Figure 2: The distribution of volatility P values across all genes in the M. tuberculosis and P. falciparum genomes.
From: Detecting selection using a single genome sequence of M. tuberculosis and P. falciparum
In the absence of differential evolutionary pressures between genes—and, in particular under completely neutral evolution—the distribution of P values would be uniform. Instead, each genome shows a characteristic U-shaped distribution, with a significant (Kolmogorov–Smirnov, P < 10-6) excess number of genes with low or high P values. The non-random tails of these distributions indicate a large number of genes under greater pressure for amino-acid substitutions than the genome as a whole, and a large number of genes under greater pressure against amino-acid substitutions.
