Category: development

Nature screws up again: touts need for severe revision of evolutionary theory while harboring a conflict of interest

November 9, 2025 • 10:00 am

Nature has shown some bad behaviors lately, and now you can add onto them two more: an ignorance of evolutionary biology and a lack of fact-checking. Both of these are instantiated in a recent book review, which, as we see so often, describes modern evolutionary biology as woefully incomplete.  The review, moreover, fails to mention all the critics of this “need for speed.” Finally, the review (of a book touting the deficiency of evolutionary theory), was written by a collaborator of several authors of the book, showing a severe conflict of interest. It’s no surprise that the authors’ colleague gave their book a glowing review.

A letter written by some well known evolutionary biologists pointing out these two deficiencies was promptly rejected by Nature.

I’ll give a critique of the book review first, and then show the letter sent to Nature that was rejected. Finally, I’ll give one of the signers’ responses to the rejection: Brian Charlesworth. I won’t give the names of the other signers of the letter (there were three), as Brian gave me permission to reproduce the letter but I haven’t asked the others.

First, the review. The book is Evolution Evolving: The Developmental Origins of Adaptation and Biodiversity, with authors Kevin Lala [formerly “Laland”], Tobias Uller, Natalie Feiner, Marcus Feldman and Scott Gilbert, published last winter by Princeton University Press, which apparently didn’t get the book vetted by competent evolutionists. The Nature review by Eva Jablonka, Israeli evolutionist and epigenetics maven, came out in January, so I’m a bit late to the party. Still, this shows that there remains a vocal minority of biologists who can’t resist showing us the many ways that evolutionary biology is wrong or incomplete, yet they’re singing the same old tune, one that’s been rebutted many times before.

Click below to read the fulsome review of the book; one that doesn’t even mention the many issues with the “new view of evolution” that have been pointed out for years.

Before I point out a few misguided statements, I urge you to read my take on a Nature paper called “Does evolutionary biology need a rethink?“, in which one group of “revisionists, with Laland (“Lala” above) being the first author, answers, “Yes, urgently”, while another group, with Greg Wray the first author, answers “No, all is well.”  As you’ll see from reading my piece, I side with the second group. Note that that exchange is already eleven years old, yet the promoters of the “rethink” view are advancing exactly the same arguments they made back then. These arguments are misguided because they are either flat wrong (e.g., their criticism of the neo-Darwinian view that mutations are “random”), or misleading (e.g., their view that development drives evolution, with development changing first and only then permitting adaptive genetic change). In her review above, Jablonka also throws in epigenetics, her speciality, which, while important in some respects, cannot form the basis of permanent adaptive evolution because environmentally-induced changes in DNA (“epigenetic” changes) persist at most for only two generations before the epigenetic marks are wiped away during gamete formation.

But I’m getting ahead of myself.

First, for the topic of “development leading evolution,” “nongenetic forms of evolution” (learning, culture, etc.), and epigenetics, all touted in Jablonka’s article, see my post above, this one, and my several discussions of the flaws of touting epigenetics as a critical and neglected factor in adaptive evolution.  I won’t repeat my arguments, but I will point out a couple of howlers in Jablonka’s review.  Her quotes are indented below.

First, on development as the guiding factor of evolution:

Under the extended evolutionary synthesis, the questions that are fundamental to the field change. Instead of just asking what genetic mutations might give one organism an advantage over its peers, the authors argue, evolutionary biologists should also focus on the developmental mechanisms and structures that underlie fitness differences.

It’s time to admit that genes are not the blueprint for life.

A developmental focus, they say, could help in understanding phenomena that are mysterious under the modern synthesis. For example, selective breeding for ‘tameness’, whether in sheep, pigs, horses, dogs or foxes, leads to the evolution of a common series of traits that are not necessarily adaptive — including smaller brains and teeth, curly tails, white patches and flat muzzles. This link, across different animal groups, bred in different ways and at different times, baffled Darwin and others for more than a century.

. . . All these features involve the same embryonic cell type (the neural crest) and their development is thus driven by similar sets of genes.

