Understanding the evolutionary history of species is at the core of molecular 
evolution and is done using several inference methods. The critical issue is to 
quantify the uncertainty of the inference. The posterior probabilities in 
Bayesian phylogenetic inference and the bootstrap values in frequentist 
approaches measure the variability of the estimates due to the sampling of sites 
from genes and the sampling of genes from genomes. However, they do not measure 
the uncertainty due to taxon sampling. Taxa that experienced molecular 
homoplasy, recent selection, a spur of evolution, and so forth may disrupt the 
inference and cause incongruences in the estimated phylogeny. We define a taxon 
influence index to assess the influence of each taxon on the phylogeny. We found 
that although most taxa have a weak influence on the phylogeny, a small fraction 
of influential taxa strongly alter it even in clades only loosely related to 
them. We conclude that highly influential taxa should be given special attention 
and sampling them more thoroughly can lead to more dependable phylogenies.