Biology Reference
In-Depth Information
homeostasis of metabolism and moved into the state of
survival without water, to cope with a major external
perturbation. This creature is robust against dehydration
stress, but this robustness is generated by the sacrifice of the
homeostasis of metabolism.
for such a state. This word does not indicate something that is
fixed and does not move, or a stasis. It means a certain state that
may change but is relatively constant.'
[5]
Stability is a concept similar to homeostasis because it is
basically judged by the degree of maintenance of a state.
Robustness, on the other hand, is focused on the mainte-
nance of functions as a criterion, not of the maintenance of
a state. That is to say, if a state changes substantially in
order to maintain the functions, it can be considered as
a kind of robustness, but not as homeostasis or stability.
With mathematical abstraction, the state of the system
can be expressed in N-dimensional phase space.
Figure 24.1
represents a simplified two-dimensional phase
space. When perturbations are imposed, the state of the
system drifts in phase space. If the degree of perturbation is
small and the system tends to be stable, then the phase
space trajectory of the system's state orbits around the
'basin' of the attractor and gradually returns to the initial
state. Bacterial chemotaxis machinery is an example of
such a case.
However, there are cases where perturbations trigger
a transition of the state from one basin of the attractor to the
other. Systems may be robust in this case by switching to
a new state that maintains functions, rather than returning
to the original state. Nature provides a satisfactory
example: the tardigrade
[6]
. This creature crawls around in
its normal state. When it enters in a high-salt environment
or a very dry environment, it becomes dehydrated, stops
metabolic activity and enters into a kind of freeze-dry state.
It survives in this state for years and can start crawling
again when water is available. In this case, it gave up the
Robustness and Homeostasis
Attention should be paid to the essential difference between
robustness and homeostasis. There is a major impact on
selection in evolution from the functions associated with
higher probabilities of survival and reproduction, not
directly from the states. However, the functions related to
maintaining the state of an individual are subject to selec-
tion only when they are linked to the possibility of survival
and reproduction. The system's behaviour of increasing the
probability of survival by moving into a completely
different state to cope with perturbations, just like the
response of the tardigrade against dehydration, may be
considered robust, but it cannot be identified as the main-
tenance of homeostasis. When the continuation of a certain
state is effective in maintaining the functions of a system,
robustness and homeostasis (or stability) are equivalent, but
when the transition of the system into a new state is an
effective response to a disturbance, a robust system will
abandon homeostasis.
In addition, there are some cases in which robustness is
maintained because a system is unstable. HIV has a very
high mutation rate and part of its gene sequence changes
frequently. Because of this, HIV escapes from immune
system control. In the case of cancer, the chromosome
becomes more unstable during advanced stages and groups
FIGURE 24.1
Robustness and stability.
Robust Adaptation
(return to a periodic attractor)
A transition to
a new attractor
Stochastic process
influences a trajectory
Robust Adaptation
(return to a point attractor)
Unstable
