genes. One suggestion is that the two are linked: while homozygotes with two copies of the genes develop the disease, heterozygotes with only one copy develop higher than usual intelligence (Cochran et al., 2006). (Mackintosh 2011, p. 285)
Aside from its scientific importance, this topic of research is also an important part of the rebuttal to antisemitic explanations for Jewish achievements. In 2006, Steven Pinker wrote in the New Republic that “Jewish achievement is obvious; only the explanation is unclear. The idea of innate Jewish intelligence is certainly an improvement over the infamous alternative generalization, a worldwide Jewish conspiracy.”
Now I’m not touting myself here as being super-smart; I just haven’t followed this very closely, though I’ve heard the claim that there is “overdominance” for a disease gene, like Tay-Sachs, so that although having two copies of the gene form (“allele”) gives you a fatal disease, having one copy gives you higher intelligence than usual. (Presumably having two copies of the “normal” allele gives you lower intelligence than having one copy, though I don’t know why that would be true.)
A similar kind of “overdominance” obtains for sickle-cell anemia. In Africa, having one copy of the disease allele (a mutant of the beta chain of hemoglobin) makes you more resistant to malaria, while two copies gives you the disease, usually fatal at a young age. Having two copies of the normal hemoglobin allele makes you susceptible to malaria. In such a situation, where the heterozygote has higher fitness than either homozygote, the gene will be maintained in the population by selection—called “balancing selection”. This is why the allele for sickle-cell anemia is so common in West Africa, as well as in U.S. blacks whose ancestors came from West Africa. (The frequency is declining in the U.S. because we don’t have malaria and also because there’s been substantial intermarriage between whites, who don’t carry the allele, and blacks.)
By the way, I used this example in my evolution course to show that evolution doesn’t create the best possible situation: the price of heterozygote advantage is having a number of people die from the disease and a number of people with two “normal” alleles die from malaria. If there was a beneficent creator, he would have endowed us with a hemoglobin allele that protects us against malaria when present in two copies but doesn’t cause sickle-cell anemia. Then everyone in Africa would be protected from malaria and not susceptible to the disease. But that hasn’t happened. This is another bit of evidence against a loving creator, for if our genes do reflect a creator’s will, he/she/it has allowed many people to die of malaria and sickle-cell anemia. (I didn’t talk about the god stuff in class.)
Anyway, I don’t know the evidence for this hypothesis for the Ashkenazi, and in truth am doubtful about it. Besides not knowing the single-gene evidence for intelligence, there has to be a correlation of intelligence with number of offspring for selection to work. Further, we need data showing that two copies of the “normal” allele give you lower IQs than the heterzygotes. I’d like to read about this issue in a brief piece, but the original article from Wikipedia has been EXPUNGED.
For reasons I don’t know—perhaps connected with antisemitism or just a general denigration of genes affecting IQ—the article, which was documented with sources, was proposed seven times for deletion from Wikipedia. Then the manipulators made it vanish:
In October 2020, Wikipedia’s “Ashkenazi Jewish intelligence” article was nominated for deletion the seventh time. The argument presented for its deletion was more or less the same one that had been made in every previous deletion proposal:
[O]ur article is some sort of pseudo-academic jaunt through fringe literature as promulgated by the IDW-sorts and the evo-psychs. Meanwhile, nary a hint is here that the true context of this is antisemitism. The article is here to wave a flag: such discussions of race and intelligence cannot possibly be race realist in the WP:NONAZI sense because look at who benefits at this article? *wink*, *wink*
This seventh attempt employed a tactic that had not been used in the other six. Rather than directly arguing for Wikipedia to cease covering the article’s topic, this deletion proposal suggested that the most effective way to address the nominator’s complaint would be to delete the article and then recreate it in an improved state. This argument succeeded where every previous deletion attempt had failed, and Wikipedia’s article about Ashkenazi Jewish intelligence was deleted on October 19th, 2020.
