At the molecular level, the rate of evolution can be characterized by the rate at which new mutations arise within a species or lineage, thus it is typically measured as the number of mutant substitutions over time. These rates vary among both genes and lineages due to gene effects (such as nucleotide composition, among-site variation, etc.), lineage effects (generation time, metabolic rates, etc.), and interactions between the two. Even at the molecular level, population dynamics (such as effective population size) must also be taken into account when considering gene substitution since the rate of fixation of a mutant allele is affected by selective advantage.