Well, as Dawkins pointed out years ago, genes are not the “blueprint for life,” but the “recipe for life,” as one needs environmental inputs to convert the DNA into an organism. As for development guiding evolution, what Jablonka and her pals apparently mean that existing developmental pathways constrain evolution: mutations can only show their effect within and already-evolved system of gene interactions. The pleiotropic effect of “tameness” mutations on several species is easily explained because you’re selecting at the same time for the side effects of tameness genes, which happen to affect morphology and color. That’s not new, and certainly doesn’t mandate a rethink of evolution.  As Brian wrote me:

“As has always been acknowledged by anyone with half a brain, the phenotypic effects of mutations are constrained by the existing developmental system. As Haldane put it, selection on humans could produce a race with the intellect of Shakespeare and the physique of Carnera, but for a race of angels we’d have to wait for the necessary mutations, both for the wings and the moral qualities.” 

But then Jablonka as well as Lala et al. (and other miscreants like Denis Noble) use this observation to claim that NEW TRAITS AND PRESUMABLY THE MUTATIONS UNDERLYING THEM ARE NOT RANDOM. From Jablonka:

The modern synthesis dictates that genetic mutations arise at random, which makes it hard to understand why these traits would consistently evolve in all these tamed animals. But seen through a developmental lens, things are clearer. . . . Thus, new traits do not arise at random. Some are more likely than others, and suites of traits often arise together. Understanding such ‘developmental biases’ can enable researchers to better understand how traits originate, what directions future evolution might take and how rapidly evolution might proceed.

They simply do not understand what evolutionists mean when they say features (and mutations) arise “at random” in evolution. The meaning is that mutations and the traits they produce occur irrespective of whether they are good or bad for the individual’s reproduction. Of course some changes are more likely than others, and mutations often have pleiotropic (“side”) effects on other traits. This means that what is subject to selection is the net effect of a mutation on the replication rate of the mutated gene.

What are examples of the “better understanding” that comes from considering development? The ones given by Jablonka, presumably from Lala et al., are not impressive. Here’s an example called “inheritance beyond genes”:

For example, certain whales learn from their mothers how to corral schools of fish into air bubbles. Desert woodrats (Neotoma lepida) eat their mothers’ faeces, which contain gut microorganisms that allow the woodrats to digest plants rich in highly toxic creosote. And molecules called epigenetic marks, which are associated with DNA and modify gene activity, are passed down through generations too. Epigenetic marks that form when mice in the laboratory are trained to link a particular smell with an electric shock, for example, have been passed down to their grandchildren — the young mice are scared of the same smell, even though they have never received the shock.

Two quick points: have the authors ever heard of “learning”? Or that learning might be primed by genes, as our learning of languages primes us to produce comprehensible syntax, but which language we speak depends on our environment? Is imitation of adaptive parental behavior (itself either genetically primed or learned) something new? Nope.  And as for epigenesis, I have heard of the mouse study, but no epigenetic trait produced by the environment can persist for more than a handful of generations, as epigenetic modifications of DNA are wiped out during gamete formation. This form of “Lamarckian” inheritance won’t work.

Here’s one more:

Furthermore, some organisms construct environments to benefit the development of subsequent generations. Dung beetles, for instance, make balls of cow dung, into which they add their own faeces as food, and lay a single egg. The nutrients and microbes in these balls influence how the larvae develop, and in turn the sizes and shapes of the beetles and how they evolve.

Is it a revolutionary insight to discover that parents do things that benefit the fitness of their offspring? Human mothers feed their babies, and sometimes what they feed them could affect their own future evolution. Big whoop!

This all shows that the insights that supposedly mandate a new theory of evolution aren’t new at all, but are comfortably part of the already-existing Modern Synthesis of evolutionary theory.  But these authors, it seems, want to make their mark by advancing the same old tired arguments that have long been refuted.

