Biomedical Engineering Reference
In-Depth Information
2.
GENOTYPIC VERSUS ENVIRONMENTAL VERSUS
FUNCTIONAL ROBUSTNESS
When speaking of robustness it is worth bearing in mind the plethora of
definitions the word attracts. For an extensive list see [www.discusss.santa.
edu/robustness]. These are to some degree domain-specific. In ecology, stability
or robustness is a measure of the preservation of species diversity upon species
removal (38), or the permanence of a configuration when perturbing some vari-
able of ecological interest. In medicine, robustness is associated with healing
and compensation, neither of which imply a return to the original phenotype but
rather a restoration of wild-type function (56). In linguistics, robustness relates
to competence and comprehensibility despite incomplete information and ambi-
guity (27). Thus structural transformation is acceptable subject to information
remaining decodable. In paleontology, robustness relates to the continuity of
lineages across geological eras (13) and the persistence of lineages during mass
extinction events. In metabolism, robustness relates to limited phenotypic varia-
tion across large changes in kinetic parameters (21,63). In cell biology, robust-
ness can describe how cell fate decisions remain constant when transcription
regulation is stochastic (25), or how conserved RNA secondary structures can
remain resistant to point mutations (15).
In each of these cases robustness relates to either (1) non-detectable or mi-
nor modification in phenotype following a large perturbation to the genotype,
(2) non-detectable or minor modification in phenotype following a large pertur-
bation to the phenotype from the environment, or (3) non-detectable or minor
modification in function following a large perturbation to the genotype or phe-
notype with or without a correlated change in the phenotype. The important dis-
tinction between genotypic and environmental robustness is that in the first case
perturbations are inherited, whereas in the second case they are not. Functional
robustness can be achieved through phenotypic invariance or phenotypic plastic-
ity. In one case the phenotype resists perturbations, and in the second case the
phenotype tracks perturbations. Genotypic and environmental robustness can be
measured through the environmental (
V
e
) or mutational variance (
V
m
) of a trait,
whereas functional robustness can be measured as the variance in geometric
mean fitness. It is often the case that a single mechanism leads to all three forms
of robustness, in which case we observe
congruence
(4) between the genotype
and phenotype.
3.
PRINCIPLES AND PARAMETERS OF ROBUST ORGANIZATION
In (31,32), Krakauer and Plotkin describe three
principles
that have arisen
in an effort to understand the evolutionary response to mutations: the principle
of canalization, the principle of neutrality, and the principle of redundancy. We
