Introduction by John Brockman in Edge:
My vision of life is that everything extends from replicators, which are in practice DNA molecules on this planet. The replicators reach out into the world to influence their own probability of being passed on. Mostly they don't reach further than the individual body in which they sit, but that's a matter of practice, not a matter of principle. The individual organism can be defined as that set of phenotypic products which have a single route of exit of the genes into the future. That's not true of the cuckoo/reed warbler case, but it is true of ordinary animal bodies. So the organism, the individual organism, is a deeply salient unit. It's a unit of selection in the sense that I call a “vehicle”. There are two kinds of unit of selection. The difference is a semantic one. They're both units of selection, but one is the replicator, and what it does is get itself copied. So more and more copies of itself go into the world. The other kind of unit is the vehicle. It doesn't get itself copied. What it does is work to copy the replicators which have come down to it through the generations, and which it's going to pass on to future generations. So we have this individual / replicator dichotomy. They're both units of selection, but in different senses. It's important to understand that they are different senses.
Now, because the individual organism is such a salient unit, biologists after Darwin got into the habit of seeing the organism as the unit of action, and therefore they asked the question, what is the organism maximizing? What mathematical function is the organism maximizing? Fitness is the answer. So fitness was coined as a mathematical expression of that which the organism is maximizing. Of course, what fitness really is, or what it ought to be if we understand it properly, is gene survival. For a long time fitness was equated in people's minds with reproduction, with having a large number of children, grandchildren, great-grandchildren. Bill Hamilton and others, but mostly Bill Hamilton, realized that you had to generalize that because, if what's really going on is working to pass on genes, offspring, grandchildren, et cetera, are not the only ways of passing on genes. An organism can work to enhance the survival and reproduction of its siblings, its nephews, its nieces, its cousins and so on.