Along with several other authors, Brian Charlesworth noted that Jablonka seems resistant to even mentioning the many objections to the “new” theory of evolution. Brian and others sent the letter below to Nature for consideration for publication. The references given in the submitted letter are included, and I’ve put in the links. Doug Futuyma’s paper is especially thorough and on the mark, and here’s his point, given in the last sentence of the abstract: “Evolutionary theory will continue to be extended, but there is no sign that it requires emendation.”

The letter:

We are writing to express our concern about the review in Nature by Eva Jablonka of the recent book by Kevin Lala et al. (Evolution Evolving)(16th January 2025 pages 539-541). The book expounds the “Extended Evolutionary Synthesis” or “EES” which is claimed by its proponents to repair problems with the science of evolutionary biology. Prof. Jablonka was a co-author with two of the book’s authors of an article promoting these claims 1, which would seem to be a conflict of interest for its reviewer. The article that accompanied that publication and refuted such claims 2, is not mentioned by Jablonka, nor are other critiques of the EES, e.g., 3. These papers make clear that several of Jablonka’s assertions are wrong, including the claim that evolutionary biologists believe that mutations “arise at random” with respect to their effects on traits, and that constraints imposed by development on evolutionary changes have been ignored by them. The review gives a false impression of the current state of the flourishing field of evolutionary biology, which owes little to the EES. It is regrettable that Nature should give a platform for such disinformation.

1          Laland, K. et al. Does evolutionary theory need a rethink? Yes, urgently. Nature 514, 161-164 (2014). https://doi.org/10.1038/514161a

2          Wray, G. A. et al. Does evolutionary theory need a rethink? No, all is well. Nature 514, 161-164 (2014). https://doi.org/10.1038/514161a

3          Futuyma, D. J. Evolutionary biology today and the call for an extended synthesis. Interface Focus 7, 20160145. http://dx.doi.org/10.1098/rsfs.2016.0145 (2017). [JAC: This is a Royal Society journal]

What transpired is that Brian says he heard nothing from Nature for a long time. He wrote back to the editor asking what happened to the joint letter. The editor explained that an automatic email response had been sent saying that if the authors didn’t hear anything within three weeks, then the letter was rejected. Brian says he didn’t see that response and admits it could have been binned without him reading it.  The editor also explained why the letter above was rejected, but I can’t reproduce that email without permission. However, you can get a sense of what the editor said from Brian’s final response here:

Dear EDITOR’S NAME REDACTED

Thank your for response. I and my co-authors do not consider it to besatisfactory, for the following reasons.

First, no automated response was received by me; our email was simply ignored.

Second, you say that “the comment piece cited in the review did include both pro and con arguments and authors from both camps”. I assume that you are referring to the reference to Laland et al. 2015, which is the only citation given by Jablonka. This was a polemical piece, arguing for the EES [“Extended Evolutionary Synthesis”] with a few dismissive references to works by mainstream evolutionary biologists.

Third, if asking someone to review a book by their close collaborators is not a conflict of interest, it’s hard to see what would constitute one.

Fourth, you say that “it didn’t make a fresh point that would be of broad interest to readers”. The point of our letter was to make it clear that Jablonka and other advocates of the EES consistently ignore the counterarguments made by ourselves and others in the evolutionary biology and genetics community. Indeed, her review contains the same tired old mistatements about randomness of mutations and developmental constraints that she and her clique keep on making. lt’s hardly our fault that these are not novel. The title of the review “A new vision for evolution is long overdue” gives the completely misleading impression that there are serious problems with our field. This is a view that is held only by a small, but extremely vocal, fringe group, most of whom (including Jablonka) have made no significant original research contributions to the field. No other field of science seems to get this kind of treatment from Nature.

Fifth, you say that “in the end the main goal of our book reviews is to set out issues in a readable way for readers across all disciplines, and we consider that Jablonka did a reasonable job here”. This seems to assign lesser importance to scientific accuracy. Indeed, you have just published a letter about the Jablonka review by a Chinese scientist trying to revive Darwin’s long discredited theory of pangenesis. He states that the theory was published in the last edition of the Origin of Species in 1859 (in fact, the last edition was published in 1872 and contains no reference to pangenesis, which was described in Darwin’s Variation in Animals and Plants under Domestication in 1868. Seemingly, the most basic fact checking is not done by Nature).

