Reviewing the 17 chapters assembled in this volume, we do not see a tightly woven web. Instead, we see diverse perspectives on a much larger nexus that is as yet largely obscure. This larger web is full of interacting molecules, neurons, brain areas, and entire organisms, all changing through development and over evolutionary time. Neuroscience as a field is already complex, but when one adds the evolutionary dimension, the complexity becomes truly awesome and certainly beyond what one can expect to capture in just a few colloquium papers. Nonetheless, some recurring themes emerge.
One idea running through several contributions is that evolution and development are linked. Historically, evolutionary neurobiologists visualized evolutionary changes as transformations between adult forms. This thinking changed with the emergence of evo-devo biology, which was slow to infiltrate neurobiology but is now ascendant (Charvet et al., 2011; Friedrich, 2011; Medina et al., 2011; Sylvester et al., 2011). According to this view, evolutionary changes must involve changes in development, which can be inferred by comparing developmental mechanisms and trajectories between species. Such comparative developmental studies can reveal the mechanistic basis of evolutionary change and thus complement studies that address the ecological and behavioral contexts in which those changes might have been adaptive.
A second theme woven into several of the chapters is that homologies at one level of biological organization may or may not be linked to homologies at higher or lower levels (Brigandt, 2002). For example,
similarities in the expression patterns of homologous genes are sometimes used to argue for the homology of the structures in which those genes are expressed, but the genes might well have existed before the higher level structures came on the scene. As long as genes can change their functions over evolutionary time, this possibility is not easily dismissed. Even complex networks of interacting genes are, as Jarvis and colleagues argue in Chapter 4, capable of becoming involved in the assembly of novel structures. If similar changes in function occur independently in multiple lineages, then the structures would be nonhomologous, even though the underlying genes are homologous. In such cases, one might say that the structures are “deeply homologous” but “superficially nonhomologous,” although this terminology is likely to engender confusion.
Analogous challenges arise in comparative neuroethological studies. One can certainly homologize behaviors, be they swimming in snails or math skills in primates, but those behavioral homologies offer only loose predictions about the homology or nonhomology of the underlying neuronal circuits. If neurons can change their behavioral functions over evolutionary time, then homologous behaviors may involve nonhomologous neurons, and nonhomologous behaviors can involve at least a few homologous neurons. This point has been made before by various authors (Striedter and Northcutt, 1991), but it continues to befuddle the unsuspecting mind. As mentioned earlier, the task of understanding how the tangled bank of molecules, cells, structures, organisms, and behaviors has managed to transform itself in evolutionary time has only just begun. Still, as this volume aims to show, some progress has been made, especially if we compare our current state of knowledge with the knowledge in Darwin’s time.
