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Point mutations in a cis-regulatory element of Sonic Hedgehog are frequently associated with preaxial polydactyly in humans, mice, and cats. The Hemingway mutant in the Maine Coon cat exhibits polyphenic effects of polydactyly that are not equally distributed. A statistical analysis of a comprehensive data base of Hemingway mutants reveals a biased and discontinuous distribution of extra digits. Further biases exist in the difference of effects in fore- versus hind-limbs and in left–right asymmetry. These non-equally distributed phenotypic effects cannot be explained by the point mutation alone. We propose a double mapping model, termed the Hemingway Model, to account for the biased distribution of supernumerary digits. The model is based on the random bistability of individual cells in the limb area affected by the mutation and on the application of the Central Limit Theorem. It proposes two kinds of mapping events that (a) transform a mutational effect of single additive changes into a continuous distribution, and (b) transform the continuous distribution into discrete character states via developmental threshold effects. The threshold widths for the occurrence of discrete extra digits are specified as units of standard deviations of the continuous variable. This makes it possible to specify the generation of empirical developmental variables (the liability of quantitative genetics) as a result of developmental parameters that give rise to biased morphological patterns and phenotypic novelty.

Most studies in evolutionary developmental biology focus on large-scale evolutionary processes using experimental or molecular approaches, whereas evolutionary quantitative genetics provides mathematical models of the influence of heritable phenotypic variation on the short-term response to natural selection. Studies of morphological integration typically are situated in-between these two styles of explanation. They are based on the consilience of observed phenotypic covariances with qualitative developmental, functional, or evolutionary models. Here we review different forms of integration along with multiple other sources of phenotypic covariances, such as geometric and spatial dependencies among measurements. We discuss one multivariate method [partial least squares analysis (PLS)] to model phenotypic covariances and demonstrate how it can be applied to study developmental integration using two empirical examples. In the first example we use PLS to study integration between the cranial base and the face in human postnatal development. Because the data are longitudinal, we can model both cross-sectional integration and integration of growth itself, i.e., how cross-sectional variance and covariance is actually generated in the course of ontogeny. We find one factor of developmental integration (connecting facial size and the length of the anterior cranial base) that is highly canalized during postnatal development, leading to decreasing cross-sectional variance and covariance. A second factor (overall cranial length to height ratio) is less canalized and leads to increasing (co)variance. In a second example, we examine the evolutionary significance of these patterns by comparing cranial integration in humans to that in chimpanzees.

The introduction of novel phenotypic structures is one of the most significant aspects of organismal evolution. Yet the concept of evolutionary novelty is used with drastically different connotations in various fields of research, and debate exists about whether novelties represent features that are distinct from standard forms of phenotypic variation. This article contrasts four separate uses for novelty in genetics, population genetics, morphology, and behavioral science, before establishing how novelties are used in evolutionary developmental biology (EvoDevo). In particular, it is detailed how an EvoDevo-specific research approach to novelty produces insight distinct from other fields, gives the concept explanatory power with predictive capacities, and brings new consequences to evolutionary theory. This includes the outlining of research strategies that draw attention to productive areas of inquiry, such as threshold dynamics in development. It is argued that an EvoDevo-based approach to novelty is inherently mechanistic, treats the phenotype as an agent with generative potential, and prompts a distinction between continuous and discontinuous variation in evolutionary theory.