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The Evolution of Complexity by Means of Natural Selection
John Tyler Bonner makes a new attack on an old the question of how progressive increase in the size and complexity of animals and plants has occurred. "How is it," he inquires, "that an egg turns into an elaborate adult? How is it that a bacterium, given many millions of years, could have evolved into an elephant?" The author argues that we can understand this progression in terms of natural selection, but that in order to do so we must consider the role of development--or more precisely the role of life cycles--in evolutionary change. In a lively writing style that will be familiar to readers of his work The Evolution of Culture in Animals (Princeton, 1980), Bonner addresses a general audience interested in biology, as well as specialists in all areas of evolutionary biology.
What is novel in the approach used here is the comparison of complexity inside the organism (especially cell differentiation) with the complexity outside (that is, within an ecological community). Matters of size at both these levels are closely related to complexity. The book shows how an understanding of the grand course of evolution can come from combining our knowledge of genetics, development, ecology, and even behavior.
What is novel in the approach used here is the comparison of complexity inside the organism (especially cell differentiation) with the complexity outside (that is, within an ecological community). Matters of size at both these levels are closely related to complexity. The book shows how an understanding of the grand course of evolution can come from combining our knowledge of genetics, development, ecology, and even behavior.
Paperback
First published August 1, 1988
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About the author
John Tyler Bonner
52 books12 followersJohn Tyler Bonner is the George M. Moffett Professor Emeritus of Biology at Princeton University, a pioneer in the use of cellular slime molds to understand evolution and development, and one of the world's leading experts on cellular slime molds.
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July 26, 2017
I found this book in the science section of a used book store. It has a sort of paper cover, and it's from 1988. Although it's a Princeton University Press book, it has the feel of a handsomely bound dissertation or mimeograph. I felt like I had found a treasure when I stumbled on it. This is exactly why I used to prowl through the science section of used book stores: hunting for books that I wouldn't normally know about. They stock the kinds of science books that were hot 10-30 years ago, and which are by now forgotten and can't be found by browsing Barnes and Nobles. This one fit the bill, and I didn't so much mind that it was outdated, because I was interested in big themes about the evolution of complexity, rather than biological particulars.
Much of the book is actually about size, rather than complexity per se, but Bonner makes a convincing (to me) case that, on average, size is correlated with complexity. Bonner begins by establishing some facts about complexity and size. First, the size of animals, especially the size of the largest animals, increases over time (the blue whale and the sequoia are the biggest animal and plant ever, and they are alive today). Moreover, the upper limit on animal size does not usually drop by big animals gradually being selected to be smaller - instead, the big guys go extinct. Second, the larger the animal, the fewer the species, but the more complex (Bonner measures complexity by the number of distinct cell types). Interestingly, there is actually an inverted U-shape to the size/species relationship: below a certain size, the size and the number of species are positively correlated. My own idea for this shape is that there are two opposing forces at work: as animals get bigger and more complex there are simply more ways to organize their cells and cell types to make different species (just you can make more words from three letters than from two), but the number of ways to get the organization wrong increases even faster. At the low end the first effect dominates so you get more species with more complexity, but as the complexity rises you get fewer and fewer.
Anyway, Bonner is interested in explaining why natural selection favors size, complexity, and diversity.
Bonner starts with size. The main advantage of going big, is that it puts you in a new (mostly unpopulated) world. It is harder to be prey and easier to be the predator. So there is some positive selection pressure to become big. At the same time, there is comparatively little pressure to become small. Since life started small (as in, single celled), for most animals shrinking means entering a world that is already heavily populated and highly adapted to life at that scale. It's very hard to compete as the new species on the block.
Of course, being gigantic has its own drawbacks. Big animals depend on a minimum threshold of food, and when environmental stresses happen, the species dies out. For example, suppose the same amount of food can support one big dinosaur or 10,000 rats. If a meteor wipes out 90% of the food, the dinosaur dies, but 10% of the rats survive to perpetuate the line. So while small animals are likely to survive cataclysms, natural selection favors becoming big only when the environment is relatively stable.
But how do you get big?
Here is where Bonner's main theme comes into play. It does not work to simply scale up small forms of life. The things that make a single-celled organism "work" depend crucially on aspects of physics and chemistry that only operate on this tiny scale (Life's Ratchet by Peter Hoffmann is a good book about this). New tricks need to be discovered by natural selection to grow larger. For example, instead of making bigger and bigger cells, evolution has isolated the cell design so that it is no longer altered by subsequent evolution. This means the easiest way of going big is to integrate many such cells into one multicellular being. Here Bonner goes into depth about how this was accomplished in biology. He is particularly interested in slime molds, which exist on the cusp of single and multicellular life.
Integration faces its own challenges. How are the integrated units to be organized? To illustrate the problem, think of a different context, that of complex machinery. Most complex machinery is also a collection of smaller components, analogous to cells, but these components have to be arranged in a precisely determined way in order for them to cooperate and do whatever it is the machine is meant to do (when I was a kid, I wanted to build a robot, and I thought it would be enough to stuff a robot model with wires. It didn't work). With technology, we can appeal to a designer to figure out how parts get precisely arranged, but with life, organization has to emerge on its own. Since there is no higher-order agent that can be appealed to, the units must have a means of communication in order to self-organize. Bonner discusses the way biology accomplishes this through chemical and other signals.