In view of these concerns about the treatment of the field of evolutionary biology by Nature, which are shared by my cosignatories (who are all regarded as leading figures in the field, and members of various national academies), I am cc-ing this email to your chief editor.

Yours sincerely,
Brian Charlesworth

Sadly the readers of Nature who are not evolutionary biologists will now think that Lala et al.’s book has indeed shown the need for a “new vision of evolution.” Given the history of the arguments made by the authors, and Jablonka’s summary of the book in her review, there is no such need. Nature blew it by rejecting the letter, which makes essential points (especially Jablonka’s failure to say that the “new vision” is deeply controversial), and also by getting a pal of the book’s authors to review it. What kind of review did they expect?

Sex variation in birds, with Emma Hilton’s analysis

August 21, 2025 • 10:20 am

Here we are dealing with sex again. But quite a few readers have written me asking me about the new paper below, which appeared in Biology Letters of the Proceedings of the Royal Society of London. (Click on the title below to read, or find the pdf here.

. . . and there is also a News and Views in Science.

The upshot of the paper is that researchers from Australia looked at 480 Australian birds across five species (rainbow lorikeet, scaly breasted lorikeet, laughing kookaburra, crested pigeon, and Australian magpie); their goal was to see how often a bird’s sex chromosomes (ZW in females, ZZ in males; in birds females are the heterogemetic sex), were discordant with that bird’s gonadal makeup (what we call “biological sex” as well as other aspects of its morphology. (Since these birds are sexually monomorphic for color and pattern, the authors looked at wing, bill, and tarsus size, which presumably do vary among the sexes on average.)

But the main object of study was whether the chromosomes—identified using two sets of DNA primers for genes that were chromosome-specific—were dicordant with the gonads. If everything’s concordant, all ZW birds should have ovaries and all ZZ birds should have had testes.

The surprising result was that there was a fair amount of discordance between sex traits (gonads) and the chromosomes, ranging between 3% and 6% of individuals depending on the species. (These individuals are called “sex reversed”, which I think is a bit confusing.)  But it’s still high. Moreover, most, but not all of those “sex-reversed” (henceforth “SR”) individuals seemed to have gonads that appeared normal, though testes in SR ZZ males were generally smaller than normal. We don’t know what percentage of the SR birds were fertile, though at least one female showed signs that she produced an egg.

The authors also found that more than a third of the SR individuals had both male and female gonadal tissue, though most of these were all likely sterile or fertile as only one sex (the authors dissected dead or injured birds sent to wildlife hospitals and thus don’t know their reproductive history).  From the paper:

. . . . . 20% of sex-discordant individuals in our study presented with some gonadal enlargement, indicative of reproductive readiness [67–69], while 36% had atypical gonadal make-up (i.e. ovotestes, both an ovary and a testis or ambiguous gonads.

My conclusion:

Since I’ll take 5% as the general proportion of SR birds, 36% of that is about 1.8%, meaning that 1.8% of the sample—if you consider these birds a random sample—had a mismatch between gonads and chromosomes, either having fairly normal gonads that were different from those predicted by the chromosomes, or having ovotestes and were true intersexes.  That is unexpectedly high. The authors do say that birds can get screwed up this way because they’re susceptible to environmental toxins, but we don’t know about these individuals.

Now before these data are scarfed up and distorted by gender activists, I have to make a few points:

1.) Humans do not have anything like this kind of discordance. How do we know? Because by now thousands of human genomes have been sequenced, both randomly by the NIH and the “thousand genome project” (now much more than 1000), as well as gene-sequencing companies like 23andMe, and if there were this kind of discordance, we would know: fertile women who submitted their DNA, for example, would hear that they had a Y chromosome. So you can’t extrapolate these bird data to humans, who are very different in both chromosomal constitution and lability to disorders of sex determination (see Emma Hilton’s tweets below).

