Within the population, breeding individuals contribute gametes
equally to a pool from which zygotes are formed. Survival of zygotes is
random, so that the contributions of families to the next generation are
not equal. The mean number of offspring per adult is 1. These conditions
result in a variance in family size of 1, so that family sizes may be
0, 1, 2, 3, 4, etc. This distribution of family sizes is described by the terms
of the Poisson distribution.