Unalignable sequences and molecular evolution

Abstract

Alignment ambiguity is a widespread problem in molecular evolutionary studies that has received insufficient attention. Most studies ignore such regions by deleting them before analyses, even though alignment-ambiguous regions can contain useful phylogenetic and evolutionary information. The alignment ambiguity might affect only one taxon, the region being readily alignable and phylogenetically informative across all other taxa. Alternatively, all possible alignments can consistently imply certain relationships. Because they are usually the most rapidly evolving regions, alignment-ambiguous regions might be those that are most able to resolve closely spaced divergences and contribute to estimates of branch lengths, evolutionary rates and divergence times. Three methods to incorporate such regions into phylogenetic and evolutionary analyses have been devised. The multiple analysis method evaluates each plausible alignment separately and seeks areas of congruence among the resultant trees, whereas the elision method combines all plausible alignments into a single analysis. Fragment-level alignment (= fixed states, INAASE) treats the entire unalignable section as a single but highly complex multistate character. Although these methods still need refining, they are preferable to discarding large portions of hard-earned and potentially informative sequence data.