2.) The prevalence of “intersex” individuals in humans is much lower than these authors observed in birds. Although “intersex” has been estimated in different ways by different people, decent estimates range around one in 5,600 people (0.018%) or, close to that, about 1 individual in 6700 (0.015%). That is much lower than 1.8%, which is nearly 2 birds in a hundred.  Extrapolations to humans are again unwarranted.

3.) These data do not tell us that the sex binary is wrong, in birds or any other animal. Even the SR birds, produced either testes, ovaries, or tissues from both: two types of reproductive tissue evolved to produce the two types of gametes that constitute the sex binary. There was no tissue that could have produced any other type of gamete, nor do we know of any such thing in birds.

4.)  These data say nothing about the prevalence of gender-nonconforming or transsexual individuals in other species, including humans. It is folly, of course, to use this kind of data from nature to address these gender-ish phenomena in humans.  What these authors have an “is” (discordance) in birds, but gender-nonconforming and transsexual people in humans still conform to the sex binary, but feel their gender is different from that of their natal sex.  And of course discussing the problems with extrapolating these data to humans is not in any way “transphobic.”

So that’s my caveat, but Emma Hilton from the University of Manchester, who knows a lot more than I do, has produced a thread of tweets about the paper with her usual wit. The tweet thread starts here, and I’ve posted them all below.

Emma’s last couple of tweets were added in response to my importuning her to say something that I could understand, because, with her knowledge, she wrote tweets I found hard to fathom. She also wrote me an email in response to my own question, which as I recall was something like, “Emma, does this mean that the proportion of true intersex birds is much higher than found in humans?”  Her response [“DSD” means either “disorders of sex determination” or its more euphemistic “differences of sex determination”].

I’m not resistant to “true intersex”, although I could introduce a resolution not often talked about – the left-right resolution 😀  [JAC: see below about developmental asymmetry]
OT-DSD (ovotestis-DSD) in humans is “true intersex”, at least when the amounts of each tissue generate meaningful conflict in downstream development, which, for the most part, they don’t. Most OT-DSD is discovered in XX individuals with residual testicular tissue that doesn’t interfere with healthy ovarian tissue function and downstream development i.e., they are uncomplicated females and furthermore, natural mothers!
(On the above, there is brilliantly-crazy paper that you might like to see, where a group of medics in Turkey – IIRC – present a panel of female OT-DSD and babies [all good info], and the whole discussion is about Jesus and the possibility of virgin births).
So birds are more plastic than humans for various reasons – the specifics of their genetic determination, the common asymmetry of development in females (that might hint at the possibility of sequential hermaphroditism), the ensuing susceptibility of the undifferentiated gonad to a “make male” trigger.
So I’d be happy to stand by the premise that OT-DSD is often “genuinely intersex” at the individual level (typically arising from a left-right conflict) and the birds are more susceptible to this particular type of conflict.
h/t: Luana

Right versus left: define them (directions, not politics)

September 11, 2024 • 11:30 am

Here is a question that keeps me awake at nights: how do you define right versus left without referring to something, like the placement of our heart, an organ that is already tilted toward one side of the body (the left except in rare cases of situs inversus)?

For example, have a look at a bilaterally symmetrical organism below, in this case one of my favorites (Merriam-Webster defines bilateral symmetry as “symmetry in which similar anatomical parts are arranged on opposite sides of a median axis so that only one plane can divide the individual into essentially identical halves”.) We know left from right because we define them consistently, and that’s because humans are NOT bilaterally symmetrical so we can all agree on which side is which.

But now I’ll ask you to answer this. (i.e., by pointing) Assume you’re talking to a person (a Martian?) who has never heard about right vs. left sides.  Tell them, using the diagram of one of my favorite organisms below, standing upright, which side is the right and which the left without referring to your own body, to any minute differences in the diagram, or to asymmetries in the environment (e.g. the world or the solar system).  Since both sides are identical, how do you know which one is right without referring to how we’ve already defined it, presumably based on our own bodies?  Explain to a Martian who is bilaterally symmetrical which side is its right and which its left, and how they would know it.

