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The controversy on the ‘unity of type’, exploited as an antievolutionist argument and based on the difficulty of revealing ancient homologies, is introduced using a historical approach. First the work of the earlier anatomists is explained. Then, the discovery of homeosis and the lessons from homeotic flies follow and serve to introduce the hox genes and their importance in the discovery of ancient homologies through to-date comparative genomics. These ancient genes allow us to establish homologies among body plans and solve the common ancestor controversy. They also explain why the evolution of form is a process of descent with modification similar to that observed in other characters that influence differential reproduction in populations, as some examples of morphological evolution from fishes and insects illustrate. This chapter gives reasons to oppose the tenet that developmental constraints in gene regulatory networks are the principal explanation for the evolution of form.

An evolutionary constraint is a bias or limitation in genotypic or phenotypic variation that a biological system produces. Striking phenotypic examples include the absence of photosynthesis in higher animals, and the general lack of teeth in the lower jaw of frogs. Constraints can influence the spectrum of evolutionary adaptations and innovations that are accessible to living things. Based on the cause of constrained phenotypic variation, one can distinguish physicochemical, selective, genetic, and developmental constraints. The latter class of constraints emerges from the processes that produce phenotypes from genotypes. This chapter examines these four causes for molecules, regulatory circuits, and metabolic networks in the genotype space framework. This framework shows that processes of phenotype formation cause the three other classes of constraints. It can help us appreciate why causes of constrained variation are often entangled and not clearly separable. The chapter also shows that the kind of evolutionary stasis that occurs during punctuated and episodic evolution is a consequence of genetic constraints, whose origins the genotype space framework can readily explain.

This chapter presents a framework for understanding the evolution of cognition and, specifically, of computational systems. The chapter adheres to a particular view on Evo Devo which, combining proposals from Complex Dynamic Systems Theory, Developmental Systems Theory, and crucially, the concept of Morphological Evolution, explains the development/evolution of phenotypes as trajectories leading to particular points within a discontinuous space of possibilities (“morphospace”) constrained by a number of parametric factors. The chapter further contends that the Chomsky Hierarchy, measuring the relative complexity of different types of “languages” (in the technical sense of Formal Language Theory), can be directly translated into a “computational morphospace,” the parametric factors of which describe the system underlying the development of the organ presented in the previous chapter.

This chapter introduces the concept of convergent evolution. It shows that parallel evolution is simply a special case of convergent evolution, and addresses the action of both functional and developmental constraints in the evolutionary process. The functional and developmental constraints that have led in convergent evolution at every level of life are described throughout the book, from the external forms of living organisms down to the very molecules from which they are constructed, from their ecological roles in nature to the way in which their minds function.

This chapter explains the effects of nutritional stress during development, with a focus on the hippocampus and spatial memory. Research work done on spatial memory in birds has provided a clear link between the ecological need to store food, a relatively enhanced spatial memory used to retrieve the cached food, and the relative volume of the hippocampus, the brain region processing spatial memory. With the importance of spatial memory for the survival of certain bird species, the author predicted that hippocampal development, and hence spatial memory, would remain intact even under nutritional stress. The data in the chapter refute that prediction and suggest that constraints during development preclude the insulation of certain brain regions from nutritional stress. Also, favoring animals with better spatial learning might provide an easier path for natural selection to select for better parents to produce high-quality offspring, rather than resolving the potential developmental constraint issue.