Around the early 2000s, while it was well-known that stress is a key factor in influencing the adaptive adjustments organisms make to changing environments, how exactly organisms accommodate stress-induced variation was poorly understood. In a review published in the Proceedings of the Royal Society B in 2005, Alexander Badyaev brought together and synthesised empirical studies of phenotypic and genotypic variation induced by stress, as well as evolutionary responses, to provide the missing developmental perspective on the intersection of stress and evolution. Twenty years after the paper was published, I spoke with Alex Badyaev about the origins of his interest in stress, his memories of the making of this review, and what we’ve learnt since about the topic.
Citation: Badyaev, A. V. (2005). Stress-induced variation in evolution: from behavioural plasticity to genetic assimilation. Proceedings of the Royal Society B: Biological Sciences, 272(1566), 877-886.
Interview conducted online on 25 February 2025; Alex Badyaev was in Tucson, USA and Hari Sridhar in Klosterneuburg, Austria.
Hari Sridhar: I’d like to start with something you said in the email you sent me. You said, “It is actually just about the best time to revisit this particular paper as this field experiences (a much more informed) renaissance and I, personally, have far greater understanding of why I was interested in this topic back then”. I want to ask you about the second part: What made you interested in this topic back then? Looking at your publication profile, before that time, you’d done a lot of work on sexual dimorphism, mostly on birds. Just from the titles of the papers, it looks like there was some work on stress in mammals and insects.
Alexander Badyaev: Yes, my previous work, at that time, was mostly on sexual dimorphism; mostly in the sense of morphological evolution. But I think stress, and its role in evolution, has been the most central topic of my entire work, regardless of whether or not it made it to the titles of the papers. Doing this review and synthesizing the components that go into it made me realize that, basically, stress encapsulates the two most essential properties of life as we know it. The first one is the dynamic, always changing, nature of it. Life is this unstoppable dynamic process—every next step is always new and different from the previous one. So, life is change by definition, lack of change is death, and yet life consists of elements that must be continuous and cannot be interrupted, they must constantly transcend this change, adaptative states, and constantly create new associations. And this brings up the second property – each new step, each transient adaptation, involves coevolution with other external elements or other fellow travellers, some very familiar, others entirely new. We typically define stress as something that exceeds the normal range experienced by a lineage. Measures of normality here essentially refer to time, to evolutionary memory of association – basically if you encounter something that you either remember, in an evolutionary sense, or not. If you don’t, you declare it to be new. If it is sufficiently new to exceed your current competence to react to it and yet you must interact with it or otherwise survive the interaction, then you declare it to be stress. There are ten thousand definitions of stress, but this is the most common denominator. So I think the reason that people constantly circle around stress in evolution in earlier works— for example, in Hoffmann and Parsons— is because it’s the best description of the dynamic aspect of the evolutionary process that we can have. Adaptation and complexity focus on stability. Concepts of stress explicitly invoke continuity of life, variable path-dependencies of elements that build it, and their necessary coevolution.
Once I realized that, I started structuring my research program around it. And because I study evolution, essentially all my work is on stress in evolution—looking at how organisms exist in the unpredictability of their evolutionary associations, how they channel and retain variation, how they suppress variation, how they express variation during particularly stressful changes. And then, also taking it all the way to the molecular basis of it – trying to figure out how you can build a multistep system that can be specific and yet not fragile, so it can subsist in stress. So basically, I’m still very much there! I’m still circling around that same fundamental question in evolution – trying to really understand how it works. I’m in good company, though.
Empirically, talking about this paper, it came out of a project on shrews. Shrews are phenomenal because they’re very short-lived. The species we study live for less than a year, which, in a seasonal environment, means that most things they encounter in the environment are new. It is true for everyone, of course, and is a question of scale, but it is particularly evident in shrews – you only see one summer, one autumn, one spring. The environment in which they grow up is pretty much the only one they’re going to get, so there’s no reason to prepare for anything else. So, they essentially must ride this unending novelty, there are no replays. There are five species of shrews in Montana, and I studied four of them as a graduate student. And the experimental measures of stress in my studies were forest management practices – essentially just clear-cuts – which certainly qualify as both unpredictable and stressful. We compared how shrews born in one type of forest deal with changes in this forest during their lifetime. Basically, when the environment of birth is a poor predictor of what you will encounter in your life. The Forest Service was mandated to monitor small mammals before and after each treatment, so they would set up these pitfall traps and shrews would fall into these accidentally, but because they must eat every 15-30 minutes, they die in those traps. And so the Forest Service ended up passively collecting them in their freezers for decades—before clearcut, after clearcut.