Partial image by Charl Hutchings, CC BY 4.0, via Wikimedia Commons

 

I’m not sure if I’m making myself clear here, so I looked in the Oxford English Dictyion for the definition of “right”. There are of course many definitions that don’t refer to the direction, but here’s what it gives for the direction:

a.  of, relating to, situated on, or being the side of the body which is away from the side on which the heart is mostly located
b.  located nearer to the right hand than to the left
c. located to the right of an observer facing the object specified or directed as the right arm would point when raised out to the side
d. located on the right of an observer facing in the same direction as the object specified

This didn’t help, because it all comes down to how humans have defined the sides based on our own asymmetries.

This problem is connected with something that’s always intrigued me: how do directional asymmetries evolve, in which an animals is predictably asymmetrical, like our hearts being more on one side or the others?  (There are some creatures with “fluctuating asymmetry”, in which right is different from left, but it’s not consistent, like lobsters in which one claw is a crusher and the other a slicer, or flatfish that develop to lie randomly on its left or right side sides as adults.  Evolving these doesn’t pose the problem I describe below.)

If we evolved from a bilaterally symmetrical (or radially symmetrical) organism, then even if front and back are genetically specified, as they are, how can you evolve from such a creature into an organism that has features consistently on the right (or left) sides?  The chemical gradients in a bilaterally symmetrical ancestor are presumably the same on both sides, so how can a gene mutation arise that consistently recognizes a given side to give rise to a feature on that side? In other words, how can a mutation KNOW whether it is on the left or right side of the body? (Of course once an initial directional asymmetry has evolved, it creates a directional cue that can be used to evolve further directional asymmetries. It’s the evolution of the first directional feature that is the difficulty.)

I’ve discussed this more clearly in two old posts on this site (here and here), which gives some partial answers residing in how asymmetrical molecules or asymmetrical beating of cilia could lead to the evolution of directional asymmetry from bilateral asymmetry.

But the problem above still nags at me: how do you tell a bilaterally symmetrical Martian which side is right and which is left without referring to our own bodies? Can it be done?

Again, this may be a non-problem, but I’ve seen no definition of “right” or “left” independent of our own bodily asymmetries.

Peter Holland lectures on the diversity of animals

July 2, 2020 • 2:30 pm

So far I’ve watched only about 30 minutes of this brand-new (virtual) lecture on the diversity of animals by Professor Peter Holland of Oxford University’s Department of Zoology, but it looks to be good. Not long ago I read his The Animal Kingdom: A Very Short Introduction, one of Oxford’s lovely small paperbacks to introduce people to new fields. It was an excellent read, despite my initial worries that a short book couldn’t begin to cover that topic.

Holland is a clear and eloquent lecturer, and his slides are very good as well.

 

Holland puts his talk within the framework of Darwin’s theory of evolution, laying out the evidence for evolution Darwin mustered in The Origin, and segues into one of his interested: evolutionary developmental biology (evo-devo).

Here’s the lecture summary:

When we think of evolution, the first person that springs to mind is Charles Darwin. In The Origin of Species (1859), Darwin presented evidence supporting evolution, proposed the useful metaphor of an evolutionary ‘tree’, and suggested an underlying mechanism: natural selection acting on variation. But there were still big questions, such as the shape of the tree (who is more closely related to whom?) and the nature of inherited variation (what are variants or mutations?)

In this talk, Professor Peter Holland explored how animal evolution is studied in the 21st century, with a focus on remarkable new insights we are gaining from molecular biology and genome sequencing.

h/t: Matthew Cobb

Directional asymmetry: how does it develop and how did it evolve? Part 1.

February 7, 2017 • 9:15 am

This post began turning out longer than I intended, so I’m going to divide it in two, with the second part up tomorrow.

When we consider major organs or features of animals, they can be bilaterally symmetrical, with the traits the same on both sides, or bilaterally asymmetrical, with differences between left and right. And there are two major forms of bilateral asymmetry.

