Table 2

Statistical tests of natural selection

β-GlobinNo. of polymorphic sites28 (31.2)*24 (27.2)20 (15.3)
DNA length284326893000
Divergence29.8 (26.6)25.7 (22.6)32.2 (37.0)
DNA length215019922241
PDHA1No. of polymorphic sites25 (21.8)24 (20.9)2 (6.7)
DNA length415341014212
Divergence45.6 (48.8)44.1 (47.2)49.2 (44.5)
DNA length414141094171
X 2 0.71940.76613.965
  • 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.