After the article’s deletion, this stated plan to recreate it turned out to be a false promise. Instead, references to high average IQ among Ashkenazi Jews were subsequently removed from every other Wikipedia article in which this topic had been discussed, including the “List of Jewish Nobel laureates” article and the general “Ashkenazi Jews” article, with edit summaries stating that the various papers, articles, and books discussing this topic were no longer reliable sources. Among the many sources rejected with this justification were papers and articles published in the Journal of Biosocial Science, Mens Sana Monographs, Commentary, and the New York Times, and the book Abraham’s Children by Jon Entine (Grand Central Publishing, 2007). Following the final removal of this material in March 2021, Wikipedia no longer covers the topic of Ashkenazi intelligence.
So it’s gone, and the ideologues have managed to suppress both data and discussion. Note as well that a change like this in one part of Wikipedia ramifies through the site, so that there appears to be no discussion of an interesting phenomenon—Ashkenazi overachievement—anywhere on Wikipedia.
As I said, I don’t know a lot about the topics covered, and nothing about Wikipedia editing, but this article does scare me about the power of ideologues to affect what has in effect become the world’s go-to source of information. (It shouldn’t be for scholars, but erroneous material on Wikipedia has made it into scientific publications.)
The author ends his/her/their article with a warning about this kind of censorship affecting the credibility of Wikipedia. (That will bring joy to the heart of Greg Mayer, who has been promising us an article on “What’s the matter with Wikipedia?” for many years. It’s even partly written.)
The ending:
The original purpose of Wikipedia was to reflect the current understanding of the topics that it covers, not to exert an influence over fields to enact social change. The fact that it performed the first function so well for most of its existence, and came to be regarded as a trustworthy source, is what has made it such an effective tool for those who wish to use it for the latter purpose. While Wikipedia may ultimately prove successful at undermining research about topics related to human intelligence, it also may undermine its own reputation in the process. Formerly trusted institutions have begun to lose society’s trust as these institutions have surrendered to “woke” ideologies, as Quillette has previously described in the case of the New York Times, and Wikipedia will not necessarily be immune to this effect.
I have one hour to analyze in detail a new article about a bird. Can I do it? The answer, yes I did!
When the New Yorker publishes a piece about pure science, as it does in this article about a South American bird, I always suspect there’s a hidden agenda. That’s because the magazine isn’t really pro-science. As I quoted a respected colleague in an earlier post:
The New Yorker is fine with science that either serves a literary purpose (doctors’ portraits of interesting patients) or a political purpose (environmental writing with its implicit critique of modern technology and capitalism). But the subtext of most of its coverage (there are exceptions) is that scientists are just a self-interested tribe with their own narrative and no claim to finding the truth, and that science must concede the supremacy of literary culture when it comes to anything human, and never try to submit human affairs to quantification or consilience with biology. Because the magazine is undoubtedly sophisticated in its writing and editing they don’t flaunt their postmodernism or their literary-intellectual proprietariness, but once you notice it you can make sense of a lot of their material.
. . . Obviously there are exceptions – Atul Gawande is consistently superb – but as soon as you notice it, their guild war on behalf of cultural critics and literary intellectuals against scientists, technologists, and analytic scholars becomes apparent.
I think a lot of that is true, so what’s the reason we get a rather confusing article about the evolutionary history of a South American bird? Well, I don’t know the author’s brief or motivation (Ben Crair is a freelance writer), but judging from the title and the content, this is an article in the “Darwin-was-wrong” genre—in this case mentioning Darwin and the tree of life as being somehow overturned by the ancestry of the hoatzin bird. It isn’t, of course, but in trying to make his case the author produces a farrago of confusion that will not only befuddle the layperson, but also confused both Matthew and me. Click to read (it’s free):
Let’s meet the subject, first. The hoatzin (Opisthocomus hoazin) is one of the world’s weirdest birds in appearance and habits. Here’s its South American range:
Below: a video of what it looks like and the very weird climbing behavior of the chicks, who have retained ancestral claws on its wings (they lose them as adults). When predators attack, the clawed chicks, whose nests are built over water, simply drop into the drink, swim to shore, and use their claws to climb back to the nest.
Here’s a video that makes the climbing behavior more obvious:
The front claws, which made people think the bird is primitive, is not the only weird thing about it (and no, it’s not the only species with claws, just one that uses them for such a bizarre reason). This excerpt is from an article by Elizabeth Deatrick on Sketch, a feature of the Audubon Society.
