The five chapters in Part II all focus on nervous system organization. This emphasis is important because, traditionally, comparative research tends to focus on similarities rather than differences (i.e., on conservation rather than variation). However, after the conserved features are known, the research focus can shift to the nonconserved features, the variable elements. In grappling with this variation, researchers often look for constraints and scaling principles (Striedter, 2005), and they seek to explain the variation in mechanistic terms.
In Chapter 4, Erin Jarvis and colleagues review the segmental variation in arthropod appendages (mainly mouthparts and limbs) and its control by hox genes. They note that hox genes also control segmental variation in the motor neurons that control the various appendages. This observation is important because it suggests that variation in hox gene expression patterns can coordinate evolutionary changes in appendage morphology with evolutionary changes in motor neurons, thus ensuring functionality. Pursuit of this idea will extend evo-devo (evolutionary developmental) biology, which has thus far focused primarily on body plan evolution, into the realm of neuroscience, which is just beginning to experience an evo-devo boom (Striedter et al., 2011).
Continuing in Chapter 5 the neuro-evo-devo theme, Luke McGowan and colleagues present results from an experiment in which they used intraventricular FGF2 injections to delay neurogenesis in the optic tectum of chicks. This manipulation increases tectum size to the point where parts of the tectum form folds, an interesting finding because delays in