Archie (1989) , Faith (1990) , and Faith and Cranston (1991) devised a test for phylogenetic structure in character data sets, which Faith and Cranston called the permutation tail probability (PTP) test. Faith (1991) introduced a re nement intended to test the strength of support for monophyly of individual clades, which he called the topologydependent permutation tail probability (T-PTP) test. Faith presented a priori and a posteriori forms of the test; here, I will discuss the a priori test.

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... ll phylogenetic permutation tests involve comparing some property of the original data set with a null distribution constructed by randomly scrambling character states within each character across taxa, generating a large number of replicate data sets. The randomized data sets have some of the properties of the real data set-particularly the number of taxa with a given state for each characterbut they certainly have no phylogenetic information. The test property used by the a priori T-PTP test for monophyly is the Bremer (1994) support or decay index: the length of the most-parsimonious tree lacking the clade of interest minus the length of the most-parsimonious tree having the clade. The Bremer index of a clade for the original data set is compared with the distribution of Bremer indices of that clade over many randomized data sets. The P-value is simply the proportion of randomized data sets for which the clade's Bremer index is as great as or greater than its Bremer index for the original data set.