Dennett is insanely difficult to summarise, because he crams so much into his books and presents much of his most interesting material as digressions. Darwinian evolution has a huge range of applications (Dennett calls it a "universal acid") and taking it as his subject gives him the opportunity to range across science and philosophy, introducing bits and pieces of all kinds which he has picked up and thinks are worth sharing. This results in the volume as a whole being a little disconnected, but more locally ideas are logically and clearly presented.
Dennett begins by explaining why he thinks Darwin deserves the prize for the "single best idea anyone has ever had" and why his idea was (and is) so revolutionary, so dangerous. He illustrates this with a brief account of pre-Darwinian ideas — with Locke as an exponent of the traditional viewpoint and Hume as someone who came very close to Darwin's insight. The key elements of Darwin's "dangerous idea" are a denial of essentialism and an understanding of natural selection as a substrate neutral, algorithmic process, applicable to an extremely wide range of phenomena and capable of achieving immense feats by slow accumulation over large extents of time and space.
Darwin's original application of natural selection was, of course, to the origin of species. Dennett explores different ways of visualising the "tree of life" and explains the problems involved in defining species (decisions about species status are necessarily retrospective). This is illustrated with an explanation of the often misunderstood "Mitochondrial Eve" phenomena.
At this point Dennett introduces a metaphor which is used throughout the book: "cranes" are devices or "good tricks" that allow design to proceed faster, but which build on existing foundations; "skyhooks" are entirely mysterious, pre-existing hooks in the sky which enable some problem to be solved or some complexity to be created entirely independently of ordinary processes of design. Dennett argues that there is no place at all for skyhooks and that the only bad reductionism is a "greedy" reductionism that tries to do without cranes.
Evolution can be seen as movement within the "Library of Mendel", the set of all possible genomes, of which only a tiny fraction actually exist. The complex constraints imposed on genomes by developmental biology and ecology reflect relative degrees of accessibility within the library — the accessible is a small subset of the possible, albeit a much bigger one than the actual. Dennett goes on to argue that this can be extended outside biology, that all design can be seen as movement through a single unified Design Space. Human creativity is no exception, and Paley's "watchmaker" analogy had more truth than it is usually credited with.
Part two of Darwin's Dangerous Idea looks at attacks on and extensions of Darwinism inside biology. Darwin himself carefully restricted the domain to which he was prepared to apply his theory, but Dennett argues that continuing to do so (at the behest of religion or otherwise) is no longer a tenable position to take. He briefly discusses two extensions: to the origin of life (focusing on the ideas of Cairns-Smith and Eigen) and to cellular automata (Conway's game of Life). Foreshadowing part three, he also mentions Nietzsche's "eternal recurrence" and the psychological consequences of a world which is self-creating and without foundations.
Though there are obvious differences between those things produced by human design and those produced by evolution, biology is engineering at some fundamental level, and "reverse engineering" is a powerful tool for biologists. This creates a connection between two difficult concepts — "function" in biology and "meaning" in philosophy. Dennett fits work by Kauffman on self-organising systems into this framework, arguing that it is an extension of Darwinism rather than a rebuttal.
A whole chapter is devoted to exploring the power of adaptionist thinking and its centrality to understanding evolution. While he rejects Leibnizian "panglossianism", Dennett sees adaptionism as a fertile source of explanations; if these are not always correct that does not diminish its general power. (This is illustrated by a brief look at the Aquatic Ape Theory, as an example of an unestablished, controversial, but interesting adaptionist hypothesis.)
This is followed by a chapter devoted almost solely to Stephen Jay Gould. Dennett continues his argument for the power of adaptionism with an attack on its most famous critique, Gould and Lewontin's famous "Spandrels of San Marco" paper. Dennett's basic argument is that Gould and Lewontin's arguments are misaimed, that "genuine" Darwinians have always shunned both panadaptionism and preadaptionism, and that "good adaptionists are always on the lookout for hidden constraints". Punctuated equilibrium is next against the wall, along with Gould's analysis of the Burgess Shale (in Wonderful Life) and his arguments for the contingency of evolution.
Dennett's conclusion from all of this is that Gould is "searching for skyhooks to limit the power of Darwin's dangerous idea". This prompted a bit of soul-searching on my part and some rereading of Gould's works, but I think that Dennett is wrong about this. While there are passages in Gould's writings and passages that can be read to support Dennett's view, it seems clear to me that Gould's overriding drive is not a search for skyhooks but rather an insistence on the complexity and diversity of the cranes involved in evolution. All the different forms of heterochrony Gould discusses in Ontogeny and Phylogeny, for example, are clearly cranes, and if he is more complimentary than some to historical figures who were clearly looking for skyhooks, that says more about his historiographical sensibilities than his own philosophy. Gould is no closer to any form of vitalism or mysticism than someone like Dawkins is to "greedy reductionism". Perhaps Dennett sees things from too high above the fray of actual biology: while he assents that cranes come in many types and that they interact in complicated ways, his cranes versus skyhooks abstraction subsumes the whole of biology into "cranes", leaving plenty of room for major disagreements which are simply invisible at this level.
