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(
A
) 'Above-the-Diagonal' group (<5%)
All RNA pairs
(
C
) Horizontal group
(
B
) 'Below-the-Diagonal' group
(> 95%)
(
D
) Vertical group
Fig. 12.20 A classification of the points on the genotypic similarity vs. phenotypic distance
(GSvPD) plot
these RNA molecules are under the control of their structural genes so that none of
their differences lie above the diagonal. These RNA pairs belong to the group of
“self-regulatory” or “self-regulated” genes. Group
C
consists of the RNA pairs
whose coordinates lie along horizontal lines, indicating that their intracellular
concentrations are similar despite the fact that their structural genes are different.
We will refer to this behavior as the
genotypic freedom with phenotypic constraint,
which may be viewed as the molecular counterpart of (or as ultimately responsible
for) the phenomenon of
convergent evolution
(see Glossary) on the macroscopic
scale. Group
D
contains the RNA pairs whose coordinates lie along vertical lines,
indicating that their intracellular concentrations can vary over a wide range despite
the fact that their structural genes are similar. This behavior may be referred to as
the
phenotypic freedom with genotypic constraints
, which may be analogous to (or
ultimately responsible for) the phenomenon of
divergent evolution
(see Glossary)
on the macroscopic scale.
Group
B
RNA pairs may seem paradoxical in the sense that they contain both
Groups
C
and
D
that exhibit no correlation between genotypes and phenotypes, thus
embodying the phenomena of the genotypic freedom with phenotypic constraints and
the phenotypic freedom with genotypic constraints, respectively. One possible expla-
nation for these seemingly paradoxical observations is that the intracellular levels of
Group
B
RNA pairs are controlled not only by their own structural genes but also by
other genes such as those encoding transcription factors, enhancers, and silencers. To
the extent that intracellular RNA levels are controlled by genes other than their own
structural genes, to that extent the points in the
genotypic similarity
vs
. phenotypic
distance
(GSvPD) plots would deviate from the associated diagonal lines.
The triangular distribution of points in the GSvPD plots (Figs.
12.17
,
12.18
)
embodies both
determinism
and
nondeterminism
, reminiscent of
deterministic chaos
in dynamical systems theories (Scott 2005). The
determinism
is reflected in the fact
that almost all the points in the GSvPD plot lie below the diagonal line, whereas the
nondeterminism is exhibited by the fact that Group D RNA pairs (Fig.
12.20
) show
unpredictable phenotypic behaviors despite their genotypic similarities. The term
nondeterminism
is interpreted here as synonymous with “unpredictability” in phys-
ics and
creativity
in linguistics. Therefore, it appears reasonable to conclude that the
triangular distribution of the points in the GSvPD plot is an experimental evidence
