Why does the variability of species decrease with the age of the phylogenetic line and why does the maximal diversity (more correctly, the maximal disparity) of phylogenetic lines occur early in the history of the lines?
Both phenomena (higher variability of early-branched species and decreasing speciation rate of clades) could have a common cause, namely, the continuous irreversible freezing of more and more traits during the evolution of a clade. It is certain that the traits differ in resistance to transition from frozen to plastic in response to reduction of genetic polymorphism. This process is likely to occur readily for some traits and can be achieved by a relatively small reduction in genetic polymorphism. For other traits, the transition from frozen to plastic is difficult or even impossible, as it requires an unrealistically small founding population and an unrealistically long period of persistence of such a small population in an extinction-prone state. On a macroevolutionary time scale, more and more traits pass into the permanently frozen state due to the universal process of sorting on the basis of stability. The stable traits (and systems and such) persist, while the unstable traits (and systems and such) pass away. In a new clade, a high proportion of species contains many traits which could melt during standard peripatric speciation or which are relatively plastic even on the level of a species (or even of a local population). Through time, more and more traits in more and more species turn into a semipermanently or even permanently frozen state. The representatives of a particular clade are not only less and less variable (more and more elastic – resistant to selection pressure), but also exhibit elasticity that is less and less affected by peripatric speciation. Originally, many representatives of a clade had the capacity to evolve new body plans after peripatric speciation. In the end, only some species retain this capacity, and even in these species some traits have a highly limited capacity to respond to selection after peripatric speciation.