On a similar note, Dennett rings a wrong note when he claims that only "greedy" reductionism (trying to do without cranes) is bad, and that attacks on reductionism are either vain attempts to find skyhooks or aimed at unrealistic portrayals of reductionism. The most widespread forms of reductionism are those that try to restrict the kinds of cranes used or that place excessive stress on particular cranes (typically privileging genetics above ecology and embryology, or physics above everything else). These kinds of reductionism may not be a problem philosophically, but they are definitely a menace elsewhere.
Dennett goes on to deal with other more harmless "heresies", though at much less length: Hoyle's idea that the Earth was seeded with life, aliens meddling with evolution, Teilhard de Chardin, and recent Lamarckian revivals. I'm not convinced most of these merited even this much attention. Dennett also offers a very brief look at the debate about the level and units of evolution. He argues that, while this is important, it doesn't impinge on the fundamentals of Darwinism as he has presented them.
One of the reasons Darwinian heresies are so widespread inside biology is that many people desperately want to stop Darwinism applying to people, and therefore seize any chance they can to undermine it. In part three of Darwin's Dangerous Idea Dennett looks at how the extension of evolutionary ideas outside biology has been resisted in fields like linguistics, philosophy, and ethics. This will be the most interesting material for many, especially those already familiar with the biological theory in parts one and two.
The application of Darwinism to culture rests on the concept of memes, concepts or ideas which are propagated from person to person and "compete" with one another. They provide a basis for culture and allow us to transcend our genetics. While Dennett doubts that a science of memetics with the power of genetics is possible, at a basic level genetics and memetics work on the same principles: design by unthinking processes of selection. Human culture is a "crane-making crane", not a set of "skyhooks"; indeed there are no "skyhooks" in culture any more than there are in biology.
When it comes to refusing to accept the consequences of evolution by natural selection, the worst offenders outside biology are people like Chomsky, Searle, Penrose, Fodor, and Putnam. Chomsky's long standing opposition to the idea that language could be the result of natural selection is an obvious target for Dennett, who spends a chapter on the origins of language and the relationship between language and intelligence. Searle's espousal of "Original Intentionality" is a perfect example of grasping for skyhooks. (Skinner, on the other hand, was a greedy reductionist, trying to explain everything in one step.)
A chapter on meaning and intentionality takes up the link between biological definitions of function and philosophical definitions of meaning introduced in part two. Dennett deploys three complex but compelling (and, as always, entertaining) thought experiments, aimed at demonstrating that there can be no distinction between "real" meaning and "artificial" meaning, that ultimately all meaning emerges from meaningless processes. Drifting a little from evolution, he then devotes a chapter to explaining why "attempts to use Gödel's theorem to prove something important about the nature of the human mind" are inherently flawed and to demolishing Penrose's "refutation of strong AI" (in The Emperor's New Mind).
Dennett spends two chapters on the origins of morals, arguing, of course, for a naturalist position (if you reject "original intentionality" you can hardly have "original sin"). While the excesses of some sociobiologists ("greedy reductionists") are deplorable, that is no grounds for rejecting an evolutionary origin for morality. Once again Dennett finds time for a quick look at the history of moral philosophy, placing Hobbes and Nietzsche as early sociobiologists. He goes on to address an important practical issue: both utilitarian and Kantian ethical systems tend to be idealised to the point where they are useless; construction of a practical "Moral First Aid Manual" will require taking into account real computational complexities.
In a brief final chapter Dennett explains how Darwin's dangerous idea has influenced his political and ethical beliefs. He sees it as a basis for assigning value to diversity, whether artistic, cultural, or biological. While some have seen it as conducive to conservative politics, Dennett thinks otherwise, ending with a vision very much in the liberal tradition.
Like Dennett's earlier books on free will and consciousness, I fear Darwin's Dangerous Idea is too complexly argued to make many converts: some will quibble at minor points and dodge the basic argument; others will become lost in the detail. While it doesn't require a technical background, it is not going to be easy reading for those without a basic sympathy for Dennett's way of looking at the world: I would recommend having read at least Dawkins' The Selfish Gene before tackling Darwin's Dangerous Idea.
For many, however, Darwin's Dangerous Idea will be a volume to read slowly and to savour. Arguably there are few original ideas in it (at least for someone who has read Dennett's earlier works and has a grounding in evolutionary biology), but few readers will fail to find something new, or perhaps some familiar ideas in new contexts. Many books have been written about natural selection, but few have applied it across such a wide swathe of philosophy. Though I disagree with the odd detail, I think that Dennett's basic argument is inexorable, inescapable, and fully as potent as he claims it is. He has produced a vastly more sophisticated version of the bonfire the positivists wanted to make of the cobwebs of metaphysics and any philosopher who wants to talk sensibly about design or meaning must pass through its flames.
Acknowledgements: thanks to Cosma Shalizi for comments on a draft of this review.