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[135
137] . Buchnera has a much simpler metabolic
network comprising only 263 reactions. During its long
association with its host and the constant environment this
host provides, Buchnera has lost the ability to survive on
a broad spectrum of nutrients. What is more, it has also lost
almost all robustness to the removal of chemical reactions
from its metabolism [135] .
A metabolism that is to sustain life in multiple different
chemical environments needs specific enzymatic reactions
to metabolize all the nutrients that these environments may
contain. It therefore needs to be more complex than
a metabolism highly specialized to one specific environ-
ment. This increased complexity endows metabolism with
robustness to the removal of reactions in any one envi-
ronment [10,57] . Although the relationship between envi-
ronmental change, increased complexity, and robustness is
not as well explored for proteins and regulatory circuits,
similar arguments can be made for them [10] .
In sum, the ability to cope with changing environments
can require increased complexity of a biological system,
which can cause robustness to genetic change in any one
environment. Such robustness is responsible for the exis-
tence of genotype networks, which can facilitate the origin
of new phenotypes. Minimally complex systems may not
display some of the core properties discussed here [98] , and
may thus not be capable of exploring a broad spectrum of
new phenotypes.
These spaces may harbor further, unrecognized similarities,
but also differences among different kinds of systems.
A combination of high-throughput genotyping with
emerging technologies for high-throughput phenotyping
[138] , and sophisticated computer models of genotype
e
e
phenotype relationships may reveal many more such prin-
ciples in the years to come. Given how vast genotype
spaces are, myriad principles of phenotypic variability may
still await discovery.
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Conclusions and Future Challenges
Genotypic changes in metabolism, in regulatory circuits, as
well as in protein and RNA molecules are involved in many
if not all evolutionary innovations. These system classes are
therefore important study objects to understand the
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The observations made here regard qualitative
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