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OCR for page 335
Epilogue
A TANGLED MULTILAYERED WEB
R
eviewing 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 dimen-
sion, 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 visual-
ized 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 homolo-
gies at one level of biological organization may or may not be linked
to homologies at higher or lower levels (Brigandt, 2002). For example,
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336 / Epilogue
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 underly-
ing neuronal circuits. If neurons can change their behavioral functions
over evolutionary time, then homologous behaviors may involve nonho-
mologous 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, espe-
cially if we compare our current state of knowledge with the knowledge
in Darwin’s time.