There are additional challenges associated with leaving the chemistry and physics of the single-celled world behind. How will the organism hold together as its weight increases? How are resources to be distributed? How are different parts of the body to communicate with each other? Bonner discusses the evolution of specialized cells and organizational schema to address these problems. The book is really good at focusing your attention on these problems, and the specific kinds of solutions adopted by biological systems.
There are also ramifications about the rate of change for big and small animals. Big animals are "built" from many steps, each of which must be completed correctly to deliver the animal. The first steps are crucial: mess with them, and in all likelihood the animal is dead. Thus, core functions of big animals are rarely going to change. Moreover, big animals hold onto and adapt earlier structures. They rarely reinvent and rediscover things.
All in all, a funny little book that gave me a lot to think about. It's pretty intense on the biology, and likely outdated, so I suppose it now has a very small audience of people like me who are interested in how people have thought about the emergence of complexity, without caring so much about the details.
Much of the book is actually about size, rather than complexity per se, but Bonner makes a convincing (to me) case that, on average, size is correlated with complexity. Bonner begins by establishing some facts about complexity and size. First, the size of animals, especially the size of the largest animals, increases over time (the blue whale and the sequoia are the biggest animal and plant ever, and they are alive today). Moreover, the upper limit on animal size does not usually drop by big animals gradually being selected to be smaller - instead, the big guys go extinct. Second, the larger the animal, the fewer the species, but the more complex (Bonner measures complexity by the number of distinct cell types). Interestingly, there is actually an inverted U-shape to the size/species relationship: below a certain size, the size and the number of species are positively correlated. My own idea for this shape is that there are two opposing forces at work: as animals get bigger and more complex there are simply more ways to organize their cells and cell types to make different species (just you can make more words from three letters than from two), but the number of ways to get the organization wrong increases even faster. At the low end the first effect dominates so you get more species with more complexity, but as the complexity rises you get fewer and fewer.
Anyway, Bonner is interested in explaining why natural selection favors size, complexity, and diversity.
Bonner starts with size. The main advantage of going big, is that it puts you in a new (mostly unpopulated) world. It is harder to be prey and easier to be the predator. So there is some positive selection pressure to become big. At the same time, there is comparatively little pressure to become small. Since life started small (as in, single celled), for most animals shrinking means entering a world that is already heavily populated and highly adapted to life at that scale. It's very hard to compete as the new species on the block.
Of course, being gigantic has its own drawbacks. Big animals depend on a minimum threshold of food, and when environmental stresses happen, the species dies out. For example, suppose the same amount of food can support one big dinosaur or 10,000 rats. If a meteor wipes out 90% of the food, the dinosaur dies, but 10% of the rats survive to perpetuate the line. So while small animals are likely to survive cataclysms, natural selection favors becoming big only when the environment is relatively stable.
But how do you get big?
Here is where Bonner's main theme comes into play. It does not work to simply scale up small forms of life. The things that make a single-celled organism "work" depend crucially on aspects of physics and chemistry that only operate on this tiny scale (Life's Ratchet by Peter Hoffmann is a good book about this). New tricks need to be discovered by natural selection to grow larger. For example, instead of making bigger and bigger cells, evolution has isolated the cell design so that it is no longer altered by subsequent evolution. This means the easiest way of going big is to integrate many such cells into one multicellular being. Here Bonner goes into depth about how this was accomplished in biology. He is particularly interested in slime molds, which exist on the cusp of single and multicellular life.
Integration faces its own challenges. How are the integrated units to be organized? To illustrate the problem, think of a different context, that of complex machinery. Most complex machinery is also a collection of smaller components, analogous to cells, but these components have to be arranged in a precisely determined way in order for them to cooperate and do whatever it is the machine is meant to do (when I was a kid, I wanted to build a robot, and I thought it would be enough to stuff a robot model with wires. It didn't work). With technology, we can appeal to a designer to figure out how parts get precisely arranged, but with life, organization has to emerge on its own. Since there is no higher-order agent that can be appealed to, the units must have a means of communication in order to self-organize. Bonner discusses the way biology accomplishes this through chemical and other signals.
There are additional challenges associated with leaving the chemistry and physics of the single-celled world behind. How will the organism hold together as its weight increases? How are resources to be distributed? How are different parts of the body to communicate with each other? Bonner discusses the evolution of specialized cells and organizational schema to address these problems. The book is really good at focusing your attention on these problems, and the specific kinds of solutions adopted by biological systems.
There are also ramifications about the rate of change for big and small animals. Big animals are "built" from many steps, each of which must be completed correctly to deliver the animal. The first steps are crucial: mess with them, and in all likelihood the animal is dead. Thus, core functions of big animals are rarely going to change. Moreover, big animals hold onto and adapt earlier structures. They rarely reinvent and rediscover things.
All in all, a funny little book that gave me a lot to think about. It's pretty intense on the biology, and likely outdated, so I suppose it now has a very small audience of people like me who are interested in how people have thought about the emergence of complexity, without caring so much about the details.
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