Biology Reference
In-Depth Information
Epigenetic mechanisms may also induce nonrandom changes in the genetic struc-
tures and genetic information, which is adaptive rather than random as assumed by
the modern synthesis:
The origin of many genetic variations, especially under conditions of stress, is not
random, is often predictable.
Jablonka (2008)
This clearly emphasizes the role of stress conditions in the induction and trans-
mission of epigenetic variation.
The supporting empirical evidence on epigenetic variation is adequate (
Jablonka
and Raz, 2009
). One of the most impressive illustrations of the role of the epige-
netic factors in evolution is the common toadflax,
Linaria vulgaris
, which sometimes
forms a variant with a very different floral system. It is believed that “epigenetic var-
iants in every locus in the eukaryotic genome can be inherited, but in what manner,
for how long, and under what conditions has yet to be qualified” (
Jablonka and Raz,
2009
). The involvement of hormones in the induction of heritable epigenetic varia-
tions is no longer mere speculation: several of the mammalian examples presented
suggest that changes in hormonal stimuli induce heritable epigenetic changes.
Despite the unquestionable contribution of the hypothesis in advancing our
understanding of the epigenetic factors in evolution, the hypothesis fails to show the
material basis, the mechanism, and the source of information necessary to produce
epigenetic marks in strictly determined DNA sites and chromosomes. The main con-
cern with the evidence, especially surrounding multicellular plants and animals, is
that epigenetic changes they present seem to have primarily deleterious effects or lit-
tle bearing on evolution.
The Hypothesis of Generic Factors
Stuart Newman and Gerd Müller developed a hypothesis on the role of physical
“generic” factors especially in the early development and in the early metazoan evolu-
tion (Müller and Newman, 2005;
Newman, 2006
). They believe that at the early stages
of evolution, about 600Mya, a period which they termed “physical” or “pre-Mende-
lian,” living systems used several physical mechanisms for embryonic patterning.
These physical factors might have been responsible for determining body plans during
the Cambrian explosion and explain the embryonic hourglass puzzle (
Newman, 2011
).
In their hypothesis, the adhesivity of unicellular organisms was the initial cause for
the forming of primitive multicellular aggregates and the emergence of such cell aggre-
gates made possible the activity of a number of physicochemical factors (viscoelastic-
ity, differential cell adhesivity, biochemical diffusion, etc.) that are active in nonliving
systems, but not in unicellulars. They believe spontaneously arising cell aggregates, in
which cells fail to separate after cell division, can form primitive multicellular organ-
isms. They posit that the formation and morphology of these cell aggregations was
determined by physicochemical factors and processes rather than by gradual changes
in genes and natural selection. These factors were the major sources of morphological
innovation in evolution at the early stages of evolution of multicellular systems.
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