Here is a question of the utmost importance for our understanding of what has been called the ‘big picture’ of evolution (Simpson 1944, 1953): are the divergences that lead ultimately to high-level sister groups, such as those that would typically be labelled as orders, classes and phyla, qualitatively or quantitatively different from those that lead to low-level sister groups, such as races, species and genera? In other words, is mega-evolution more than just accumulated micro/macro-evolution, or alternatively is evolution effectively ‘scale-independent’ (Leroi 2000)?

This question can be approached in three ways. We can choose to compare the magnitude of changes involved in high- and low-level divergences, the type of changes, or the timing (in development) of changes. Here, I argue that previous work on the first of these has been unproductive and has generated more heat than light; but that the second and third offer better prospects for shedding light on this important issue. However, in an unusual strategy, I also play devil's advocate with my own argument at the end of the chapter. This helps to take us in an interesting, final (for now) direction.

Because the designation of high-level sister groups as, for example, orders or classes, is a subjective rather than an objective process, I will, wherever possible, use specific examples rather than general levels of taxon.