Higher living beings are an open system with many relationships to the environment. Changes in the environment trigger reactions in the system or directly affect it, leading to internal disturbances in the system (or the cell). Such disturbances are normally kept within narrow limits because automatic adjustments within the system take effect and in this way wide oscillations are avoided, the internal conditions being kept almost constant. The coordinated physiological responses that maintain most of the body's dynamic equilibria are so complex and particular to living organisms that it has been suggested that a particular designation be used for these reactions: that of homeostasis.
The concept of homeostasis basically refers to the notion of stationary state. Life can be defined as linked to maintained states, despite the tendency towards increasing entropy, thanks to the biological "history" itself. Cybernetic control mechanisms allow formalization tests.
It is probable that less sticklebacks will have pelvic spines in Bear Paw Lake, as it is a smaller region without as much competition. Predators may be much lower in such an environment, eliminating a need for such protection. In the ocean, however, there is much biodiversity and competition, so the fish need pelvic spines to protect themselves.
