Measure of phylogenetic dissimilarity between two communities, is calculated as the noncommon portion of the phylogenetic tree (distance) that includes all organisms from both communities. UniFrac, thus measures the phylogenetic distance between groups of taxa in a phylogenetic tree as the proportion of the branch length of the tree that results in descendants from either environment, but not both.
UniFrac can be used to determine whether communities differ significantly, to compare many communities simultaneously using clustering and ordination techniques, and to measure the relative contributions of various factors, such as chemistry and geography, to similarities between samples. In microbiology, the utility of UniFrac is demonstrated by comparing the published 16S rRNA gene libraries of cultured isolates and environmental clones of bacteria. UniFrac offers a way to characterize microbial communities by exploiting the abundance of environmental rRNA sequences and providing quantitative insight into the factors underlying the distribution of lineages in different environments.