Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 253

M.J. West-Eberhard, an inherent potential of animals to switch to alternative devel-
opmental states in response to external or internal factors. Weak regulatory linkages
enable the animal to adaptively respond to various external or internal stimuli, by
switching between two or more alternative states at different levels from the cellu-
lar to organismic levels. In this developmental plasticity, the authors see the “cryptic
source” of evolutionary change ( Kirschner and Gerhart, 2005 ).
Such phenotypic changes may be stabilized by the emergence of genetic accom-
modation. So, facilitated variation posits that evolutionary novelties are generated
primarily by developmental processes, while natural selection plays no role in the
emergence of evolutionary novelties.
They believe the facilitation evolved as a “by-product of properties of core pro-
cesses such as robustness, adaptability, modularity, exploratory behavior, and capac-
ity for regulatory weak linkage” ( Gerhart and Kirschner, 2007 ). According to the
hypothesis, the robustness and adaptability are essential for using core processes in
various combinations and quantities, thus reducing the need for regulatory changes
in the process of the evolution of novelties.
The supporting evidence for the hypothesis is hardly adequate or plausible. For
instance, the illustration of the exploratory behavior of the core components with the
example of neural crest cells is hardly convincing because these cells do not have
“many different options”; before leaving the neural tube, they are provided with
information on where to go and what to do. The stabilization of microtubules within
the cell could also hardly be considered a “futile cycle of outgrowth and shrinkage”
but is based on a processing of information on the state of the cytoplasm.
Although the hypothesis of facilitated variation considers the phenotypic vari-
ability rather than random mutational changes as the main source of evolutionary
change, it does not provide the source of the information that is used to select among
the developmental “options,” the one option that is adaptive under the specific exter-
nal or internal conditions.
The Developmental Plasticity Hypothesis
M.J. West-Eberhard's hypothesis marks a considerable progress in understanding
the origin of diversity and mechanisms of evolution ( West-Eberhard, 1989, 2003,
2005 ). Her studies on developmental plasticity culminated in the publication of the
influential Developmental Plasticity and Evolution (2003). West-Eberhard believes
evolutionary changes and speciation result from the reorganization of the pheno-
type in response to changes in the environment. The phenotypic reorganization may
be followed by selection and genetic accommodation of the new phenotype via the
occurrence of relevant mutations or the existing genetic variation. Ultimately, this
may lead to reproductive isolation and the evolution of new species. While genetic
changes occur spontaneously and randomly, phenotypic reorganization is an adaptive
response to external/internal stimuli.
New phenotypes can arise in response to genetic or environmental inputs in liv-
ing organisms which developmentally are very plastic. These inputs may lead to the