In antisymmetry (which can be considered a macroscopic form of “fluctuating asymmetry” see here), there is no directionality to the trait, so the asymmetry is random with respect to the side of the body. One example of this is the lobster claws, in which one becomes a “crusher” claw and the other a “cutter”, as in this individual.

cmfwcrl

Now this asymmetry is adaptive in the sense that it’s useful for a lobster to have one claw that can crush and another that can cut; it’s like a crustacean Swiss Army knife that can do multiple things. But if you look at lobsters, you’ll find that the crusher claw is on the right as often as it is on the left; in other words, the asymmetry is random in direction among individuals.

This is still an evolved trait, as it’s clearly adaptive to have the two functions, but it doesn’t really matter to the lobster which side does which.

We know how this asymmetry develops—at least proximally. What happens is that the claw that is used most often after the fourth molt develops into the crusher claw, and the other one into the cutter. (I recommend having a look at the link, which details some clever experiments.) This means that there is some developmental program in the lobster’s genes that turns on “crusher” genes in the most stimulated claw, and that, in turn, may activate genes (or repress genes) on the other side of the body leading to the development of the cutting claw.  In this case the environment itself, or rather the behavior of the animal interacting with the environment, activates the genetic program, and since it’s apparently random which claw is most stimulated, we get half the lobsters with a crusher on the left, and half with the crusher on the right.

Here’s another example of antisymmetry, the big vs. small claws of the male fiddler crab, Uca deichmanni  (the females’ claws are the same size). There are equal proportions of right-clawed and left-clawed males:

8102a06fig1
Source. Photo by A. Anker.

Another type of asymmetry is directional, that is, the left and right sides differ, but always in the same direction. We’re familiar with this in our own bodies, in which the heart and viscera are directionally asymmetrical. The bulk of the heart, for instances is on the left side of the body, which is why you feel your heartbeat on that side. Quirks of Human Anatomy gives more examples:

Our right lung has three lobes but our left lung only two. Our heart is shifted to left, our spleen is located on the left, and our stomach bulges to the left, whereas our liver is shifted to the right. Our colon curls into a question mark, although its exact path can vary from person to person.

There are rare individuals in which every directionality like this is reversed due to a condition called situs inversus; these individuals are usually normal, but their innards are mirror images of the much more frequent “normal” individuals.

These kinds of directional asymmetries are not infrequent. Male narwhals, for instance, have a grossly enlarged canine tooth (up to 3 meters long) that forms a tusk, and it’s always on the left side, as shown in this photograph below. (Females don’t usually have tusks, which might imply sexual selection via male-male competition, but it looks as if the tooth/tusk is a sensory organ that males, use to communicate with each other when they rub tusks.)

monodon_monoceros_cranio_narvalo_narwhal_skull_museo_di_bologna-1

Another example of directionality are some nocturnal owls in which the ear openings are asymmetrical; this helps them localize prey. In the barn owl, the left ear opening is higher than the right. Here’s another owl, the boreal owl, showing directional asymmetry in the skull:

owl_skull_jim_duncan

Flounder species show both forms of asymmetry. In some species of flounders, which begin swimming vertically, they subsequently flatten so that they lie on their left side, with the left eye migrating over the head to the right side, while other species lie on their right side with the eye migrating the other way. These species are directionally asymmetrical, but in opposite ways. Still other species of flounders also flatten, but in a random direction, so some individuals lie on their left sides, and others on their right; this, of course, is antisymmetry.

Like antisymmetry, directional symmetry is often adaptive in that it’s useful to have only one side enlarged, and if you have to enlarge a tooth to make a tusk, it’s got to lead to asymmetry. Here, however, we face a genetic problem: the induction of the tooth on a given side is not due to random environmental stimuli, but is somehow to the genes themselves. There must be a genetic program in narwhals, for example, that says “make left tooth grow large,” regardless of the environment. And that means this: those genes know whether they’re on the right or left side of the body!  It’s easier to envision genes knowing whether they’re in the front or back half of the body, as an anterior-posterior gradient is set up in the egg or early zygote. But such gradients aren’t obvious for the right versus left sides of the body in animals that are, by and large, bilaterally symmetrical.