And I learned of that phenomenal collection of thousands and thousands of poor shrews when I was teaching mammalogy at the University of Montana. So, I analysed it and discovered that if you look at the stress-induced variation in their jaws, you find that it is orthogonal to divergence between species. So, basically, you have these parallel rails on which species travel, and stress enables species to jump from one track to another. And I remember, that I was really struck by this—how stress-induced developmental variation that you must suppress to function normally is so important when you change course and need to jump to an alternative set of tracks. It was just a beautiful morphological pattern. I think it’s what got me thinking about how variation is structured, how it’s suppressed in order to maintain functioning within a species, how it is preserved, how it’s called upon to accomplish change under stress—that whole framework. The study came out as a cover paper in Proceedings B in early 2000. We did much follow-up on this since; Rebecca Young – one of the first doctoral students in my lab led most of this work, specifically looking at the interaction between muscle attachment, bite force, and bone formation (e.g., 1,2,3) that produces these patterns, but this paper was one of the first.
Another study I did back then with some undergrad researchers was on bumblebees. In bumblebees, you have this interesting system where you have an individual who is sitting by the entrance to the nest and is in charge of ventilating an underground chamber. If it doesn’t do a good job, CO2 accumulates in the nest which can lead to some developmental abnormalities. That gave us the opportunity to manipulate things—you can either slow down that ventilator or accelerate it. And so, we did that and looked at the fluctuating asymmetry between the two pairs of wings and discovered that there, too, you had stress-induced variation, but in that case, it was compensated within the organism. That paper came out in Am. Nat. in 2001 and was led by Chris Klingenberg, who was at Cambridge at the time. I guess these two empirical projects, one where stress-induced variation is accommodated, and another where stress-induced variation leads to species divergence, made me realize that there’s something interesting there. Experiencing the clarity of these empirical patterns made me think of synthesizing the mechanisms. And the rest is history!
HS: Do you know why the forest service was collecting all these shrews?
AB: It was part of their protocol to monitor changes in mammalian fauna in relation to proposed treatments. But these treatments had major effects on shrews in particular. Cutting the forest dries up the soil, changing the insect composition. Within a shrew lifetime, you could go from the prevalence of soft and moist earthworms to an abundance of hard beetles and not much else. That’s a big deal in terms of the bite force that you need to capture and consume prey. Plus, shrews are very local in relation to the size of these clear-cuts. They can’t just leave, they either live or die, so it is a major stress that leads to post-production rearrangement of their foraging apparatus. A sort of replay of their development but with new functional goals. Basically, evolution as we know it, but at shrew speed.
HS: Did the work on shrews happen during your PhD?
AB: Yes, it was a side project, in the middle of my PhD. I found this collection when I was a teaching assistant for mammalogy for Dr. Kerry Foresman. I was rummaging through Forest Service huge freezers—it is amazing what you can find in there—and discovered these thousands of solidly frozen shrews that everybody had passively collected. So I thought – what a waste. But it was a heroic project, with me and a whole team of undergrads. We had to clean the skulls with dermestid beetles, photograph them, measure them…
HS: So, both projects were done with undergrads?
