⁵While the idea of evolution as fitness maximization can be found in many textbooks, some biologists would refute the whole idea of evolution optimizing any single function; see a recent review by Birch (2016). To some extent, this controversy may also have arisen because of some semantic confusion about whether that would mean that evolution has already optimized the function (which would be a very strong statement) or whether it is in the process of optimizing it (which may be more plausible); see Parker and Smith (1990). Regarding the precise definition of fitness, it seems impossible to find a consensus opinion (Rosenberg and Bouchard, 2011; Grafen, 2008). A standard textbook definition would be along the lines “expected number of offspring”, but this may have to be complemented by the concept of inclusive fitness treated in Chapter 6. — In this book, I tend to anthropomorphize evolution rather unashamedly, often comparing it to a human programmer. I believe that is a useful pedagogical device since humans find it easier to think of natural phenomena in terms of agents that have goals instead of a more abstract description such as a dynamical system. See e.g. Dawkins (1986) for a rather strictly anti-anthropomorphic view on evolution.