Evolution of complexity

My cats are jittery. The end of the world must be near.

In this post I will outline the questions I wish to answer, the existing approaches to these questions and the intuitions I wish to formalize. Thus I’m going to break all the promises I made in the last post — but really, this should have been the last post, somehow I got sidetracked…

There is an intuition that life has gotten more complex over geological time. More hierarchies have been built, more specialization of parts, possibly even more intelligent. This intuition is not only true for life, but for societies. Governments, bureaucracy, culture, social divisions — all seem to become more complex with time. The modern division of labor is the pinnacle of this. Why?

Orthodox evolutionary theory does not have an answer to this. And well it does not, because the answer is both nasty — and, I will argue, deathly wrong. Orthodox evolutionary theory proposes that natural selection is the major driver of evolutionary change, that organisms change their forms and types because of differences in fitness. Ergo, things must have become more complex, specialized, etc. — because they are more fit. Humans, as such recent players in evolutionary history, are very complex, very specialized, and very smart — hence we are the evolutionary climax, the last of the rungs of the evolutionary ladder to perfection…

This answer is the disowned natural bastard of orthodox evolutionary theory. It is never given out in modern polite scientific discourse, and most biologists would argue militantly that it is not true. Like all disowned bastards of royalty, however, its head constantly pokes up with alarming frequency, each time with new insurgent allies looking for a coup d’etat: eugenics, racism, social darwinism, all the bad movies using evolution as deus -ex-machina (X-men!) etc. Hear hear, it seems to say, Evolution is making us better, and we should hasten her along.

I’m fully convinced that this bastard child is incredibly wrong and we ought to have its head on a pike. The trouble is, parts of its parents might have to go also. The parts that lead to this bastard child, at any rate — I will argue that the long trends of macroevolution have little to do with natural selection.

For simplicity, let’s focus on complexity alone for the moment. Current (polite, modern, scientific) discussion about the evolution of complexity hinge around if it is increasing at all — it turns out “complexity” is an awfully slippery thing to measure. McShea did a great deal of work (1991, 1996, 2005) here, although him and colleagues aren’t the only group (e.g. Heylighen 1999). In any case, there’s a vast literature on this. I would carefully submit, however, that complexity has in fact increased, since we don’t find any pre-cellular living forms (the earliest living form cannot have been a fully formed bacteria!) and even our oldest Archeabacteria are well removed from the earliest living form (say, a self-replicating RNA). Nor did anything as complex as mammals show up right at the beginning of life. Although this work on the definition and measurement of complexity is incredibly valuable, we don’t need a thermometer to tell us that boiling water is hotter than ice.

The question is therefore why. To use the answer of orthodox evolutionary theory, that natural selection drove the extinction of simple organisms and made organisms more and more complex, is intensely unsavory. It’s more than just the political and cultural distastefulness of the answer and the capacity for people to abuse this fact, I — and many others — do think it’s actually wrong. But then there must be another force outside of natural selection that can drive evolutionary trends on the geological scale. What is this other force? I’ll summarize the existing arguments — and my opinions (why else write a blog?) — below:

Gould (Gould 1997) has famously argued that complexity, on average, has nothing to do with natural selection. The increase in complexity over time, he says, is simply the result of a random walk. Sometimes complexity is good for the organism and it grows more complex. Sometimes complexity is less good and organisms grow less complex. However, for biochemical reasons, the first life forms had to be very simple. On the other hand, complexity cannot go below zero. Thus, the evolution of complexity is that of a random walk with an absorbing state of zero. The average complexity then naturally increases, but complexity itself, per se, doesn’t do anything for fitness at all — what is good for fitness is entirely environmentally dependent.

Gould thus posits no positive force for the increase in complexity. McShea, who concluded that complexity was increasing after all, considers drift to be a positive, but weak, force for the increase of evolution. With Robert Brandon, they coauthored a book arguing that this is the case. A positive review is here. The idea is that mutation is a natural driver for diversity, which is synonomous with complexity. After all, one does not mutate into the same thing that one was. Thus, in the complete absence of selection, evolution progresses into greater complexity. McShea and Brandon called this the Zero Force Evolutionary Law (hmmm… I pick up a hint of Asimov here).

