main article: effective population size
In population genetics an idealised population, also sometimes called a Fisher-Wright population after R.A. Fisher and Sewall Wright, is a population whose members can mate and reproduce with any other member of the other gender, has a sex ratio of 1 and no overlapping generations. As such, each individual has the same chance to contribute its own genetic material to the next generation (no selection).
Deviation from the idealised population results in the effective population size being smaller than the total population size.