AB: Oh, yeah. I was a graduate student myself and I had this dream team of the most extraordinary undergrads ever – Suzanna Sowry, Byron Weckworth and Celeste Fiumara. John Glen, Leslie Jonart and Miguel Fernandes joined us later too. All wrote strong undergrad theses and published their work (e.g., 1, 2, 3); Suzanna and Celeste collected essentially a full array of top research awards at the university at the time, Leslie went on to do an important work in stem cell biology, Byron became a leading biologist in his field, legendary for his work on snow leopards and Siberian tigers. But the shrew project back then was particularly heroic. Only one lab on campus had a microscope suitable for the kind of work we needed, and we could only use it at night when it was not used by others. So, we would meet in that lab at 11 at night and work all night photographing thousands of shrew skulls under this microscope. I remember we used a computer capture card called “Snappy” that came with that massive parallel port contraption. Snappy was an absolute lifesaver but it would drain its 9V battery after acquiring every 3-4 images, so we would go through boxes and boxes of 9V batteries and always run out. One memory I have from that project is Celeste periodically disappearing into the winter night and coming back all frozen and covered with snow but with yet another box of fresh batteries. To this day I have absolutely no idea where she managed to find all those batteries and she is not telling. That was Missoula, Montana in 1997 – obviously nothing was open at 2am, much less anything selling batteries.
I’m very lucky, in retrospect, to have found this system—shrews are basically channelled by their environment into particular developmental patterns. Parts of their jaws remain in cartilage when they are born and so they literally ossify into functional use. It’s a really cool system. They basically let the environment to literally shape them, and they would have gotten it right every time, should the environment stay constant. So, you get a perfect imprint of stress on morphology and can measure it. Anyway, that’s how this whole thing started empirically.
HS: What was the topic of your PhD?
AB: The evolution of sexual dimorphism. I did a series of comparative studies, primarily based on the avifauna of the Pamir Mountains and the Himalayas, where I did my previous research. I then tested the patterns I uncovered within a species — in Montana’s house finches. They’re all cardueline finches, but in Montana I studied a finch species that, at the time, was thought to originate from Carpodacus rosefinches of the Himalayas – the centre of rosefinch radiation. Most of the variation in rosefinch morphology, life history, migration patterns, and population structure there was associated with variation in altitude. The mountain ranges of Central Asia have this altitudinal variation in abundance, obviously, so I studied these patterns there. And then I tested the evolutionary mechanisms driving dimorphism divergence across house finch populations across North America. Fundamentally, it is the same story, how evolving lineages deal with changing environments and demographics, how phenotypic and genetic variance is structured and how it channels evolutionary change.
HS: Coming back to the paper, what motivated you to write it? In the Acknowledgements you also talk about publishing an expanded version of this in a book.
AB: Well, I wanted to understand how general those patterns were. The unusual thing about this review is that it combined fields. I talk about everything there from directional mutations to the bowtie control organization of stress pathways in physiology, to the priming effect of stress on memory, to evolutionary transitions out of stress states. There was very little of this crosstalk at the time. Even now, even though everyone, just about, studies stress, people very rarely talk about it across disciplines and mean the same thing. You go to a workshop on stress and discover that people rarely converge on their definition of stress. So, as I just mentioned, to me stress is a window into the dynamic process of evolution – the one that resets the path-dependency of coevolving associations between components that make an organism. That is when ubiquitous external unpredictability actually becomes a stress. But many others don’t see it that way. They say, no, stress is a specific challenge to current functioning that doesn’t have evolutionary significance aside from the evolution of buffering. Fundamentally it is a clash between dynamic and static views of evolution and adaptation – we certainly have a predisposition to associate precision and complexity with some measure of stability that should be buffered and protected. So, fundamentally, this review was an initial attempt to reconcile these views and identify some general principles. It goes interchangeably, between the evolution of migration strategies and the channelling of stress variation in development, for example. I was fearless back then!
You mentioned that stress did not appear often in the titles of my papers after this review. I think this is because it just migrated into the main texts of my papers. It was the realization that it’s a background process in evolution and it is everywhere. It is endlessly variable in meaning, so it would be equivalent to putting the word “life” in every title. But if you actually did any kind of word cloud by the meaning of stress in my papers—there would probably be one enormous “stress” and “evolution” surrounded by a cloud of many other tiny words. It’s a realization that it is a fundamental reflection of the evolutionary process. I think that was the biggest breakthrough for me at that time.
