Related to the idea of stabilizing selection is that of the requirement that organisms function adequately in their environment. Thus, where stabilizing selection acts because of the particular niche that is occupied, mechanical and physico-chemical constraints act in a more general manner. For example, the acceleration caused by gravity places constraints on the minimum bone density and strength for an animal of a particular size. Similarly, the properties of water mean that tissues must have certain osmotic properties in order to function properly.
