The structure and function of every living organism is determined by genes it inherits from its ancestors. The rules for combining genes from parents to form offspring are well understood, and usually the attributes of a child can be traced to its parents genes. However, in what appear to be random events a mutation will be introduced. This mutation will occur somewhere in the genetic material of a new individual and will be a feature that is not traceable to either parent.
In the vast majority of cases mutations are harmful. In other words, the new individual is less likely to survive to reproduction age than its parents. In those cases where an individual does survive, it is slightly less fit than others in its generation and is less likely to mate and pass on its mutation.
When the size of the population is fixed, however, the situation changes slightly. Fixed size populations are found on islands, for example, where there is no chance for migration from other populations to add to the gene pool and there is a maximum number of individuals that can be supported by the environment. In these situations surviving individuals with harmful mutations are more likely to mate since other individuals have restricted choices. This increases the probability that harmful mutations are passed down to the next generation.