HS: Was the idea for the book already there when you were writing this paper? Because you mention the book in the Acknowledgements. Was this paper motivated by the expanded chapter you were going to write, or was the invitation to write the chapter based on the editors already having seen a version of this paper?
AB: The stress review in Proceedings B definitely crystallized what I would later say in the book. The book also had more empirical examples, as far as I remember. The subject of the book was variation, and the patterning of variation; essentially the relationship between the stability needed for functionality and the variation needed for exploration. One of my recent empirical papers has the press release titled something like “When the boundaries become bridges in evolution”. So, it is the same question—when you have a stress buffering mechanism that becomes a bridge to the next adaptation. Basically, border guards are the best at border crossings when the political regime changes. But the main question is whether they specifically evolved for this role or whether they are just a byproduct of functionality itself, that either crystallizes and synchronizes variability or controls it through various feedback mechanisms. Anyway, this review, unlike the book, also talks more about physiology, which has very interesting insights. Such as mild stressors maintaining functionality of stress pathways—”what doesn’t kill you makes you stronger” kind of thing. And then there is that whole idea of bowtie organization in which you have multiple inputs that are converted to a single currency in order to induce a stress response or stress preparedness in subsequent evolutionary states. We have done some work on this in relation to maternal effects, also some papers with Tobias Uller on how diverse stressors experienced by one generation are interpreted by the next generation. What kind of organization do you need to have in order to convert current stress to a common currency that will be understood later on? So I think the review synthesizes and integrates all these ideas and examples from diverse disciplines and tries to bring them to a common denominator. In my head, I saw that common denominator.
HS: The way you remember it, you feel that you were invited by the journal to write this review by the journal?
AB: I don’t remember. I was trying to look up the history of that paper, but I’ve been through too many computers since then!
HS: One clue to this is that the turnaround time is really quick. It was submitted in August and accepted in December.
AB: Well, maybe it was just a very good paper! Plus, back then the peer-review process actually worked. But I really don’t remember. All I can say is that the timing of it was perfect. There was such a huge interest. So, I did save the peer reviews of that paper, and one indicator of how intense the field was at the time is that one of the reviewers was admonishing me for not citing two papers that, however, were published after my manuscript went out for review! Plus, Mary-Jane West-Eberhard’s Developmental Plasticity and Evolution had just come out. I got the book right away, obviously, but planned to read it after finishing my review. But then I ended up reading it soon after anyway – I couldn’t wait. And many times since. But lots of books on this general topic were published during these few years. It was an explosion of interest, but these ideas have all been around for centuries. I think the novelty of the stress review at the time was my attempt to synthesize these ideas across fields that are normally not integrated. But you can easily trace most of these concepts and principles to Development and Evolution by James Baldwin, for example. 1902. It’s all right there, pretty much, just in different words.
HS: Was this literature you were well aware of even before you started working on this review?
AB: Yes, of course. That’s why I wrote the review. But fundamentally I was as interested in the empirical resolution of these problems as in the conceptual ones. I am very well aware of how biased we can be in our intuition about how life works, plus, fundamentally, I am an empirical biologist, I really do need to know how the gears work and really do not like to have too many black boxes in my logic. The empirical component slows you down, but it also makes you much more confident once you find something. In the shrew study, for example, we spent a lot of time studying the timing of ossification induced by muscle attachment. Trying to understand whether stress-induced variation is channelled by the rearrangement of functional modules or by some stochastic noise. I’m actually still there but with a different system. We have this new project that specifically addresses how you can reconcile specialization with continuity in evolution. Basically, how you can have specificity in outcomes produced by non-specific regulation. We looked at it in the most proximate way – how temporary protein networks are linked to each other during developmental stages, so they have transient specialization at each stage and yet don’t get stuck. This study is actually now coming out as cover paper in the next issue of Proceedings B, 25 years after the shrew paper that started this work. So a full circle in a way.
