Chapter 26: Development & Evolution

Development & Evolution posits that evolution is fundamentally a change in developmental processes over time, relying on the inherent modularity of an organism and the parsimony of a "genetic toolkit" shared across the animal kingdom. By examining how specific DNA regions known as enhancers can be modified, the text illustrates how genes can be expressed in new locations or at different times without disrupting the entire organism's viability. Key concepts like deep homology demonstrate that the basic instructions for building body plans—such as the signaling pathways for the central nervous system—are conserved between protostomes and deuterostomes. The narrative breaks down morphological divergence into four distinct categories: heterotopy, where gene expression shifts to new sites like the migration of ribs in turtle shells; heterochrony, involving shifts in the timing of growth; heterometry, which alters the amount of a gene product to produce variations like the different beak shapes of Darwin’s finches; and heterotypy, which involves changes to the actual protein-coding sequences, such as those that enabled the evolution of the mammalian uterus or the specific limb count of insects. The chapter also addresses developmental constraints, explaining why certain body plans never appear due to physical laws or the complex patterns generated by reaction-diffusion mechanisms, such as Turing’s model for zebra stripes. Moving beyond traditional genetics, it delves into epigenetic inheritance, covering how epialleles and symbiotic relationships provide selectable variation. Finally, it discusses genetic assimilation, a process where phenotypes initially triggered by environmental factors become genetically fixed, allowing the environment to serve as a catalyst for permanent evolutionary change.