A matter of time - understanding the limits of the power of molecular data for delimiting species boundaries

The complexity of species delimitation has not decreased with the availability of molecular data and careful interpretation of data is needed for classification at the species level. Recent studies have revolutionized our understanding of species delimitations in lichen-forming fungi, including the discovery of a bewildering number of cryptic species. However, molecular sequence data has also illustrated the difficulty of separating some lineages that are well characterized by phenotypical traits. Without acknowledging the temporal aspect, species delimitation using molecular data will fail across a wide range of scenarios. In light of the prevalent use of molecular sequence data in species delimitation research of lichen-forming fungi, here we aim to highlight the major strengths and limitations of using genetic data to inform species-level taxonomy. Using examples from our research in Lecanoraceae and Parmeliaceae, we underscore how understanding the temporal component of diversification and statistical inference under the multi species coalescent model can help to inform taxonomic decisions. We use examples from Bryoria (Parmeliaceae) and Thamnolia (Icmadophilaceae) to demonstrate the power of statistical inference in helping to inform taxonomic decisions in closely related species groups. We conclude with a general discussion of some guidelines to incorporate evidence from molecular sequence into species-level taxonomy.