But, fundamentally it is the same question in evolution ever since Edward Cope’s “Law of the Unspecialized“— is specialization a dead end in evolution? It’s the perennial question—how do you reconcile the continuity of evolution with the specialization of its stages? How do you produce something that is crystalized, functional and specialized, and yet not have it be the end of it? Obviously, organisms solve it, but we need to know how they do it. How you can have it both ways—specialized and yet continuously changing. And stress tells us how. That was, I think, the motivation.
HS: When you were writing this paper—this must have been in 2004—were you already on the faculty at the University of Arizona?
AB: Yes, for a year.
HS: Did you take out time to do this, or was this just something you were doing on the side?
AB: Oh, very much a side activity together with everything else you do when starting a new faculty position. Lots of new research projects, lots of teaching, new students. Plus, reading, reading, reading. But yeah, I was just starting in Arizona.
HS: I’d like to go over the people you mention in the Acknowledgements, just to get a sense of who they were and how they contributed to this paper.
AB: Let’s see whom I mention there… Some were graduate students in my lab at the time – Becca Young, Kevin Oh, Dana Seaman. Others — Laura Carsten, Heather Maughan, Laura Reed – were grads in the Evolutionary Developmental Biology course I taught that year. So, basically, they were forced to read and discuss this manuscript! The syllabus from that first year is still surviving online – I’m looking at it now – what a reading list! Gunter Wagner, of course, was a professor at Yale. He was a very inspiring colleague with whom I’d had a number of interactions before. Gunter’s work had been very influential for my growth for decades. I think I first met him when I was a beginning graduate student, and then just sort of continued to stay in touch. Becca Young from my lab became his postdoc at Yale later on.
Let’s see who else I mention there. Joanna Masel is a faculty here at the University of Arizona. She had, I think, just started, right before that paper was published. She had done some important work on capacitance in evolution, and we discussed a manuscript of her future paper on this in that class. Basically again, the same question of the patterning of variation in relation to evolutionary trajectories. Let’s see — Joachim Hermisson was working on similar questions of the evolution of genetic architecture, phenotypic robustness, and the accumulation of neutral variation in relation to the complexity of adaptations. Greg Gibson and Gunter Wagner had a very important paper then on the population genetic theory of canalization, basically again how the complexity of adaptations harbours and expresses variation. Hermisson and Wagner had just published their paper on genetic robustness and how the extent of suppression of mutational variants by the current adaptation shapes that variation. They likened selection to a vacuum cleaner, and the robustness of adaptation to the size of the rug and introduced that moving-rug effect that links robustness to accumulating neutral variation that becomes newly available to selection when you either shrink the rug or move it during evolution. The general point is basically the same as in Baldwin’s writing – past adaptations prepattern the variance available for subsequent evolution. Benedikt Hallgrímsson and Brian Hall were the editors of the Variation book and gave me a lot of feedback on ideas that would eventually make it to my chapter in their book. So these were the acknowledgments.
HS: How did you know Hermisson?
AB: I think we first met at one of the evolution congresses. At least I vividly recall listening to his talk at Evolution. It was a packed room, I was a beginning graduate student – it must have been one of my very first Evolution meetings – and I was very inspired by it. It had a big impact on me; I still remember that talk actually… But I don’t remember how he ended up commenting on the review – I must have sent it to him and asked “What do you think about it?” Or maybe we just talked about it when he visited, and he gave me some suggestions.
HS: Any other memories you have around this paper?
AB: As far as technically producing it?
HS: That, as well as its reception after it was published? Were there any responses directly to you?
AB: Yes, there were some invitations to give presentations at various symposia and conferences on stress in evolution – I think I ended up giving six or seven of these shortly after the review came out. Fundamentally, though, I write these papers for myself, in the sense that they structure my own understanding and force me to make some logical leaps explicit. I must complete them this way in order to know whether to keep or discard ideas and intuitions. So, it ends up being a sort of scientific diary—imprints of your understanding at each particular step.
HS: Since you read the paper again now, I wanted to ask you whether your thinking on what you say in the paper is more or less the same today? Or are there places where you would say you think about things differently today?
