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
neurogenesis have likely led to cortical folding in large-brained mammals. However, the FGF2 injections also disrupt the normally smooth pattern of tectal lamination, which is unlikely to be adaptive. Intriguingly, McGowan et al. suggest that the laminar disruptions are causally linked to ruptures in the overlying pia mater. Collectively, these findings imply that evolutionary increases in the size of brain regions must be coordinated with expansions of the associated pia mater, which may be difficult when neural expansion is caused by a delay in neurogenesis.
In Chapter 6, Leah Krubitzer and Adele Seelke focus on variability in cortical organization, both within species and across mammalian taxa. In addition to describing this variability, they analyze its phylogenetic pattern and underlying mechanisms. In particular, they suggest that the cerebral cortex is constrained to vary in specific ways rather than being freely variable. This finding would explain why many features of cortical organization are broadly conserved and why some variants evolved repeatedly and independently in diverse lineages. What sorts of mechanisms generate this variation and its constraints? As Krubitzer and Seelke review, both intrinsic genetic and extrinsic activity-dependent mechanisms are at play. Furthermore, variation in one part of the nervous system can induce changes in distant, functionally related brain regions. For example, removal of the eyes during early development causes a dramatic reduction and functional respecification of the primary visual cortex. A similar cascade effect has been observed in blind mole rats. Thus, experimental manipulations of brain development can mimic at least some aspects of natural variation.
Jon Kaas continues in Chapter 7 the discussion of mammalian cortical variation, but his chapter is focused more explicitly on neocortical modules, which include cortical areas, patches, bands, stripes and interstripes, blobs and interblobs, and columns and minicolumns. Within each module, adjacent neurons tend to be activated by similar stimuli at similar locations or, for movement-related neurons, to control similar behaviors. Between modules, activity patterns change abruptly. These findings suggest that cortical modules are generated by Hebbian plasticity, which strengthens connections between neurons that fire simultaneously or nearly simultaneously. Although this form of plasticity is most often invoked as a mechanism for generating topographic maps within the brain, it can also explain the formation of abrupt boundaries, because such boundaries can maximize the overall probability that adjacent neurons fire concordantly. As Kaas suggests, the mechanisms for topographic map and module formation seem to exist throughout mammalian neocortex but also in some other brain regions, such as the frog’s optic tectum.
In Chapter 8, Suzana Herculano-Houzel steps back from the organizational details of mammalian brains and focuses instead on the number of
neurons and nonneurons (primarily glia) found in the major brain regions of various mammals. Using the isotropic fractionator method, which involves homogenizing brain regions and counting stained cell nuclei in samples from the resulting homogenate, she discovered that neuron numbers scale differently (against brain region mass) in primates and rodents. This finding may explain why primates tend to be more intelligent than other mammals, even when brain mass is held constant: as brain size increases, primates have more neurons per gram of brain tissue than other mammals. Accordingly, Herculano-Houzel argues that absolute neuron number is a better predictor of “intelligence” than absolute brain size. She also points out that human brains contain almost exactly the number of neurons that one would predict, given the primate scaling rules. This conclusion would have pleased T. H. Huxley, if not Darwin himself. Moving beyond these findings, Herculano-Houzel proposes interesting ideas on the evolution of brain energy costs and their relationship to feeding behavior.