During diversification, rather different modes of organization were achieved, such as unicellularity, cellular differentiation, or development that proceeds by induction. These modes of organization put constraints on what further kinds of innovation were likely to occur. Evolution in bacteria, for example, tends to involve minor changes in the code, RNA, which in turn affects metabolic pathways. Flowering plants are developmentally simple and morphologically plastic, and often speciate by multiplication of chromosome number. They are essentially constrained from evolving nervous systems by the cellulose walls that enclose each cell.
Mammals have complex, interactive development.
Their evolution frequently involves regulatory genes that affect developmental timing and differential sensitivity of different parts of the neuroendocrine system. A small difference early leads to a big difference in adult structure and function. This means that evolution can be expected to occur with differing tempo and mode at different times during the history of life and in different taxonomic groups. As we learn more and more about molecular genetics and developmental biology, we can make more and more refined predictions about which groups are likely to speciate a lot and under what circumstances, and what sorts of novelty will appear in the daughter species. Deepened understanding will permit new tests of the validity of the theory.