Is the microbial tree of life verificationist?

The field of microbial phylogenetics has questioned the feasibility of using a tree-like structure to the describe microbial evolution. This debate centres on two main points. First, because microorganisms are able to transfer genes from one to another in zero generations (horizontal gene transfer, or HGT), the use of molecular characters to perform phylogenetic analyses will yield an erroneous topology and HGT clearly makes the evolution of microorganisms non tree-like. Second, the use of concatenated gene sequences in a total evidence approach to phylogenetic systematics is a verificationist endeavour, the aim of which is to bolster support. However, the goal of the total evidence approach to phylogenetic research is based in the idea of increasing explanatory power over background knowledge through test and corroboration, rather than to bolster support for nodes in a tree. In this context, the testing of phylogenetic data is a falsificationist endeavour that includes the possibility of not rejecting the null hypothesis that there is no tree-like structure in molecular phylogenetic data. We discuss several tests that aim to test rigorously the hypothesis that a tree of life exists for microorganisms. We also discuss the philosophical ramifications of background knowledge and corroboration in microbial studies that need to be considered when suggesting that HGT confounds the tree of life. © The Willi Hennig Society 2009.