Chapter Chapter-by-Chapter Anaylsis of The Selfish Gene (1989 edition) - Study Guide by Richard Dawkins

Using several examples from nature, including snails and flukes, the microscopic protozoan parasite called Nosema and flour beetles, and crabs and Sacculina, Dawkins describes how one individual can influence the behaviour of the other to their advantage. This influence may be of benefit or cost to the host, depending on whether the host and the parasite "want the same thing". It has been suggested that the host will benefit from the presence of the parasite if the parasite’s genes are to be transmitted to future generations via the same vehicle as the host’s genes. If this is not the case, one would expect the parasite to damage the host in one way or another. For example, the Sacculina effectively castrates the crab so that the crab diverts the energy and resources, that would otherwise have gone towards reproduction, into its own body which the Sacculina then sucks out for its own nourishment. However, there are cases when both individuals "share" the same vehicles to propagate their genes into the next generation. One example of this is when the wood-boring ambrosia beetle is parasitised by bacteria that not only lives in the host’s body, but also uses the host’s eggs as their transport into a new host. In this case, the bacteria does more than simply cooperate with the beetle and in fact play an essential role in "pricking the unfertilised egg into action, provoking them to develop into male beetles.

Dawkins takes this idea and argues that parasites such as these will cease to be parasitic and become mutualistic, with the two bodies ultimately merging into the "host" body completely. He then applies this theory to human genes. From above, we have already established that individuals will cooperate if they share the same impartial exit channel into the future. He follows this through to argue that our own genes cooperate with one another purely for this reason – that they share the same outlet, and that the individual genes have ceased to be parasitic and have become mutualistic, giving the appearance of a coherent "whole".

He uses this theory to finish his book by asking three questions. Why did genes gang up in cells? Why did cells gang up in many-celled bodies? And why did bodies adopt what he calls a "bottlenecked" life style?

He answers the first question by using an analogy of a pharmaceutical factory. The synthesis of a useful chemical needs a production line because the starting chemical cannot be transformed into the desired end-product without a series of intermediates arranged in a strict sequence. In the same way, single enzymes (made by one gene) cannot achieve the synthesis of a useful by-product without the "help" of intermediates and so the enzymes (or genes) will have had to cooperate with each other in order to achieve their goal. This will have started as elementary co-operation between the replicators in the primeval soup, but eventually a cell wall will probably have formed in order to keep the useful chemical together and prevent them leaking away. And this is how cells came together.

The second question is just as simple for the selfish gene theory to explain. Each cell will benefit from co-operating with other cells because this will mean that each cell will be able to specialise and therefore become more efficient at performing its particular task, while at the same time taking advantage form the efficiency of the other specialists. In economics, this principle is called Economies of Scale.

And thirdly, why do bodies participate in a "bottlenecked" life cycle? By bottlenecked, Dawkins means that "no matter how many cells, of no matter how many specialised types, cooperate to perform the unimaginably complicated task of running an adult body, the efforts of all those cells converge on the final goal of producing single cells again – sperms and eggs." He argues three reasons why this occurs: "back to the drawing board", "orderly timing" and "cellular uniformity". All these three mechanisms benefit the vehicles in evolution and additionally result in the vehicles becoming more discrete and vehicle like, to the extent that biologist often believe that the organisms use the DNA to reproduce themselves, in much the same way as they use an eye to see, which in fact is the truth turned upside-down!