Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

It is quite natural to expect that the rate of evolution should be greater for organisms with a shorter generation period than for organisms with a longer generation period.However, real data do not confirm this assumption and, in fact, very frequently show quite the opposite.For example, elephants are one of the lines with the greatest taxonomic rate; on the other hand, some lines of small fauna with short generation times evolve very slowly (Simpson 1961).

            Several possible explanations have been proposed for this paradox.One of them is that organisms with a long generation time are simultaneously organisms with large body dimensions and large organisms also have relatively small numbers of individuals in their populations.Because of the greater effectiveness of genetic drift and lower effectiveness of stabilizing selection in small populations, species in small populations can evolve faster (see  IX.2).Another explanation is based on the idea that most evolutionary changes constitute a response of the organisms to a change in the environment (Gillespie 2001).Thus, the rate of evolution is actually limited, not by the ability of the population to respond rapidly to a change in the environment, but by the rate of these changes.In organisms with a long generation time, greater changes occur in the environmental conditions during this generation time and thus the intensity of selection pressures acting on them is greater on an average than for organisms with a shorter generation time