Let me add first that while it may be important to be asymmetrical, as with the lobster, there probably aren’t many cases in which directional asymmetry is more important than antisymmetry (can you think of examples?). In these cases which form of asymmetry evolves may just be a result of whether the genetic variation promoting asymmetry is of the antisymmetric or the directional sort.

When I was younger I pondered this question at length.  Yes, you can determine front and back in the egg, and then top vs. bottom (dorsal versus ventral), but, unless there’s some directional left-right gradient set up in the egg  (and I wasn’t sure how that would work), I couldn’t see how a gene would know, from its internal environment, which side of the body it was on. (Draw a box with a front-back and a top-bottom chemical gradient; you’ll see that the concentrations of the “morphogen” chemicals are the same on the left and the right.) How, then, I wondered, could directional asymmetry, which must involve genes taking cues from their local environments, ever evolve?

Well, if we start with a single trait being directionally asymmetrical, that would be all that is required for subsequent traits to cue on that, or on each other, to themselves evolve directional asymmetries. And even organisms that look pretty bilaterally symmetrical, like Drosophila, can have subtle directional asymmetry (flies have asymmetrical guts and the male genitalia rotate in a given direction during development.)

But that still leaves a problem: Assuming that organisms evolved from a common ancestor that was completely bilaterally symmetrical (right vs left), how did the very first directional asymmetry evolve? With gradients the same on both sides of the organisms, how could gene variants accumulate that would be activated (or silenced) on a consistent side of the body?

I’ll leave this for readers to ponder. If you’re a biologist, you may already know some of the answers. I’ll discuss some solutions (and some selection experiments) in the next installment.

For more on directional versus antisymmetry, go to Rich Palmer’s website at the University of Alberta.

Fruit fly embryo development visualized in real time

June 13, 2012 • 11:07 am

From Nature News we have this amazing video, “Fruitfly development, cell by cell.” It’s based on two new papers (references below) that produce a three-dimensional image of animal development:

Current light-sheet microscopy techniques involve illuminating one side of the sample. Either one side of a developing organism is imaged continuously, or two sides are viewed alternately, with the resultant data reconstructed to form a three-dimensional view. However, viewing from one side at a time means that the cells cannot be tracked as they migrate from top to bottom, and rotating the sample to view both sides takes so much time that when the next image is taken the cells have changed, so that they no longer line up.

Simultaneous multi-view imaging solves this problem by taking images from opposing directions at the same time and piecing data together in real time. This required massive computing power; the data sets were as large as 11 terabytes (the amount of data on about 2500 DVDs) in one of the studies1. Now every cell in a D. melanogaster embryo can be visualized as the animal develops from a fertilized egg into hatching larva. . .

Keller says that the techniques allow researchers to see what is happening in an entire animal through every stage of development, and what goes wrong as a result of different mutations. “Until now, developmental biology was a qualitative field, describing different mutations and their effect during development. But we couldn’t see what individual cells were doing in an individual embryo,” he says. Keller and his colleagues are now using the technique to follow the growth and differentiation of neurons in the developing brain of D.melanogaster and other species.

Below is the development of a Drosophila melanogaster embryo within the egg. You can see the classic insect segmentation form as the cells move about.  After about a day, this egg will hatch into a larva (the “maggot”), which after about five more days will crawl out of its food (they’re reared in vials of agar-based medium), pupate on the wall of the vial, and then begin the transformation into an adult fly. At 25 degrees C (about 78F), it takes about 8-10 days from when a fly lays an egg until that egg becomes an adult fly (and another 12 hours or so before the adult female can lay another egg), so one can go through 30 or more generations per year. That’s why flies are so good for genetic and evolutionary work.

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Tomer, R., Khairy, K., Amat, F. & Keller, P. Nature Methods http://dx.doi.org/10.1038/nmeth.2062 (2012).

Krzic, U., Gunthur, S., Saunders, T. E., Streichan, S. J. & Hufnagel, L. Nature Methods http://dx.doi.org/10.1038/nmeth.2064 (2012).