Evolutionary theory predicts the recent spread of primate immunodeficiency viruses (PIVs) to new human populations to be accompanied by positive selection in response to new host environments and/or by random genetic drift. I assess evidence for positive selection in human and chimpanzee PIVs type I (PIV1s), using ratios of synonymous to nonsynonymous nucleotide change based on branch lengths and outgroup rooting. Ratios are smaller for PIV1s from humans than for PIV1 from a chimpanzee for the pol, gag, and env glycoprotein 120 (gp120) regions, indicating greater effects of positive selection in PIV1s from humans. Parsimony-based relative rate tests for amino acid changes showed significant differences between PIV1s from humans and chimpanzees in 18 of 48 pairwise comparisons, with all 18 showing faster rates of change in PIV1s from humans. This study indicates that in some instances, the recent evolution of human PIV1s follows a speciational pattern, in which increased diversification of taxa is correlated with greater amounts of character change appearing and being maintained through time. This extends the generality of the speciational pattern to a group of organisms (viruses) having the fastest known rates of anagenetic change for nucleotide characters and indicates that comprehensive understanding of PIV1 evolution requires consideration of both anagenetic change within viral lineages and the relative historical success of different viral clades. Phylogenetic analyses show that neither PIV1s infecting humans nor those infecting chimpanzees represent monophyletic groups and suggest multiple host-species shifts for PIV1s.