Scientists usually study natural selection at a single level, such as genes or individuals or even a population, says biophysical complexity researcher Maya Paczuski – but it takes place at all these levels simultaneously, and what happens at each scale resonates through the web of life in ways we're just beginning to comprehend.

I talked to Paczuski, founder of the University of Calgary's Complexity

Science Group, for a recent Wired.com story on the expansion of evolutionary theory to include complexity and emergence. These phenomena don't replace the classic mechanisms of genetic mutation and natural selection, but work with them; and accompanying this expanded conception of evolution is the multi-scale perspective espoused by

Paczuski.

Here's what she told me:

You always get into trouble if you say these things out loud with creationists around. It's not that the whole theory of evolution is going to come tumbling down – there's no doubt that evolution has taken place on the earth, and that we definitely understand certain parts of it and that they fit together. But there really are fundamental conceptual problems with the whole notion of fitness and therefore the whole notion of selection, and therefore in how evolution actually takes place in evolutionary systems of which biology is one example, but there are others – the meme, the evolution of societies or of technology, or in principle any kind of system where new things are created that didn't exist before. Most people understand the notion of selection at the level of the individual. If you don't reproduce, then you are not fit, and you don't leave your genes behind. On the other hand, Richard Dawkins has made to some extent valid arguments that selection doesn't happen at the level of the individual, but rather the gene. But you can also make arguments that selection happens at the level of groups of individuals: if you haven't enough genetic diversity, you have a whole population wiped out by an epidemic, by some bacteria or virus that comes through. That's why we have all these genetic variants that aren't necessarily increasing the fitness of any individual, but have at the same time presumably increased the fitness of the population. You have selection at the level of the individual, the level of the gene, and at the level of populations of individuals, and can even make good arguments that selection happens at the level of ecosystems. The whole ecosystem has got to be functional to keep going. And these organisms evolve in relation to all the other organisms that they exist with, because their environment is made up of all the other species and individuals that they encounter. So when you talk about selection, selection probably happens at all scales, from gene to individual to species to collection of species to ecosystem to we don't even know what. That's my personal point of view.

And when you get into the lab and you try to measure fitness, you're only going to measure it at some particular level, such as which bacteria outcompetes another on a food source. That gives you a quantititve measure of that system, but it's not clear how that explains the complex evolution of the biosphere. One of the things that complexity theory teaches us is that you have emergent properties – like ecosystems – so you have to think of selection happening at many different scales. That problem hasn't been addressed in any coherent way in scientific literature. It's one of the great complex problems of the future. Neo-Darwinian evolution isn't fixated at one level. It's being applied at different levels – but in a given study, only at one level. There's been arguments: Dawkins argues that it happens at the gene, others at the level of individuals, and others at the level of species – but there's now more of a growing consensus that it happens at all these different levels, and we don't understand how that comes about. When you get into the notion of different levels, you deal with problem of selection at different time scales – for instance, when you talk about a particular individual, it's about what happens during their lifetime. They either make or don't make children. When you talk about human populations and human dynamics ... what's the time scale over which selection is happening there? It's not individual anymore. It's a longer scale altogether.... What time scales are relevant? It seems you end up with lots of different possible time scales. How do you unify all these different mechanisms taking place? And why are there all these different levels? That's the fundamental thing that makes life complex. And those points aren't accounted for by Darwinian evolution. In any complex system, what happens at the larger scale can affect what happens at the smaller scale. It's not just that everything goes up from genes to individuals to groups of individuals to ecosystems and species. You also have feedback going back the other way, all the way down. That's part of the whole selection mechanism, and we don't really understand that. These kind of questions are absolutely the most fantastic questions you can ask nowadays.

I then asked Paczuski about non-linearity and emergence – the dynamics underlying the sort of sudden, radical jumps of complexity seen in (to pick a particularly vivid example) honeybee colonies.

You can't get anything new from linear systems. Linear systems don't have property of emergence. What are the rules for the jumps of complexity? The best answer is that these are just things that just appear from the dynamics of the system itself. It's not predictable what they're going to be. You can't describe life by describing where the atoms are of everything on earth.

It wouldn't give you the relevant information you're interested in. What are the right levels to study the jumps? The levels where you can make the most compact description of things that you're interested in.

It's a subjective thing. You look at the system, see things going on, and figure out what's interesting to you, and those are the levels. You do have cells, which have boundaries, eukaryotes and so on – they really are distinct structures. When you talk about individuals, it's pretty obvious that these cells get together and move around and reproduce together. You can still make good decisions about what a species is. Then it gets a little more complicated, because then you have to talk about, what's a community? What's a society? Definitions rely on information at the lowest level. On boundaries – the notion of a self-replicating unit. But at the higher level, it's not really so clear anymore really what they are relying on. At least it isn't to me. I suppose if you talked to some other people, they'll come up with some highfaluting theory, some absolute definition for the relevant emergent structures, but we really don't know all that well. If you have a complex system, you should expect that there's going to be many different levels at which selection acts. The whole landscape of selection is getting more integrated, more complicated, and that's why you can't say that selection acts on one particular level. And once you admit that, there's no end to the emergent forms, the ones where we haven't yet realized that selection is acting. Biologists have success showing selection in defined domains – but none of those domains give a well-defined picture of how you can get all this complexity.... What are the real mechanisms over which selection acts at the level of species? At the level of families? At the levels in the tree of life? You've had episodes in geological time where whole phyla went extinct – those were much higher levels. It's not just random how extinction events happen. They happen in bursts.

That suggests many different levels of selection. I have to say that I really don't know what drives the jumps in complexity. Now we're talking about a fundamental problem in physics:

we cannot, as physicists, explain how something new happens. Something that didn't exist before. We know that in evolution, this is a very structured process. This problem of emergence is very much related to the problem of selection, the levels at which selection happens, and how that creates space or niches for new species to emerge in the biosphere. Those two things are very much related to each other. Its' a fascinating problem, but a frustrating one. I haven't seen a promising way forward at the moment. But the most important thing as a scientist is to know the difference between something you don't understand and something you do. The first step is that we don't have a conceptual understanding of many-level selection. This whole notion of emergence, of many levels – that you cannot describe life at any given level at all in particular, which is the notion that came out of complexity theory – it's a pretty new field.

There was very little work in complexity until the advent of the personal computer, where you could actually do the simulations. Now you have people out there, students and people who work with research institutes, and people trying things on there own who can make their own little world with rules and see what happens. We still don't have a very good grasp of it, but it's a necessary field to develop as part of understanding biology, life, and the organization of human societies and how signaling communication networks evolve.

Phew! There's a lot going on there. But if it caught your interest, be sure to check out the story.

Image: Domenico Nardone

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