In the abstract model we will represent the severity of a mutation by a number between 0 and 1. A value of 0 means will indicate a neutral (harmless) mutation, and a value of 1 will represent a lethal mutation. All of the environmental factors that contribute to survival -- whether an individual can gather food effectively, or survive temperature extremes, etc -- are abstracted into this single value.
In real organisms each mutation will have a different effect, depending on which part of the genome it changes. In the mathematical model, however, we will make a simplifying assumption that all mutations have a constant and slightly negative effect on the health of an individual, no matter where they occur in the genome. We will use a single parameter named s, where , to represent the average effect of each mutation. S is known as the selection coefficient.