River Out of Eden, by Richard Dawkins. The review provides summaries of the main arguments from each chapter, and a discussion, in particular, of the different thresholds mentioned by Dawkins.
The book River Out of Eden is divided into five chapters, entitled The Digital River, All Africa and Her Progenies, Do Good By Stealth, God's Utility Function, and The Replication Bomb.
In the first chapter, The Digital River, Dawkins introduces the idea of evolution by comparing it to a river, metaphorical, and changing over time. The river, he tells us, contains all of the DNA that has survived up until that point, and through the course of time, the river has deviated from its original path, producing different routes for the river, different branches, as he calls them. On each of these different branches of the river, he says, different combinations of DNA have been put together, and different genes have been constructed, giving different forms of life on each of the different pathways.
As Dawkins says, "The river of my title is a river of DNA, and it flows through time, not space. It is a river of information, not a river of bones and tissues: a river of abstract instructions for building bodies, not a river of solid bodies themselves. The information passes through bodies and effects them, but is not affected by them on its way through" (Dawkins, 1995).
Later in the same chapter, Dawkins compares these different genes (or those different sequences of DNA within the different branches of the river) to a computer program. He points out that humans rely on many kinds of signals to carry out our bodily functions, from digital to analog, but he says that the genetic code uses only digital signals: DNA preserves its codes in distinct categories, with only four possible 'categories' (i.e., bases (CGAT)). As he says, "Our genetic system, which is the universal system of all life on the planet, is digital to the core" (Dawkins, 1995). He argues that this digital system is the best system for ensuring error-free replication of the DNA, and goes on to show how the codes made up by the myriad combinations of these bases can produce all of the proteins we need to construct cells, to produce enzymes, basically, all of the elements we need in order to be able to live. He compares the process of building these elements to a computer's start up commands, and shows that through this system, all of the different cells of the human body assume their functions, and correctly carry out their roles within the body.
He explains his idea thus, "Every cell in your body contains the equivalent of forty six immense data tapes, reeling off digital characters via numerous reading heads working simultaneously. In every cell, these tapes - the chromosomes - contain the same information, but the reading heads in different kinds of cells seek out different parts of the database for their own specialist purposes. That is why muscle cells are different from liver cells. There is no spirit-driven life force....Life is just bytes and bytes of digital information" (Dawkins, 1995).
Later, Dawkins explains how it is possible to trace the origins of man, through tracing backwards from each person's mother and father. He says, however, that sex confuses the issue of trying to trace true common ancestors, as sex mixes up the genetic code, therefore confusing the pattern of inheritance, and presenting problems for those people who try to trace the origins of any particular section of DNA. As he says, continuing his river metaphor, "The river is not only uninfluenced by the experiences and achievements of the successive bodies through which it flows. It is also uninfluenced by a potential source of contamination that, on the face of it, is much more powerful: sex."
In chapter two, he points out that using mitochondrial DNA can circumvent these problems, as mitochondrial DNA is passed directly down the female line, and hence not muddled up during sex. He discusses how mitochondrial DNA has been used to suggest the common ancestor of man arose from Africa several hundred thousand years ago.
In the remaining chapters, Dawkins uses clear arguments to explain complex issues in evolutionary biology, such as looking at models of gene survival in terms of economic models, or continuing the metaphor of the river to explain the evolution of complex systems, such as vision, as he does in chapter three.