Most species that we know from the paleontological record existed for the order of a million to several million years. It is very striking how little the phenotype of a species changes during its existence. While species change during their existence cyclically or acyclically, phenotype changes rarely exceed the limits of normal intraspecies variability estimated on the basis of inter-population differences. It is only thanks to this lack of variability that the geological science known as biostratigraphy can exist. In the biostratigraphic determination of the age of rocks, the existence of evolutionary stasis makes it impossible to utilize changes in the phenotype of individuals occurring within a certain species over time. However, it makes it much easier to utilize changes in the numbers of the individual (phenotype-invariable) species in the particular geological layers (Gould 2002).
Simultaneously, the phenotypes of species remains unchanged even when there is a drastic change in the climate in the region and thus at a time when they are exposed to substantial changes in selection pressures. In fact, it even seems that species change most slowly at the time of the most drastic climate changes, for example at the time of alternation of glacial and interglacial periods (Erwin 2000). If the phenotype of a species changes, then these changes are related primarily to traits that react directly to the physical conditions in the environment of the particular individual, i.e., for example, a change in body weight and overall body dimensions. Most of the changes observed during the existence of a species thus most probably correspond to ecophenotype changes, i.e. changes of a nongenetic nature.
A period in which no anagenetic processes occur in a particular species is termed evolutionary stasis. Evolutionary stasis is apparently not only a consequence of the absence of selection pressures and the absence of evolution, but is rather a certain type of active evolutionary process. Even in the complete absence of selection pressures, the average phenotype of the members of a certain species should have fluctuated through the action of random processes (e.g. drift) more than they actually fluctuated, as is apparent from the paleontological record. Thus, with great probability, evolutionary forces act on species and their populations, making them resistant to random changes, to be more exact determining the tendency of the population or species to the to the original phenotype after a random change. This force could, for example, be normalization selection. However, because the species retains its typical phenotype even at times of substantial climate changes and over its entire area of occurrence with heterogeneous natural conditions, this explanation is not sufficient.