I think Gould’s idea is very clever, but it contradicts empirical evidence. There are traits that seem to make its carrier fitter over a wide swath of environments; the eusociality of ants must have contributed to their dominance in the world’s ecology. Might complexity be such a trait? Probably not, considering the dominance of bacteria… but we cannot reject that complexity has an effect on fitness overall, as Gould does. Besides, according to Gould, if all of complexity is driven by environmental conditions, then all lineages should show theoretically unlimited movement in complexity. Thus, according to this theory, whenever environments favor the loss of mitochondria in eukaryotes, eukaryotes should lose them and good riddance. Unfortunately, for all strains of eukaryotes, losing mitochondria is death — regardless of the environment, so there is no strain of prokaryotes with eukaryotes as their ancestor. This is the intuition I hope to tighten later. Similarly, of the many billions of cases of cancer that has occurred throughout evolutionary history, there is no species of single-celled organism that had a amphibian, reptile, or mammalian ancestor (I’m not sure about sponges). Once the organism dies, the cancer dies (unless it’s kept alive in a lab). None of the cancer cells could revert to unicellularism, although it’s undoubtedly advantageous for them to do so. Although multicellular to unicellular evolution has certainly occurred, they don’t seem to have ever occurred in species where the viability of the organism depends on an intense and obligate integration evolved over billions of years. Thus, unlike Gould’s claim, there seem to be some plateaus of complexity that, once stepped onto, cannot be descended from.

McShea’s idea, on the other hand, I’m skeptical about. It reminds me of a flavor of mutationism and orthogenesis that has been soundly routed in the course of the history of evolutionary thought, with good reason. Natural selection is an awfully powerful force, strong enough to beat the second law of thermodynamics every time. Most mutations increase diversity, yes, but most mutations also make us closer to a ball of gas, and yet we aren’t balls of gas. The authors seem to believe that it is a gentle breeze of a force blowing in the background, such that if the selection for or against complexity averaged out to nearly zero over time, then this force is sufficient to provide a long term trend. With no mathematical model behind their reasoning, I cannot formed an informed opinion of whether this might be true — but I highly doubt it. What I think is much more likely to happen is that the evolution of complexity would be precisely as dictated by natural selection, whether it is a random walk or not, and only increased slightly at all points in time by the gentle breeze. Consider the following 2 minute drawing in GIMP:

Different prediction for the evolution of complexity

It looks terrible, I know — and the axes are unlabeled. Okay okay, x axis is time and y axis is some measure of complexity. Let’s say that natural selection is the black line, so sometimes complexity is selected up and sometimes down, but there’s no overall trend (well, there should be, since it’s a random walk with an absorbing condition — but bear with me here). Red is what I think McShea and Brandon is proposing, that there’s a gentle background force moving complexity up. But I think that the gray line is what would happen — the evolution of complexity, according to McShea and Brandon’s force, would exactly reflect natural selection, with no trend. The force would nudge complexity to be a bit higher than what it would otherwise be, but that’s it.

Wow this post has gotten long already — you can’t say very much in a post! Next post — and I won’t break any promises this time — will deal with my own thoughts on increasing complexity and its links to the holey landscapes model.

16 Responses to Evolution of complexity

  1. I think a good physical example for your drawing would have been a simple harmonic oscillator in a uniform gravitational field. The oscillations don’t change, but the centre is displaced slightly by the force of gravity.

    What I don’t understand is how any of the three pictures relate to the irreversible evolution and plateaus your sketched at the beginning of the post. Or was that just a tangent?

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  4. Maybe a general rule:

    More available resources -> better conditions for complexity to rise.

    If you study Ecology, you can see that organisms who are not at the very bottom of the food chain are more complex. The evolution of life on this planet produces a compounding effect or a positive feedback loop in which new organisms thrive on the old and the dead.

    • You would have to formalize this argument much more to make it convincing. If you take microbes and put them into a more nutrient rich environment, there will be more of them, but there will not be any obvious increase in complexity. Of course, with more individuals, you could argue that there will be more opportunity for variation on which complexity can piggy-back. However, at the same time, with a less austere environment the selective pressures will be much lower and that tends to slow down evolution (although not always). You would need to go into more detail in order to explain which factor would dominate and under what conditions.