AB: That’s a good question. Re-reading this review now, I see a number of themes that I pretty much continue to explore. What seems particularly essential now is to recognize that a system of organization that enables life to exist in constant change, in constant transitions, appears at that change as well, it is not preexisting. Similar things are happening in development. Why exactly is stress such an essential part of development? Fundamentally again, it is a way to contrast a myopic view of local selection, local optimization, and the fact that you don’t ever get to live in a static context or fully predictable static environment, so the overall goals of development are always very different from local goals. Stress essentially sets new goals for development, forces coordination of local goals among components, synchronization of variation among cells, for example, by giving them some novel, that is external, force to react to. That’s why we exercise. Setting new goals or exceeding the normal range of the environment reawakens latent capacities in cells and tissues, forces the system to develop new ways of integrating for new goals, prevents it from settling on local optimization and decaying its capacity to change and thus persist when context changes. So, all of a sudden you realize it’s not about stress, it’s about time and life’s continuity. And stress might be just a shortcut that we invented to make sense of it, because we have a hard time thinking about dynamic processes, and we love to interpret things around us as somewhat static, somewhat predictable, to protect them from failure. We would rather endlessly explain away the tension between this imagination and reality.
I think another interesting aspect in this paper, which is not fully developed but basically hints at it, is how do you evolve stress resistance strategies? The tension is a bit artificial there – if something is so rare, such that it basically doesn’t exist in your lineage memory and you therefore cannot implement an appropriate response, how do you evolve a response? So essentially it is often the evolution of resetting mechanisms or responses that produce dormancy, relocation, stress-induced polymorphism that enables migration and other ways of skipping an unfamiliar context instead of dealing with it. Or the evolution of an alert, neurotic phenotype that is tuned to the unexpected. The physiological literature is full of examples of generational transference of stress signals without the transference of the stressors themselves.
Basically I’m glad I wrote it back then and stayed at that general conceptual level, structuring it around these classical themes, like stress buffering as a byproduct of developmental complexity or stress avoidance strategies. If I had seen the somewhat circular nature of arguments around stress and evolution, the way I see it now, I might not have written it. Alternatively, maybe the explicit integration of a dynamic perspective, emphasizing continuity, perennial novelty and predictable unpredictability of life – what Susan Oyama called “cycles of contingency” in her amazing book, which had a major effect on my views – is exactly what was needed. But then it would be a different review, so maybe I should write that one now!
HS: Why should a young student or researcher read this paper today?
AB: Fundamentally, to see if they like it or not! I guess, to see if they agree with this attempt at synthesis of common themes across disparate disciplines. Some themes survived and are still stimulating a lot of research. A lot of research now in physiology, for example, is centred on the evolution of control architecture that adjusts the system state or maintains homeostasis. Basically, how systems remain controllable while transiting between environments and changing coevolving players. Which is essentially a description of any evolutionary change. This paper wasn’t published that long ago, and so a lot of the themes are still with us. Like the whole idea of priming transitions, that goes back to Waddington (1957), the idea that variation has to be primed in order to accomplish transitions in evolution. And stress is a very important primer of both that crystallization and transition. There, I also make a distinction between the priming effect of stress, when it structures existing variation, and the maintenance of adaptive states. This is still very much an active dichotomy in contemporary literature on stress. Another very active field still is investigation of what produces the alignment of various sources of developmental variance. How despite all of their complexity, organisms do not suffer the curse of dimensionality that, we think, should prevent change of their components. Much of this research is under the general theme of evolution along the lines of least resistance – which means different thing to different people – from shared projection of variance, to weak links in complex systems, to long-term developmental or functional coevolution and many other approaches. We recently provided a different take on it too, showing that in protein networks these lines can be actually due to directions of the most stochastic connectivity. So fundamentally all the themes mentioned there remain relevant. Maybe it will make someone investigate a particular aspect. Maybe someone will conclude that some generalizations weren’t warranted, So, it still might be good for stimulating interest in this field.
HS: We are coming to the end of the interview. Moving away from the paper a little bit, I wanted to ask you whether you identify yourself as an evo-devo biologist?
