Frequency spectrum and mismatch distribution from a world sample of 718 Y chromosome sequences. There are 20 segregating sites ascertained at approximately 20,000 positions. Ascertainment was done in two samples, one with 21 chromosomes and one with 53. If a site has population frequency *x*, then the probability that it will be detected in a sample of size *n*, the ascertainment function, is 1 − (*x*^{n} + (1 − *x*)^{n}). Diamonds show the expected number of sites in each frequency class under the hypothesis of constant population size, computed by multiplying the ascertainment function by the distribution in Eq. . The excess of low frequency sites is consistent with a Pleistocene population expansion. Because the whole nonrecombining portion of the Y chromosome is a single locus, these sites are not independent, and there is no simple statistical test of the constant size hypothesis.

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