  5. I have trouble following your arguments, I’m not a sophisticated enough a biologist. I believe in a kind of “Divine selection” like the one mentioned in the New Testament. There Jesus and John the Baptist talk about how metaphorically every tree that did not bear fruit would be uprooted and destroyed. If you apply this directly and non-metaphorically to biology throughout the history of life, which I believe is their deepest intention, selection would not favor those that can best take advantages of the resources of their environment but instead favor those that provide good sustenance to other species.

    The history of life might then be one similar to Walt Disney movies with good and bad individuals and species where the exploiters, the opportunistic, the selfish and evil would be at war with the guardians and fosterers of resource abundance, conservation and increase, with the good eventually winning out.

    Furthermore following the Sermon on the Mount, it would not be the fittest that survive in the end. That is it would not be the ones who are prospering the most in their niches. These fittest individual and species would be like the rich, well fed and individuals of good repute mentioned in the Sermon. Analogously then it would be those species and individuals on the margins, the unfit individuals, the hungry, the poor etc. in the environment who would pass on their genes in the end. This is because seasons and environments are subject to rapid changes and the individuals and species flourishing in one age would be too big to get through sieve of environmental cataclysm into the next as in the end of the dinosaurs. Only the small and floundering like the mammals and small birds can were able to “Squeeze through the narrow door” into then next age.

    Also perhaps toward an end of an age there are too many selfish opportunistic organism and not enough good fruit producers and this causes a collapse of the ecosystem because the foundation of any ecosystem and its engine is not the opportunistic but instead is the good fruit producers who have achieved a kind of selfless devotion to the needs of others (not producing good fruit merely to better spread their seeds). This would entail radically changing our understanding of what motivates animals and what they know or can sense and that animals might be able to make good husbandman decisions in regards to the and individuals on which they dine.

    As for the evolution of complexity it might be driven in the husbandman/peak predators by the complexity of the environment and the ever growing complexity of the decisions required to make it continue to flourish and grow despite environmental changes. In the good fruit producer themselves complexity might be driven the needs of servicing those who consume their good fruits.

    • Thank you for the comment. Sorry that it took so long to respond. I was hoping Julian would field this, but he is a bit busy right now, so I will take a stab. In general, basing your hypotheses about nature on biblical texts is not very scientific. In particular, some of what you write is almost self-contradictory:

      selection would not favor those that can best take advantages of the resources of their environment but instead favor those that provide good sustenance to other species.

      Providing sustenance to other species usually means dying and not reproducing any more, this would obviously result in lower fitness and those individuals would become out-competed by others that don’t become food and have a chance to leave more offspring for the future. In general, your way of presenting your ideas makes me wary of how comfortable you are with evolution. In almost all cases, your hypotheses would be wrong, and demonstrably so.

      That being said, there are cases where the dynamics you describe can produce the meekness effect. In particular, if you look at group selection on the level of ecologic systems. In this case, meekness would be a form of cooperation, and you could use standard group-selection arguments to show how it can emerge even though it is locally mal-adaptive. I might write a post about that in the future.

      Unfortunately, your discussion of complexity is fundamentally unsatisfying. The goal of biologists studying (and others attempting to study) complexity is usually to show how unbounded growth in complexity can be achieved. Your suggestion does not do this, since it is limited by the complexity of the environment. Sure, if that increases over time then you could rescue your approach, but then you are just pushing the question instead of answering it. Now you would have to explain why environments have unbounded increasing complexity instead of individual organisms. I suspect that is a harder road to take.

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  9. Eric Bolo says:

    Neat post! I admit that I had taken the “bastard child” Idea as a given.

    Have any simulations been made to model complexity trends alongside “natural” selection?

    • Julian Xue says:

      Hi Eric, what do you mean?

      • Eric Bolo says:

        It may be a bit crazy, but I mean mathematical models (maybe game theory models) that could allow for players to compete in a selective environment while also acquiring or losing complexity (e.g. Assembling with other players in a symbiotic way analogous to eukaryotic cells + mitochondria).

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