Clock models for evolution of discrete phenotypic characters

Clock models, which consider rates of character change through time and across lineages, are widely used in molecular phylogenetics for inferring the pattern and timing of evolutionary divergences. However, clock models are also potentially relevant to phenotypic analyses, such as those involving morphological characters. Important hurdles need to be overcome, including biases in character sampling (e.g., changes along terminal branches are often not sampled) and strong among-lineage heterogeneity in evolutionary rates (e.g., 'living fossils' vs rapidly evolving taxa). These caveats notwithstanding, an increasing number of empirical studies have applied clock models to good effect in phylogenetic analysis, especially in combined analyses of DNA and morphological data from fossil and living taxa (total-evidence tip-dating). These studies have improved our estimates of the shape and chronology of the tree of life.