The main features of the biosystem "organism" are as follows: integrity, hierarchy, openness, functionality, stability, adaptability to the environment.
The main features of biosystems:
6. adaptability to the environment.
I would not single out the main principle. There are several of them, and they complement each other. Take 2 traits: heredity and variability. Remove heredity, and the organism will not be able to pass on to its descendants new traits obtained in the course of evolution. This means that the sign of variability becomes useless.
An organism is an open living system, a living being, an elementary structural unit of life.
I believe that the main feature of an organism is the interconnection of its individual parts: organs, tissues, cells. If this relationship fails, the body will die.
The interconnected work of organs is the integrity of an organism that functions as a living system.