Novelty is a topic of broad interest, with two distinct approaches within evolutionary biology. The dominant approach since Darwin has been transformationist, with novelty arising through gradual changes in morphology. The Modern Synthesis emphasized the importance of ecological opportunity rather than the source of variation, and this view has many adherents today. Yet, since well before Darwin, an alternative view has held that novelties could arise by rapid changes and many not necessarily be connected to ecological opportunity. The rise of comparative evolutionary developmental biology since 1990 has led to a resurgence of these arguments. Many case studies have documented novelties and there have been rigorous efforts to define the attributes of novelty, but there have been few attempts at a more general model. In contrast, studies of technological innovation have been replete with qualitative models since the 1930s. In this article I consider several possibilities for constructing a general model of novelty and innovation: (1) A general formal theory. (2) Commonalities between different levels, such as genes and morphology, but with sufficient differences between domains that any formal theory would be level specific. (3) Commonalities across levels but for various reasons developing a formal theory even within domains is improbable. A final alternative is that novelty and innovation may be so deeply historical that any general framework is impossible. I conclude that a common conceptual framework can be developed and serve as the foundation for simulation studies, but the importance of feedbacks and potentiating factors renders a formal model implausible.
