Gene Trees and Species Trees: Molecular Systematics as One-Character Taxonomy

Reconstruction of phylogenies from molecular data has become an important and increasingly common approach in systematics. The product of such studies is a gene tree, hypoth- esizing relationships among genes or genomes. This gene tree may be fundamentally incongruent with the true species phylogeny, due to various biological phenomena such as introgression, lineage sorting, or mistaken orthology. In such circumstances all of the gene tree characters defining the relationships of molecular taxa (haplotypes) may be necessarily correlated, and the gene or genome may behave as a single species tree character. In these circumstances robustness of the gene hypoth- esis is meaningless as a measure of confidence in the species phylogenetic hypothesis. Incongruence between a phylogenetic hypothesis based on numerous, presumably independent, non-molecular characters and a single gene tree should not be assumed to be due to noise in non-molecular data. As with other characters, a character phylogeny, in this case a gene tree, can be tested best by a parsimony analysis in which other characters are included. If independence of molecular characters is assumed, then each is an equivalent phylogenetic hypothesis, as is each non-molecular character, leading to the suggestion that direct combination is appropriate. Swamping becomes an issue when a large molecular data set may be behaving as a single character. To alleviate this problem, a gene tree may be treated as a single multistate character, either ordered or unordered, and included with non-molecular data to obtain a globally parsimonious result. An example is given using published molecular and non-molecular data from the Asteraceae.