Although I received my doctoral training within the neo-Darwinian tradition, in a university department (at Nottingham) that was largely devoted to population genetics, there is a view of evolution adopted by some neo-Darwinians that I have always rebelled against. This is the view that evolutionary processes can be understood in terms of only two levels of biological organization – the gene and the population. At its worst, this view is associated with actually defining evolution in those terms alone. For example, in their 1971 book A Primer of Population Biology, the American biologists Edward O. Wilson and William H. Bossert defined evolution as ‘a change in the gene frequency of a population’. Evo-devo can be seen as a rebellion against this overly reductionist approach.