Table 2.

Statistical tests of natural selection

Locus	Measure	Total	African	NonAfrican
β-Globin	No. of polymorphic sites	28 (31.2)*	24 (27.2)	20 (15.3)
	DNA length	2843	2689	3000
	Divergence	29.8 (26.6)	25.7 (22.6)	32.2 (37.0)
	DNA length	2150	1992	2241
PDHA1	No. of polymorphic sites	25 (21.8)	24 (20.9)	2 (6.7)
	DNA length	4153	4101	4212
	Divergence	45.6 (48.8)	44.1 (47.2)	49.2 (44.5)
	DNA length	4141	4109	4171
	X2	0.7194	0.7661	3.965
	Probability	0.396	0.381	0.046†

The test (33) generates a statistic, X², that is expected to be chi-square distributed with one degree of freedom. DNA lengths are the average length of sequences (excluding insertions/deletions) for the regions compared, for polymorphism and divergence, respectively. Divergence was calculated as the average number of pairwise differences between the human sequences and the chimpanzee sequences. For the β-globin data (4), the chimpanzee sequence (44) was not as long as that for most of the human sequences, so the sequence lengths for the average divergence values are less. For the β-globin data, there were 222 non-African sequences and 127 African sequences. 

^*The expected values of polymorphic sites and divergence are shown in parentheses. 

^†The probability of observing the X² value, by chance, is less than 0.05.