Phenotypic evolution is a complicated process compared to molecular evolution, and fossils are all phenotype with no genotype. The phenotype has many causal factors, which are often grouped into genetic and environmental components. In phenotypic evolution, only the genetic components matter, whereas in molecular sequence evolution everything is ?genetic?. Similarities do exist between phenotypic and molecular evolution, though. One is that the rate of evolution is directly related to the accumulation of homoplasy, a phenomenon sometimes called the ?saturation? rate. While it is recognized that some molecules evolve faster than others and that slow molecules are better for phylogeny reconstruction of deep branches while fast ones are better for shallow branches, this concept has scarcely been investigated in relation to phenotypic evolution. The relevance of a given type of morphological data (discrete characters, shape variables, size variables) to a particular phylogenetic problem is probably less a matter of systematic philosophy and more a matter of evolutionary rates, themselves determined by heritability, selection, constraints, and drift.

Investigation of mammalian molar teeth, the mainstay of mammal paleontology, gives some interesting insights into these issues. Molar size evolves more quickly than molar shape, which in turn evolves more quickly than discrete trait acquisition and loss. Phylogenetically, discrete traits will be more valuable for deeper divergence times, size for very shallow ones, and shape intermediate. Among the metric traits (as measured in the Least Shrew, Cryptotis parva), heritability is lowest in molar length (h²=0.35), intermediate in molar width (h²=0.42), and highest in molar shape (h²=0.50). Consequently, molar shape would be expected to evolve more quickly; however, size is a univariate trait, which is more readily ?homoplastic? than multivariate shape. The components of genetic variation are not parallel to the phenotypic components, and neither are parallel to evolutionary ?directions?. Compared to cytochrome b mtDNA, molar shape evolves faster making it relevant for reconstructing phylogeny for divergences between 10,000 and 10,000,000 years. Molar size evolves more quickly, and discrete traits more slowly.