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hypothesis, a metabolic novelty may arise more often via the emergence of new
combinations of enzymes in metabolic networks rather than via the evolution of
new enzymes through gene mutations. The problem with this system class is that
it is investigated by computational methods alone, because it is currently unfeasible
through experiment. The validation of the mechanism has to wait until empirical evi-
dence becomes available.
The second system class involves regulatory circuits comprising primarily
transcriptional regulatory circuits, in which a transcriptional factor regulates the
expression of another. The hypothesis posits that mutations in regulatory proteins/
sequences can produce inherited changes in relationships between the transcriptional
regulators in a circuit that can induce changes in the gene expressing phenotype with
all the ensuing consequences in development and evolution. The evidence supporting
this idea is ambiguous and it does not explain how changes in relationships between
regulators (not in genes or gene sequences) may be inherited.
The third system class comprises the established neoDarwinian concept of gene
mutations, discussed earlier.
When the hypothesis posits that external stimuli may induce an adaptive change
in the phenotype, it is not clear if it implies random changes or adaptive nonrandom
changes. If the first is the case, then it faces the same statistical unlikelihood. If the
latter is the case, then the source of information is beyond the three system classes.
To the extent that the hypothesis relies on random mutational events, it faces all the
difficulties that modern synthesis does.
The Hypothesis of Epigenetic Variation
About two decades ago (1995), Eva Jablonka and Marion Lamb concluded that
the modern synthesis was unable to account for a number of nongenetic phenom-
ena such as DNA methylation, X-chromosome inactivation, genome imprinting, and
chromatin modifications that influence patterns of gene expression. They postulated
the existence of an epigenetic inheritance system (EIS), which would expand the
concept of biological inheritance and acknowledge a Lamarckian role in evolution
( Jablonka and Lamb, 1995 ). They and others (Avital et al.) further developed the
hypothesis in its present form ( Jablonka and Lamb, 2008; Jablonka and Raz, 2009 ).
Living organisms are endowed with varying degrees of developmental plasticity
that they express in response to environmental stimuli. Developmental plasticity is
the primary source of heritable non-DNA epigenetic variations arising in response to
induced stimuli that are transmitted to later generations of individual cells or organ-
isms. These variations fall into one of three main categories, epigenetic, behavioral,
and symbolic, with the latter relevant for human evolution only.
Epigenetic variations are related to an EIS that is responsible for the cellular epi-
genetic inheritance. It comprises self-sustaining feedback loops or transcriptional cir-
cuits, structural inheritance of the type of the ciliate cortical inheritance, chromatin
markings, which include both the DNA methylation and histone modifications and,
finally, the RNA-mediated inheritance .
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