Hoatzins are the only birds in the world that eat nothing but leaves, which, compared to seeds and fruit, aren’t very nutritious, and are hard to digest. So to accommodate this diet, the Hoatzin has evolved a multi-chambered digestive tract with lots of little “stomachs,” where the leaves can sit for a while and be digested by friendly bacteria. During the digestion process, the bacteria release methane that the bird then belches out, producing an olfactory aura that’s landed the Hoatzin a less-than-flattering nicknamed: the stinkbird. So much for fitting in.
It’s also called the “skunkbird.”
So we have some interesting facts. But what intrigues Crair is that when you try to place the hoatzin on the family tree of birds, it’s very hard. Its structure and morphology aren’t useful, because it diverged from other birds so long ago (it may occupy a long, old branch of its own). So people turn to DNA, which, if you use enough of it, should, via inspecting similarities and differences, tell you what species the hoatzin is most closely related to. But because it’s so long diverged, that’s been problematic too. Various analyses, depending on which parts of the DNA you look at, have suggested that it’s most closely related to turacos, or maybe to cuckoos, or maybe to rails, or maybe, as this article suggests, to the ancestor of cranes and shorebirds. The problem is that the DNA is so long diverged from that of other bird species that, depending on what part of the DNA you look at and which bird species you use for comparison, you get different answers.
But this is not your typical bird, and we can usually place a bird near its closest relatives if we use a lot of DNA. That kind of analysis sometimes yields surprising results: one I like to mention is that peregrine falcons are more closely related to parrots than to other birds of prey (“raptors”) like hawks, ospreys, and eagles. That’s based on a lot of DNA, and that is the correct placement of falcons on the bird evolutionary tree.
Of course the hoatzkin kind of difficulty does not invalidate the idea of a “branching tree of life”, for we are using a phylogeny of genes (one branching scheme) to determine the phylogeny of species (or populations), which can be a different branching scheme. And if you look at some animals and some genes, you’ll find that the family tree of genes, whose own evolutionary history branches when they mutate, does not match the phylogeny of the species themselves: the evolutionary history of the organisms that contain the genes.
These discrepancies between the “tree” of some genes and of the organisms that contain them have several causes.
The first is what we call “incomplete lineage sorting”. Let me explain. Suppose that an ancestral species has two mutant forms of a gene or DNA segment. Let us call them A, and B. Let us then suppose that the ancestor branches in such a way that it produces species X, Y and Z, and the branch that eventually splits to produce Y and Z comes off separately from the branch that leads to X. Now assume that each of these ancestors has all the A and B mutants, but then species Z loses the A form via genetic drift, while species X and Y lose the B form. This happens, for genes can change their frequencies by random processes not involving natural selection (“genetic drift”).
The history of the species themselves shows that Y and Z are more closely related to each other than either is to X, since they share a more recent common ancestor. But if you look at the one gene that had the two forms A and B, you’ll see that species X and Y are more closely related than either is to Z in terms of that gene, for they both have form A, while Z has form B. In other words, a tree for this gene shows an evolutionary history different from that of the tree for the species themselves. And if you look at another gene, which drifts independently, you may find that species X and Z are more closely related to each other than either is to species Y.
Since there are a gazillion genes to look at, it’s not unlikely that you would find such discrepancies. The genes can show three different evolutionary histories while there is only one for the species themselves, based on which populations evolved into new species. Gene trees and species trees can be discordant.
The way to solve this, of course, is to use lots and lots of genes, for together they should show a preponderance of phylogenies that match the tree of species themselves, since genes (with two exceptions mentioned below) stay within the borders of species. (The definition of “biological species” involves barriers to gene exchange.)
And, in general, that’s what we find. When we sequence whole genomes of species, like humans, chimps, and gorillas, we find what we knew from other data: humans and chimps (I lump bonobos with chimps) are more closely related to each other than either is to the gorilla, which is the more distant ancestor or “outgroup”. And all four of these species are yet more distantly related to the orangutan. The more genes we use, the closer we get to reconstructing the true evolutionary history of the species themselves.
