Problems of macroevolution (molecular evolution, phenotype definition, and canalization) as seen from a hierarchical viewpoint

Abstract

As seen from a hierarchical viewpoint, macroevolution is neither a functional process nor a series of events in the past. It is a record only. For this reason macroevolutionary laws are all statistical laws.Natural selection is a process that operates from one generation to the next at the population level in the hierarchy. Yet structures at the organism level are found to "evolve." It is possible to formulate only a tautological form of the concept of natural selection at the population level alone; the bridge between levels in this case is the phenotype. The phenotype (i) exists at the boundary between the organismic and population levels of the hierarchy; (ii) is a functional manifestation of the interaction between the genotype and the local environment only during the period of a single generation; (iii) should ideally be defined so as to exclude traits not reviewed by natural selection; (iv) is factorable into many individual functional traits if one views viability selection as being instituted by a sequence of environmental catastrophes, each of which emphasizes a particular set of traits as being temporarily important to survival.It is reemphasized that the action of natural selection on continuously distributed, nonpolymorphic traits curtails variability in proportion to the intensity of selection. The necessity for coadaptation within the organism imposes a bell-shaped curve upon surviving variability. Canalizing selection is proposed as the process that modifies these bell-shaped curves into lognormal parametric distributions. It is also proposed that the per cent variability of the sample populations can serve as a measure of the intensity of natural selection (normalizing and directional together) that has most recently been acting upon the traits in question in the populations used to establish the parametric distributions. © 1975 American Society of Zoologists.