AB: Well, I’m very interested in mechanisms that make evolution possible, in processes that drive and direct development. Like I said before, I’m very interested in leaving as few black boxes as possible in my thinking, if only so I don’t depend on them. That makes studying development mandatory for what I do. Plus, reality is the best antidote to our reliance on various beliefs and shortcuts in thinking that we internalized as axioms from our education or culture. We certainly know much more about the molecular details of development than even recently, what kind of biology goes into transcription factors finding their DNA targets, for example, what goes into gene co-expression in transcription and translation, cell communication… But interestingly it does not yet translate into understanding how these mechanisms evolve to work the way they do. Aside from knowing that we were spectacularly incorrect in some of our previous assumptions, especially in our assumption of required specificity in regulatory interactions. So, I think it is just about absolutely the best time to study development; in many ways we are back to square one, but with far better tools and more open minds. That’s what makes studying development absolutely fascinating and a very powerful way of advancing evolutionary understanding. So yeah, I consider myself somebody who studies the evolution of development.
HS: This is a good point at which to ask you about the other thing you said in your email, that it is actually just about the best time to revisit this particular paper, as this field experiences a much more informed Renaissance. Do you mean evo-devo, or evolutionary biology more broadly?
AB: I meant stress in evolution, but both themes are experiencing a renaissance, actually. In stress research it is the understanding that the concept of stress forces us to seriously consider the dynamic nature of life – its fundamental singularity, irreversibility and unstoppable nature in contrast to our implicit treatment of predictability and stability in our research and thinking. Stress makes these tensions explicit and opens a new window into the process of life and the theory of evolution. Life obviously evolves to continue, first and foremost, so its changeability in dealing with unpredictable external contexts is another definition of something being alive, not a specifically evolved property. In evo-devo we are also witnessing an increasing realization that physical, informational, chemical and biological approaches all study the same underlying phenomena and so we better integrate them if we want to understand life better. There is no particular hierarchy among these approaches. I think we are past the time when we are surprised that physical transitions are coopted into biological regulation and vice versa, for example. I’ve always been exposed to strong biomechanics and biophysics schools, such as that of Lev Beloussov whose lectures I attended as an undergraduate at the Lomonosov Moscow State University in Russia. However, for me, it was key papers by Gerd B. Müller and Stuart A. Newman that integrated these fields, and completely changed my perspective on evolutionary and developmental implications of physico-chemical processes, and on competences and communications of cells and tissues (e.g.,1,2,3) – at about the same time as I wrote that review, actually.
But despite all of this, we are still very naïve about what moves development or regeneration forward, what sets the targets and goals of development, what stops it, how integration between cells and tissues actually evolves, coordinated and transferred across different levels of organization. The very same basic questions that have been with us for centuries actually. It seems that top-bottom, open-ended, or distributed processes are more important than we realized before, but we still don’t understand how to integrate them into our evolutionary theory or practice. I think the whole view of unfolding regulation in development needs rethinking – there are just too many black boxes there. I mean we can make sense of many regulatory processes in retrospect, but we are often in the dark about the rules of their recruitment in evolutionary sequences. Or how general these rules might be. The concept of time in development and evolution needs rethinking, time flows at different rates at different scales among coevolving players and we need to think about how to integrate these. And so on.
HS: Would you count this as one of your favourite papers?
AB: It’s a difficult question. Yeah, probably. Although, I see some gaps in it. I personally tend to like a bit more contrarian things. But often you can be spectacularly wrong or spectacularly naïve, and the ideas don’t survive the test of time. And often when you look back you realize that you were trying to really sharpen the tension for yourself to force yourself to solve it. Sometimes it works really well and you learn a great deal, sometimes it doesn’t. So in this sense, this paper is well balanced. I managed to capture interesting general motifs that evidently resonate with many people. But then life is endlessly complex and diverse and remarkably forgiving of our attempts at generalizations. As long as we don’t overdo it on reductive determinism – then nature goes on strike and proves us wrong right away. But, otherwise, you always learn something new empirically across many conceptual frameworks and models of reality that we create to make sense of it for us. So that was my attempt at one such generalization, and for that reason, it is probably one of my okay papers, yes.
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