The Structure of Evolutionary Theory

I've always enjoyed reading Stephen Jay Gould's essays about paleontology and evolution. A lot of his writing made it on to audiobooks, which made it easy for me to listen to it in the car. So when I had a chance to pick up a copy of his book "the Structure of Evolutionary Theory," naturally I did.

This book is massive. I mean huge, at around 1400 pages. Consciously paraphrasing Darwin's words, Gould describes the book as "a single extended argument." He separates the argument into three branches: "efficacy," "agency," and "scope."

"Agency" is basically the question: what the locus of natural selection? Darwin viewed natural selection as operating only on individuals, not on larger groups such as populations or species. Gould takes a different view and believes that selection can operate on these higher levels.

"Efficacy" refers to the ability of natural selection to create the various life forms we see. Gould points out that nearly all of Darwin's critics accepted that natural selection could be a negative force, removing certain individuals from the population. A big part of the genius of Darwinism was realizing that natural selection could also be a positive force leading to new life forms over time, without the need for some external guiding hand.

Here, too, Gould proposes some changes to Darwin's theory. Darwin believed that all adaptations were equally likely. He used the analogy of the descendants occupying a sphere of possibilities extending around the point in space where the parent resided. In contrast, Gould belives that "structural constraints" make some evolutionary pathways easier for organisms to travel on than others.

Finally, "scope" refers to the claim that the processes of natural selection that we see around us can be extrapolated to longer periods of time. This was another controversial point in Darwin's day: some people believed that evolution could be creative in a local or small sense, but not explain the progression of species over millions of years. Darwin argued that the accumulated action of many small changes occuring over millions of years produced the natural world we see today.

Gould points out that the "many small changes over time" theory ignores a lot of the paleontological evidence. The fossil record tends to show sudden changes, like the mass extinction of the dinosaurs, or the "Cambrian explosion" of different life forms and body plans almost half a billion years ago. Darwin understood this, but argued that the fossil record was incomplete.

Gould presents a different argument, often called "punctuated equilibrium." Basically, the argument is that occassionally, during certain brief periods, evolution operates under different rules. For example, if a comet hits the earth and blocks out the sun for a year, suddenly the "rules of the game" have changed for many organisms. The species that survive this event will have to have food reserves of some kind, hibernation strategies, or food sources that do not rely on the sun. Once this period of alternate selection has passed, a new equilibrium will result among the surviving species. Some of them will diversify into evolutionary niches previously occupied by newly extinct species.

So much for the basic premises. A large part of the book is devoted to an examination of historical views about natural selection. For example, Gould takes a closer look at Lamarck's theory, and finds that the standard textbook view of what Lamarck believed is wrong. Although Lamarck was still wrong, he was wrong in a different way than most people realize. Gould also tries to repair the historical reputation of the "catastrophists," who believed that the earth underwent catastrophic changes in the past. Catastrophism has become associated with modern-day "young earth" creationism, and attempts to explain the past using biblical accounts of Noah's flood. But Gould points out that in the 19th century, catastrophists such as Georges Cuvier. were not trying to graft the bible on to paleontology, but rather, just tring to account for the actual uneven fossil record.

Gould also spends a lot of time discussing Darwin's thought process, the details of his arguments, and how he came to his conclusions. The result is often interesting... as Gould points out, Darwin often spotted difficulties in his own theory that others had difficulty seeing. For example, Darwin saw clearly that Lord Kelvin's (erroneous) estimate that the earth was only 100,000 years old would not leave enough time for his mechanisms of gradual change to work their magic.

Gould gives an interesting summary of the changing currents of thought in biology. In 1909, roughly 100 years after Darwin's birth, Darwinism was still a controversial topic... many leading scientists believed that natural selection was only a subsidiary force. Alternate explanations were still scientifically respectable. In 1959, 50 years later, the "modern synthesis" was in full swing-- a sort of "hardening" of biological thought around the theory of natural selection as proposed by Darwin. Gould points out that though the "modern synthesis" played a useful role in dismissing antiquated competitors to natural selection, it put too much weight on the power of selection. He refers to this as "panselectionism" and it is a kind of Panglossian perspective on evolution... if an organism has feature X, feature X must be the optimum thing for that organism, because we live in the best of all possible worlds. Starting in the 1980s and continuing up to the present day, evolution has become an interesting topic again as views similar to Gould's gain credence.

This is a really long book and I skipped over a few chapters. Gould goes into a huge amount of detail about the history of the theories and the men who proposed them. Sometimes the personal details are interesting. I guess science has always been a contentious place, even for someone as conciliatory and well-spoken as Darwin.

It's interesting to contrast Gould's views with those of another prominent modern evolutionist, Richard Dawkins. Dawkins' views are a lot closer to those of the "modern synthesis" that Gould deplores. Dawkins doesn't believe in hierarchical selection. In fact, Dawkins believes that all selection happens at the level of genes. He is famous for predicting the existence of "selfish genes," which create more copies of themselves in the genome, despite allegedly doing nothing useful. I'm not sure where Dawkins stands on the issues of internal constraint and gradualism. These topics aren't really mentioned in the last two books of his I read, "The Selfish Gene," and "The Ancestor's Tale."

In general, I feel like the three branches of Gould's argument are a unified package. If you accept that organisms have internal constraints on their evolution, then it makes sense to view species as a unit of selection. If there were no internal constraints, then we would expect the species that exist to simply reflect the external constraints of the environment. In "The Selfish Gene," Dawkins presents a lot of arguments that group selection and species selection can't work. They're basically all game-theory arguments based on the idea that a "cheater" will reap great rewards in the short term that will totally prevent group selection from having time to take place. But if we have internal constraints, nobody can "cheat."

Similarly, punctuated equilibrium relies on the idea of internal constraints. The same set of conditions on the earth won't automatically produce the same organisms. If the climate heats up again to the temperature it was 65 million years ago, we won't necessarily get dinosaurs again. Existing organisms will evolve to fill those evolutionary niches, and they will look different than the dinos did.