  The site frequency spectrum describes variation among a set of n DNA
sequences. Its i'th entry (i=1,2,...,n-1) is the number of nucleotide sites at
which the mutant allele is present in i copies. Under selective neutrality,
random mating, and constant population size, the expected value of the spectrum
is well known but somewhat puzzling. Each additional sequence added to a sample
adds an entry to the end of the expected spectrum but does not affect existing
entries. This note reviews the reasons for this behavior.