Since the hoatzin is so long removed from other species of birds, it’s evolved nearly independently for nearly sixty million years, and so, depending on which species and which genes you look at, you could find that the hoatzin has genetic similarities that are discrepant if you use different comparison groups. There is no species closely enough related to the hoatzin to allow us to show a general similarity between its genes and that “sister” species. Ergo we don’t know the closest relative of the hoatzin. And we may never know.
I haven’t done a great job explaining this, but perhaps you’ll understand. Still, author Crair doesn’t try to explain this at all, referring in one sentence to incomplete lineage sorting as a “kind of genetic scrambling.” The key, though, is to understand that the evolutionary history of individual genes or segments of the genome is not the same thing as the evolutionary history of the populations of organisms that contained those genes—the species themselves.
The other factors that causes discrepancies between gene trees and species trees are hybridization, which can transfer genes between species that aren’t all that closely related, or horizontal gene transfer (“HGT”) via vectors like viruses. Both of these transfer bits of DNA into species that don’t reflect their evolutionary history, and trying to suss out species’ history from such wide gene exchange is confusing. That, too, could have been a problem with placing the hoatzin, but I doubt it.
Here’s one example from humans. Because of ancient hybridization between Homo sapiens and Neanderthals—I won’t get into the issue of whether they’re different species, though I don’t think they are—if you looked at the right gene in me, Jerry, and compared just that one gene to other gene forms in my species and to the Neanderthal genome, you might find that “for gene X Jerry is more closely related to Neanderthals than to his fellow H. sapiens.” And that would be true. Most of us probably carry a different set of gene bits from Neanderthals, and if you wanted to make a tree using just those bit, you’d find out that every one of us is more Neanderthal than sapiens. But of course this is only for the bits of genome that we’ve inherited after ancient hybridization between Neanderthals and our own ancestors. If you look at whole genome analysis, you’ll see that this small discrepancy washes out and you get the correct answer: all of us are more closely related to each other than to Neanderthals.
It is these issues that allow the author to conclude that the idea of Darwin’s branching evolutionary tree may be way overrated. Here’s how he states it:
The tree is so ingrained in evolutionary biology that scientists encourage “tree thinking.” By learning to think in terms of trees, students can avoid the common fallacy of reading evolution as a ladder in which simpler organisms become more complex, as in the famous image “The Ascent of Man,” which shows a knuckle-walking ape evolving into an upright human. For all its pedagogical value, however, the tree also embeds subtle assumptions about evolution. The tree tends to downplay the genetic variation within species, which can obscure the fact that common ancestors are actually diverse populations that can pass on different versions of a gene to different descendants. It tells a story of endless partition and diversification, with branches that diverge and never reticulate.
Now doesn’t that imply that the idea of evolutionary trees is dubious? In fact, the idea of trees for species, which is the way Darwin meant it to be construed, is doing fine; it just doesn’t always comport with trees for some genes within a species. Tree thinking is well and good and isn’t likely to go away.
And yet. . . and yet Crair admits this in one place:
The outlines of animal evolution still look a lot like a tree in many places, which is why scientists continue to spend so much time developing and debating different branches. But, if tree thinking taught biologists that everything is connected, genes are suggesting that the connections can run even deeper than a tree can capture. To gain a more complete picture—and to answer questions like how such an unusual mix of traits came together in the hoatzin—scientists may need to think outside the tree.
If Crair was careful to distinguish between species trees on the one hand and gene trees on the other, he wouldn’t have to create these apparent “discrepancies”. We’ve known about this issue for years; in fact, I wrote about it in my book Speciation with Allen Orr, and that came out in 2004. (See the Appendix.)
I don’t know if sloppy editing exacerbated the confusions in this article, or whether the author didn’t clarify them (I think he understood them). In the end, we’re simply left with these facts:
a. The hoatzin is a damn weird bird.
b. We haven’t been able to deduce its closest bird relatives.
c. There are evolutionary/genetic reasons for this difficulty.
But that wouldn’t make as click-worthy an article as the one that the New Yorker published—to wit:
a. The hoatzin is a damn weird bird.
b. We haven’t been able to deduce its closest bird relatives.
c. Therefore there must be something wrong with Darwin’s idea of a “tree of life”.
“a” and “b” are what you should remember, and also remember about why the hoatzin smells bad and the weird claws its chicks use to climb up trees.
