The bark of a tree is impervious to gases and water because bark cells are dead cells of the epidermis and compactly arranged without intercellular spaces. They contain a chemical called suberin in their walls that make them impervious to gases and water and it doesn't allow gases or water into the tree as bark cells are in the bark outer most layer of the tree.
Answer:
The Hardy-Weinberg law states that in a sufficiently large population, in which matings occur randomly and that is not subject to mutation, selection or migration, gene and genotypic free frequencies are kept constant from one generation to another, once a state of equilibrium has been reached, which in autosomal loci is reached after one generation.
It is said that a population is in equilibrium when the alleles of the polymorphic systems maintain their frequency in the population throughout the generations.
Hierarchy - I don’t think I have the full context of the question but the word fits the best of its biology related
