Neutral genetic variation in a population is eventually lost to drift. The probability, at any given time, that an allele will eventually become fixed equals the frequency of the allele at that time. The rate at which these events occur is greater for small populations. For an allele which eventually becomes fixed, the average time to reach fixation is 4N . The effective population size is the size for which expected heterozygosity matches observed heterozygosity.
Individuals vary greatly in number of progeny.
Numbers of males and females are not equal. Population size fluctuates. Subdivided population structure. Successive generations are a random sample from the parental gene pool. That is, indeed, the Hardy-Weinberg expectation.