Genetic differences exist within every species. Selective pressures mean that certain genetic features may be more or less advantageous depending on an organism's environment. Through NATURAL SELECTION the genetic makeup of a species will change over time to favor more advantageous traits.
When humans select for certain traits which we consider desirable, we call it ARTIFICIAL SELECTION. Dog breeds are a good example of this process. In cannabis, we call this process PHENO HUNTING.
During the pheno hunt, our cultivators plant hundreds of seeds from a given cultivar [a.k.a. strain]. Each of these seeds contains slightly different genetic material and will respond in slightly different ways to its environment, thereby producing hundreds of individual phenotypes of that cultivar. Ritual cultivators then evaluate each phenotype (each individual plant) for desirable traits.
The most desirable traits in flower that will be dried for smoking or vaporization are different from the traits we look for in flower that will be washed and pressed into hash rosin.